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During star formation, the accretion disk drives fast MHD winds, which usually contain two components, a collimated jet and a radially distributed wide-angle wind. These winds entrain the surrounding ambient gas producing molecular outflows. We report a recent observation of ¹²CO (2–1) emission of the HH 46/47 molecular outflow by the Atacama Large Millimeter/submillimeter Array, in which we identify multiple wide-angle outflowing shell structures in both the blueshifted and redshifted outflow lobes. These shells are highly coherent in position–position–velocity space, extending to ≳40–50 km s⁻¹ in velocity and 10⁴ au in space, with well-defined morphology and kinematics. We suggest these outflowing shells are the result of the entrainment of ambient gas by a series of outbursts from an intermittent wide-angle wind. Episodic outbursts in collimated jets are commonly observed, yet detection of a similar behavior in wide-angle winds has been elusive. Here we show clear evidence that the wide-angle component of the HH 46/47 protostellar outflows experiences variability similar to that seen in the collimated component.

We investigate how the dynamical state of molecular clouds relates to host galaxy environment and how this impacts the star formation efficiency (SFE) in the Milky Way and seven nearby galaxies. We compile measurements of molecular cloud and host galaxy properties, and determine mass-weighted mean cloud properties for entire galaxies and distinct subregions within. We find molecular clouds to be in ambient pressure-balanced virial equilibrium, where clouds in gas-rich, molecular-dominated, high-pressure regions are close to self-virialization, whereas clouds in gas-poor, atomic-dominated, low-pressure environments achieve a balance between their internal kinetic pressure and external pressure from the ambient medium. The SFE per free-fall time of molecular clouds is low, ∼0.1%–1%, and shows systematic variations of 2 dex as a function of the virial parameter and host galactic environment. The trend observed for clouds in low-pressure environments—as the solar neighborhood—is well matched by state-of-the-art turbulence-regulated models of star formation. However, these models substantially overpredict the low observed SFEs of clouds in high-pressure environments, which suggest the importance of additional physical parameters not yet considered by these models.

Strongly lensed quasars with time-delay measurements are well known to provide the "time-delay distances" ${D}_{{\rm{\Delta }}t}=(1+{z}_{L}){D}_{L}{D}_{S}/{D}_{{LS}}$, and the angular diameter distances to the lens galaxies D_L. These two kinds of distances give stringent constraints on cosmological parameters. In this work, we explore a different use of time-delay observables: under the assumption of a flat universe, strong lensing observations can accurately measure the angular diameter distances to the sources D_S. The corresponding redshifts of the quasars may be up to z_S ∼ 4 according to the forecast. The high-redshift distances would sample the Hubble diagram between SNe Ia and the cosmic microwave background, model-independently providing direct information on the evolution of the nature of our universe, for example, the dark energy equation of state parameter w(z). We apply our method to the existing lensing system SDSS 1206+4332 and get ${D}_{S}={2388}_{-978}^{+2632}\,\mathrm{Mpc}$ at z_S = 1.789. We also make a forecast for the era of Large Synoptic Survey Telescope. The uncertainty of D_S depends on the redshifts of the lens and the source, the uncertainties of D_Δt and D_L, and the correlation between D_Δt and D_L. Larger correlation would result in tighter D_S determination.

Brown dwarfs and directly imaged giant planets exhibit significant evidence for active atmospheric circulation, which induces a large-scale patchiness in the cloud structure that evolves significantly over time, as evidenced by infrared light curves and Doppler maps. These observations raise critical questions about the fundamental nature of the circulation, its time variability, and its overall relationship to the circulation on Jupiter and Saturn. Jupiter and Saturn themselves exhibit numerous robust zonal (east–west) jet streams at the cloud level; moreover, both planets exhibit long-term stratospheric oscillations involving perturbations of zonal wind and temperature that propagate downward over time on timescales of ∼4 yr (Jupiter) and ∼15 yr (Saturn). These oscillations, dubbed the quasi-quadrennial oscillation (QQO) for Jupiter and the semiannual oscillation (SAO) on Saturn, are thought to be analogous to the quasi-biennial oscillation (QBO) on Earth, which is driven by upward propagation of equatorial waves from the troposphere. To investigate these issues, we here present global, three-dimensional, high-resolution numerical simulations of the flow in the stratified atmosphere—overlying the convective interior—of brown dwarfs and Jupiter-like planets. The effect of interior convection is parameterized by inducing small-scale, randomly varying perturbations in the radiative–convective boundary at the base of the model. Radiative damping is represented using an idealized Newtonian cooling scheme. In the simulations, the convective perturbations generate atmospheric waves and turbulence that interact with the rotation to produce numerous zonal jets. Moreover, the equatorial stratosphere exhibits stacked eastward and westward jets that migrate downward over time, exactly as occurs in the terrestrial QBO, Jovian QQO, and Saturnian SAO. This is the first demonstration of a QBO-like phenomenon in 3D numerical simulations of a giant planet.

We present a search for H i in the circumgalactic medium (CGM) of 21 massive ($\langle \mathrm{log}{M}_{\star }\rangle \sim 11.4$), luminous red galaxies (LRGs) at z ∼ 0.5. Using UV spectroscopy of QSO sightlines projected within 500 kpc (∼${R}_{\mathrm{vir}}$) of these galaxies, we detect H i absorption in 11/21 sightlines, including two partial Lyman limit systems and two Lyman limit systems. The covering factor of $\mathrm{log}N({\rm{H}}\,{\rm{I}})\geqslant 16.0$ gas within the virial radius of these LRGs is ${f}_{c}(\rho \leqslant \,{R}_{\mathrm{vir}})={0.27}_{-0.10}^{+0.11}$, while for optically thick gas ($\mathrm{log}N({\rm{H}}\,{\rm{I}})\geqslant 17.2$) it is ${f}_{c}(\rho \leqslant \,{R}_{\mathrm{vir}})={0.15}_{-0.07}^{+0.10}$. Combining this sample of massive galaxies with previous galaxy-selected CGM studies, we find no strong dependence of the H i covering factor on galaxy mass, although star-forming galaxies show marginally higher covering factors. There is no evidence for a critical mass above which dense, cold (T ∼ 10⁴ K) gas is suppressed in the CGM of galaxies (spanning stellar masses $9.5\lesssim \mathrm{log}{M}_{\star }\lesssim 11.8$). The metallicity distribution in LRGs is indistinguishable from those found about lower-mass star-forming galaxies, and we find low-metallicity gas with $[{\rm{X}}/{\rm{H}}]\approx -1.8$ (1.5% solar) and below about massive galaxies. About half the cases show supersolar [Fe ii/$\mathrm{Mg}\,{\rm{II}}$] abundances as seen previously in cool gas near massive galaxies. While the high-metallicity cold gas seen in LRGs could plausibly result from condensation from a corona, the low-metallicity gas is inconsistent with this interpretation.

We systematically investigate the mid-infrared (MIR; λ > 3 μm) time variability of uniformly selected ∼800 massive young stellar objects (MYSOs) from the Red Midcourse Space Experiment Source survey. Out of the 806 sources, we obtain reliable 9 yr long MIR magnitude variability data of 331 sources at the 3.4 μm (W1) and 4.6 μm (W2) bands by cross-matching the MYSO positions with ALLWISE and NEOWISE catalogs. After applying the variability selections using ALLWISE data, we identify five MIR-variable candidates. The light curves show various classes, with the periodic, plateau-like, and dipper features. Out of the obtained two color–magnitude diagram of W1 and W1−W2, one shows "bluer when brighter and redder when fainter" trends in variability, suggesting change in extinction or accretion rate. Finally, our results show that G335.9960−00.8532 (hereafter, G335) has a periodic light curve, with an ≈690 day cycle. Spectral energy density model fitting results indicate that G335 is a relatively evolved MYSO; thus, we may be witnessing the very early stages of a hyper- or ultra-compact H ii region, a key source for understanding MYSO evolution.

Spectrometers provide our most detailed diagnostics of the solar coronal plasma, and spectral data is routinely used to measure the temperature, density, and flow velocity in coronal features. However, spectrographs suffer from a limited instantaneous field of view (IFOV). Conversely, imaging instruments can provide a relatively large IFOV, but extreme-ultraviolet (EUV) multilayer imaging offers very limited spectral resolution. In this paper, we suggest an instrument concept that combines the large IFOV of an imager with the diagnostic capability of a spectrograph, develop a new parametric model to describe the instrument, and evaluate a new method for "deconvolving" the data from such an instrument. To demonstrate the operating principle of this new slitless spectroscopy instrument, actual spectroscopic raster data from the Hinode/EUV Imaging Spectrometer (EIS) spectrometer is used. We assume that observations in multiple spectral orders are obtained, and then use a new inverse problem method to infer the spectral properties. Unlike previous methods, physical constraints and regularization derived from prior knowledge can be naturally incorporated as part of the solution process. We find that the fidelity of the solution is vastly improved compared to previous methods. The errors are typically only a few km s⁻¹ over a large IFOV, with a width of a few hundred pixels and an arbitrarily large height. These errors are not much larger than the errors in current slit spectroscopic instruments with limited IFOV. A further benefit is that the performance of candidate instruments can be optimized for specific scientific objectives. We demonstrate this by deriving optimum values for the spectral dispersion and signal-to-noise ratio.

We combine Galaxy Evolution Explorer and Gaia DR2 catalogs to track star formation in the outskirts of our Galaxy. Using photometry, proper motions, and parallaxes we identify a structure of ∼300 OB-type candidates located between 12 and 15 kpc from the Galactic center that are kinematically cold. The structure is located between l = 120° and 200°, above the plane up to ∼700 pc and below the plane to ∼1 kpc. The bulk motion is disklike; however, we measure a mean upward vertical motion of 5.7 ± 0.4 km s⁻¹, and a mean outward radial motion of between 8 and 16 km s⁻¹. The velocity dispersion along the least dispersed of its proper-motion axes (perpendicular to the Galactic disk) is 6.0 ± 0.3 km s⁻¹, confirming the young age of this structure. While spatially encompassing the outer spiral arm of the Galaxy, this structure is not a spiral arm. Its explanation as the Milky Way warp is equally unsatisfactory. The structure's vertical extent, mean kinematics, and asymmetry with respect to the plane indicate that its origin is more akin to a wobble generated by a massive satellite perturbing the Galaxy's disk. The mean stellar ages in this outer structure indicate the event took place some 200 Myr ago.

L1521F is found to be forming multiple cores and it is cited as an example of the densest core with an embedded VeLLO in a highly dynamical environment. We present the core-scale magnetic fields (B-fields) in the near vicinity of the VeLLO L1521F-IRS using submillimeter polarization measurements at 850 μm using JCMT POL-2. This is the first attempt to use high-sensitivity observations to map the sub-parsec-scale B-fields in a core with a VeLLO. The B-fields are ordered and very well connected to the parsec-scale field geometry seen in our earlier optical polarization observations and the large-scale structure seen in Planck dust polarization. The core-scale B-field strength estimated using the Davis–Chandrasekhar–Fermi relation is 330 ± 100 μG, which is more than 10 times the value we obtained in the envelope (the envelope in this paper is the "core envelope"). This indicates that B-fields are getting stronger on smaller scales. The magnetic energies are found to be 1 to 2 orders of magnitude higher than nonthermal kinetic energies in the envelope and core. This suggests that magnetic fields are more important than turbulence in the energy budget of L1521F. The mass-to-flux ratio of 2.3 ± 0.7 suggests that the core is magnetically supercritical. The degree of polarization is steadily decreasing toward the denser part of the core with a power-law slope of −0.86.

The three-dimensional (3D) shape of a galaxy inevitably is tied to how it has formed and evolved and to its dark matter halo. Local extremely metal-poor galaxies (XMPs; defined as having an average gas-phase metallicity <0.1 solar) are important objects for understanding galaxy evolution largely because they appear to be caught in the act of accreting gas from the cosmic web, and their 3D shape may reflect this. Here, we report on the 3D shape of XMPs as inferred from their observed projected minor-to-major axial ratios using a hierarchical Bayesian inference model, which determines the likely shape and orientation of each galaxy, while simultaneously inferring the average shape and dispersion. We selected a sample of 149 XMPs and divided it into three subsamples according to physical size and found that (1) the stellar component of XMPs of all sizes tends to be triaxial, with an intermediate axis ≈0.7 times the longest axis and that (2) smaller XMPs tend to be relatively thicker, with the shortest axis going from ≈0.15 times the longest axis for the large galaxies to ≈0.4 for the small galaxies. We provide the inferred 3D shape and inclination of the individual XMPs in electronic format. We show that our results for the intermediate axis are not clouded by a selection effect against face-on XMPs. We discuss how an intermediate axis significantly smaller than the longest axis may be produced by several mechanisms, including lopsided gas accretion, non-axisymmetric star formation, or coupling with an elongated dark matter halo. Large relative thickness may reflect slow rotation, stellar feedback, or recent gas accretion.

We investigate the possibility that the dwarf galaxies Crater II and Hercules have previously been tidally stripped by the Milky Way. We present Magellan/IMACS spectra of candidate member stars in both objects. We identify 37 members of Crater II, 25 of which have velocity measurements in the literature, and we classify three stars within that subset as possible binaries. We find that including or removing these binary candidates does not change the derived velocity dispersion of Crater II. Excluding the binary candidates, we measure a velocity dispersion of ${\sigma }_{{V}_{\mathrm{los}}}={2.7}_{-0.4}^{+0.5}$ km s⁻¹, corresponding to ${\text{}}M/L={47}_{-13}^{+17}$M_⊙/L_⊙. We measure a mean metallicity of $[\mathrm{Fe}/{\rm{H}}]=-{1.95}_{-0.05}^{+0.06}$, with a dispersion of ${\sigma }_{{\rm{[Fe/H]}}}={0.18}_{-0.08}^{+0.06}$. Our velocity dispersion and metallicity measurements agree with previous measurements for Crater II, and confirm that the galaxy resides in a kinematically cold dark-matter halo. We also search for spectroscopic members stripped from Hercules in the possible extratidal stellar overdensities surrounding the dwarf. For both galaxies, we calculate proper motions using Gaia DR2 astrometry, and use their full 6D phase space information to evaluate the probability that their orbits approach sufficiently close to the Milky Way to experience tidal stripping. Given the available kinematic data, we find a probability of ∼40% that Hercules has suffered tidal stripping. The proper motion of Crater II makes it almost certain to be stripped.

Because of the plasma pressure gradient across the lunar wake boundary, the interplanetary magnetic field is enhanced in the lunar wake. In previous works, the solar wind ions that enter into the near lunar wake are found to have an unimportant influence on the magnetic field within the lunar wake. In this study, two cases of a 22%–70% reduction of the magnetic field in the near lunar wake are first observed by the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun mission (ARTEMIS). The magnetic field depressions are caused by the refilling of dense plasma clouds (with densities of 0.20–0.47 cm⁻³) into the near lunar wake. The ions of the plasma clouds that originate from the reflected solar wind ions in the lunar dayside are accelerated into the near lunar wake by the solar wind convection electric field. The source regions of the reflected ions can be traced back to the lunar magnetic anomalies over the sunlit surface. Pressure (magnetic pressure + thermal pressure) balances are roughly maintained for both cases at the boundary between the plasma cloud and the ambient plasma. Our results imply that the interaction between the solar wind and lunar magnetic anomalies drastically disturbs the near-Moon electromagnetic environment.

We present axisymmetric hydrodynamical simulations of accretion-induced collapse (AIC) of dark matter (DM) admixed rotating white dwarfs (WD) and their burst gravitational-wave (GW) signals. For initial WD models with the same central baryon density, the admixed DM is found to delay the plunge and bounce phases of AIC, and decrease the central density and mass of the protoneutron star (PNS) produced. The bounce time, central density, and PNS mass generally depend on two parameters, the admixed DM mass M_DM and the ratio between the rotational kinetic and gravitational energies of the inner core at bounce ${\beta }_{\mathrm{ic},{\rm{b}}}$. The emitted GWs have generic waveform shapes and the variation of their amplitudes h₊ show a degeneracy on ${\beta }_{\mathrm{ic},{\rm{b}}}$ and M_DM. We found that the ratios between the GW amplitude peaks around bounce allow breaking of the degeneracy and extraction of both ${\beta }_{\mathrm{ic},{\rm{b}}}$ and M_DM. Even within the uncertainties of the nuclear matter equation of state, a DM core can be inferred if its mass is greater than 0.03 M_⊙. We also discuss possible DM effects on the GW signals emitted by PNS g-mode oscillations. GWs may boost the possibility for the detection of AIC, as well as open a new window into the indirect detection of DM.

We present a machine-learning method for the reconstruction of the undistorted images of background sources in strongly lensed systems. This method treats the source as a pixelated image and utilizes the recurrent inference machine to iteratively reconstruct the background source given a lens model. Our architecture learns to maximize the likelihood of the model parameters (source pixels) given the data using the physical forward model (ray-tracing simulations) while implicitly learning the prior of the source structure from the training data. This results in better performance compared to linear inversion methods, where the prior information is limited to the two-point covariance of the source pixels approximated with a Gaussian form, and often specified in a relatively arbitrary manner. We combine our source reconstruction network with a convolutional neural network that predicts the parameters of the mass distribution in the lensing galaxies directly from telescope images, allowing a fully automated reconstruction of the background source images and the foreground mass distribution.

The afterglow of GRB 170817A/GW170817 was very unusual, slowly rising as ${F}_{\nu }\propto {t}_{\mathrm{obs}}^{0.8}{\nu }^{-0.6}$, peaking at ${t}_{\mathrm{obs},\mathrm{pk}}\sim 150\,$ days, and sharply decaying as $\sim {t}_{\mathrm{obs}}^{-2.2}$. Very-long-baseline interferometry observations revealed an unresolved radio afterglow image whose flux centroid apparently moved superluminally with v_app ≈ 4c between 75 and 230 days, clearly indicating that the afterglow was dominated by a relativistic jet's compact core. Different jet angular structures successfully explained the afterglow light curves: Gaussian and steep power-law profiles with narrow core angles θ_c ≲ 5° and significantly larger viewing angles θ_obs/θ_c ∼ 3−5. However, a top-hat jet (THJ; conical with sharp edges at θ = θ₀) was ruled out because it appeared to produce an early flux rise much steeper ($\propto {t}_{\mathrm{obs}}^{a}$ with a ≳ 3) than observed. Using 2D relativistic hydrodynamic simulations of an initially THJ, we show that the initial steep flux rise is an artifact caused by the simulation's finite start time, t₀, missing its flux contributions from t < t₀ and sometimes "compensated" using an analytic THJ.  While an initially THJ is not very physical, such simulations are particularly useful at ${t}_{\mathrm{obs}}\gtrsim {t}_{\mathrm{obs},\mathrm{pk}}$ when the afterglow emission is dominated by the jet's core and becomes insensitive to its exact initial angular profile if it drops off sharply outside of the core. We demonstrate that an initially THJ fits GW170817/GRB 170817A's afterglow light curves and flux centroid motion at ${t}_{\mathrm{obs}}\gtrsim {t}_{\mathrm{obs},\mathrm{pk}}$, for θ_obs/θ₀ ≈ 3 and may also fit the earlier light curves for Γ₀ = Γ(t₀) ≳ 10^2.5. We analytically express the degeneracies between the model parameters, and find a minimal jet energy of ${E}_{\min }\approx 5.3\times {10}^{48}\,$ erg and circumburst medium density of ${n}_{\min }\approx 5.3\times {10}^{-6}\,{\mathrm{cm}}^{-3}$.

We present an 870 μm Atacama Large Millimeter Array polarization observation toward the Class II protoplanetary disk around AS 209, which has concentric, multiple gaps and rings. We successfully detect the polarized emission and find that the polarization orientations and fractions have distinct characteristics between the inner and outer regions. In the inner region, the polarization orientations are parallel to the minor axis of the disk, which is consistent with the self-scattering model. The mean polarization fraction in the region is ∼0.2%, which is lower than the expected value when the grains have the maximum polarization efficiency, which corresponds to λ/2π ∼ 140 μm in grain radius. In the outer region, we detect ∼1.0% polarization and find that the polarization orientations are almost in the azimuthal directions. Moreover, the polarization orientations have systematic angular deviations from the azimuthal directions with Δθ ∼ 45 ± 16. The pattern is consistent with a model in which radially drifting dust grains are aligned by the gas flow against the dust grains. We consider possible scenarios of the grain dynamics at the AS 209 ring that can reproduce the polarization pattern. However, the directions of the observed angular deviations are opposite to what is predicted based on the fact that the disk rotates clockwise. This raises a question regarding our understanding of the alignment processes and/or grain dynamics in protoplanetary disks.

We present Keck/OSIRIS infrared IFU observations of the z = 3.153 sub-DLA DLA2233+131, previously detected in absorption to a background quasar and studied with single-slit spectroscopy and Potsdam Multi Aperture Spectrophotometer integral field spectroscopy (IFU). We used the Laser Guide Star Adaptive Optics and OSIRIS IFU to reduce the point-spread function of the background quasar to FWHM ∼ 015 and marginally resolve extended, foreground DLA emission. We detect $[{\rm{O}}\,{\rm{iii}}]\lambda 5007$ emission with a flux ${F}^{[{\rm{O}}{\rm{iii}}]\lambda 5007}\,=(2.4\pm 0.5)\times {10}^{-17}$ erg s⁻¹ cm⁻², as well as unresolved $[{\rm{O}}\,{\rm{iii}}]\lambda 4959$ and Hβλ4861 emission. Using a composite spectrum over the emission region, we measure dynamical mass $\sim 3.1\,\times \,{10}^{9}$M_⊙. We made several estimates of star formation rate (SFR) using $[{\rm{O}}\,{\rm{iii}}]\lambda 5007$ and Hβλ4861 emission, and measured a SFR of ∼7.1 − 13.6 M_⊙ yr⁻¹. We map $[{\rm{O}}\,{\rm{iii}}]\lambda 5007$ and Hβλ4861 emission and the corresponding velocity fields to search for signs of kinematic structure. These maps allow for a more detailed kinematic analysis than previously possible for this galaxy. While some regions show slightly red and blueshifted emission indicative of potential edge-on disk rotation, the data are insufficient to support this interpretation.

We present a laboratory rotational study of, and astronomical search for, lactaldehyde (CH₃CH(OH)CH(O)), one of the simplest chiral molecules that could reasonably be seen in the interstellar medium (ISM), in the millimeter and submillimeter wave regions from 80 to 460 GHz. More than 5000 transitions were assigned to the most stable conformer, and a set of spectroscopic constants was accurately determined. Lactaldehyde is involved in numerous metabolic pathways used by life on Earth, and is a logical step up in complexity from glycolaldehyde (CH(O)CH₂OH) which is being detected with increasing regularity in the ISM. We present an accompanying radio astronomical search for lactaldehyde in three high-mass star-forming regions (NGC 6334I, Sgr B2(N), and Orion-KL) as well as in the publicly available data from the ASAI Large Project. Neither molecule is detected in these sources, and we report corresponding upper limits to the column densities. We discuss the potential utility of lactaldehyde in combination with other members of the [C₃,H₆,O₂] isomeric family in probing pathways of chemical evolution in the ISM.

We present new Hubble Space Telescope imaging of a stream-like system associated with the dwarf galaxy DDO 68, located in the Lynx-Cancer void at a distance of D ∼ 12.65 Mpc from us. The stream, previously identified in deep Large Binocular Telescope images as a diffuse low surface brightness structure, is resolved into individual stars in the F606W (broad V) and F814W (∼I) images acquired with the Wide Field Camera 3. The resulting V, I color–magnitude diagram (CMD) of the resolved stars is dominated by old (age ≳ 1–2 Gyr) red giant branch (RGB) stars. From the observed RGB tip, we conclude that the stream is at the same distance as DDO 68, confirming the physical association with it. A synthetic CMD analysis indicates that the large majority of the star formation activity in the stream occurred at epochs earlier than ∼1 Gyr ago, and that the star formation at epochs more recent than ∼500 Myr ago is compatible with zero. The total stellar mass of the stream is ∼10⁶M_⊙, about 1/100 of that of DDO 68. This is a striking example of hierarchical merging in action at the dwarf galaxy scales.

It has been found that the Kelvin–Helmholtz instability (KHI) induced by both transverse and torsional oscillations in coronal loops can reinforce the effects of wave heating. In this study, we model a coronal loop as a system of individual strands, and we study wave heating effects by considering a combined transverse and torsional driver at the loop footpoint. We deposit the same energy into the multistranded loop and an equivalent monolithic loop, and then observe a faster increase in the internal energy and temperature in the multistranded model. Therefore, the multistranded model is more efficient in starting the heating process. Moreover, higher temperature is observed near the footpoint in the multistranded loop and near the apex in the monolithic loop. The apparent heating location in the multistranded loop agrees with the previous predictions and observations. Given the differences in the results from our multistranded loop and monolithic loop simulations, and given that coronal loops are suggested to be multistranded on both theoretical and observational grounds, our results suggest that the multistrandedness of coronal loops needs to be incorporated in future wave-based heating mechanisms.

Planet atmosphere and hydrosphere compositions are fundamentally set by accretion of volatiles, and therefore by the division of volatiles between gas and solids in planet-forming disks. For hyper-volatiles such as CO, this division is regulated by volatile sublimation energies, and by the ability of other ice components to entrap. Water ice is known for its ability to trap CO and other volatile species. In this study we explore whether another common interstellar and cometary ice component, CO₂, is able to trap CO as well. We measure entrapment of CO molecules in CO₂ ice through temperature-programmed desorption experiments on CO₂:CO ice mixtures. We find that CO₂ ice traps CO with a typical efficiency of 40%–60% of the initially deposited CO molecules for a range of ice thicknesses between 7 and 50 monolayers, and ice mixture ratios between 1:1 and 9:1. The entrapment efficiency increases with ice thickness and CO dilution. We also run analogous H₂O:CO experiments and find that under comparable experimental conditions, CO₂ ice entraps CO more efficiently than H₂O ice up to the onset of CO₂ desorption at ∼70 K. We speculate that this may be due to different ice restructuring dynamics in H₂O and CO₂ ices around the CO desorption temperature. Importantly, in planet-forming disks, the ability of CO₂ to entrap CO may change the expected division between gas and solids for CO and other hyper-volatiles exterior to the CO₂ snowline.

All four circumbinary (CB) protoplanetary disks orbiting short-period (P < 20 days) double-lined spectroscopic binaries (SB2s)—a group that includes UZ Tau E, for which we present new Atacama Large Millimeter/Submillimeter Array data—exhibit sky-plane inclinations i_disk that match, to within a few degrees, the sky-plane inclinations i_⋆ of their stellar hosts. Although for these systems the true mutual inclinations θ between disk and binary cannot be directly measured because relative nodal angles are unknown, the near coincidence of i_disk and i_⋆ suggests that θ is small for these most compact of systems. We confirm this hypothesis using a hierarchical Bayesian analysis, showing that 68% of CB disks around short-period SB2s have θ < 30. Near coplanarity of CB disks implies near coplanarity of CB planets discovered by Kepler, which in turn implies that the occurrence rate of close-in CB planets is similar to that around single stars. By contrast, at longer periods ranging from 30 to 10⁵ days (where the nodal degeneracy can be broken via, e.g., binary astrometry), CB disks exhibit a wide range of mutual inclinations, from coplanar to polar. Many of these long-period binaries are eccentric, as their component stars are too far separated to be tidally circularized. We discuss how theories of binary formation and disk–binary gravitational interactions can accommodate all these observations.

The isolated binary evolution model for merging neutron stars (NSs) involves processes such as mass transfer, common-envelope evolution, and natal kicks, all of which are poorly understood. Also, the predicted NS–NS merger rates are typically lower than the rates inferred from the LIGO GW170817 event. Here, we investigate merger rates of NS and black hole–NS binaries in hierarchical triple-star systems. In such systems, the tertiary can induce Lidov–Kozai (LK) oscillations in the inner binary, accelerating its coalescence and potentially enhancing compact object merger rates. However, because compact objects originate from massive stars, the prior evolution should also be taken into account. Natal kicks, in particular, could significantly reduce the rates by unbinding the tertiary before it can affect the inner binary through LK evolution. We carry out simulations of massive triples, taking into account stellar evolution starting from the main sequence, secular and tidal evolution, and the effects of supernovae. For large NS birth kicks (${\sigma }_{{\rm{k}}}=265\,\mathrm{km}\,{{\rm{s}}}^{-1}$), we find that the triple NS–NS merger rate (several hundred ${\mathrm{Gpc}}^{-3}\,{\mathrm{yr}}^{-1}$) is lower by a factor of ∼2–3 than the binary rate, but for no kicks (${\sigma }_{{\rm{k}}}=0\,\mathrm{km}\,{{\rm{s}}}^{-1}$), the triple rate (several thousand ${\mathrm{Gpc}}^{-3}\,{\mathrm{yr}}^{-1}$) is comparable to the binary rate. Our results indicate that a significant fraction of NS–NS mergers could originate from triples if a substantial portion of the NS population is born with low kick velocities, as indicated by other work. However, uncertainties and open questions remain because of our simplifying assumption of dynamical decoupling after inner binary interaction has been triggered.

The morphological asymmetry of leading and following sunspots is a well-known characteristic of the solar surface. In the context of the large-scale evolution of the surface magnetic field, the asymmetry has been assumed to have only a negligible effect. Using the surface flux transport (SFT) model, we show that the morphological asymmetry of leading and following sunspots has a significant impact on the evolution of the large-scale magnetic field on the solar surface. By evaluating the effect of the morphological asymmetry of each bipolar magnetic region (BMR), we observe that the introduction of asymmetry to the BMR model significantly reduces the contribution to the polar magnetic field, especially for large and high-latitude BMRs. Strongly asymmetric BMRs can even reverse regular polar field formation. The SFT simulations based on the observed sunspot record show that the introduction of morphological asymmetry reduces the root-mean-square difference from the observed axial dipole strength by 30%–40%. These results indicate that the morphological asymmetry of leading and following sunspots has a significant effect on the solar cycle prediction.

The origin of the disk and spheroid of galaxies has been a key open question in understanding their morphology. Using the high-resolution cosmological simulation New Horizon, we explore kinematically decomposed disk and spheroidal components of 144 field galaxies with masses greater than ${10}^{9}\,{M}_{\odot }$ at z = 0.7. The origins of stellar particles are classified according to their birthplace (in situ or ex situ) and their orbits at birth. Before disk settling, stars form mainly through chaotic mergers between protogalaxies and become part of the spheroidal component. When disk settling starts, we find that more massive galaxies begin to form disk stars from earlier epochs; massive galaxies commence to develop their disks at z ∼ 1–2, while low-mass galaxies do after z ∼ 1. The formation of disks is affected by accretion as well, as mergers can trigger gas turbulence or induce misaligned gas infall that hinders galaxies from forming corotating disk stars. The importance of accreted stars is greater in more massive galaxies, especially in developing massive spheroids. A significant fraction of the spheroids come from the disk stars that are perturbed, and this becomes more important at lower redshifts. Some (∼12.5%) of our massive galaxies develop counter-rotating disks from the gas infall misaligned with the existing disk plane, which can last for more than a gigayear until they become the dominant component and flip the angular momentum of the galaxy in the opposite direction. The final disk-to-total ratio of a galaxy needs to be understood in relation to its stellar mass and accretion history. We quantify the significance of the stars with different origins and provide them as guiding values.

One of the greatest challenges in solar physics is understanding the heating of the Sun's corona. Most theories for coronal heating postulate that free energy in the form of magnetic twist/stress is injected by the photosphere into the corona where the free energy is converted into heat either through reconnection or wave dissipation. The magnetic helicity associated with the twist/stress, however, is expected to be conserved and appear in the corona. In previous works, we showed that the helicity associated with the small-scale twists undergoes an inverse cascade via stochastic reconnection in the corona and ends up as the observed large-scale shear of filament channels. Our "helicity condensation" model accounts for both the formation of filament channels and the observed smooth, laminar structure of coronal loops. In this paper, we demonstrate, using helicity- and energy-conserving numerical simulations of a coronal system driven by photospheric motions, that the model also provides a natural mechanism for heating the corona. We show that the heat generated by the reconnection responsible for the helicity condensation process is sufficient to account for the observed coronal heating. We study the role that helicity injection plays in determining coronal heating and find that, crucially, the heating rate is only weakly dependent on the net helicity preference of the photospheric driving. Our calculations demonstrate that motions with 100% helicity preference are least efficient at heating the corona; those with 0% preference are most efficient. We discuss the physical origins of this result and its implications for the observed corona.

The Gaia era opens new possibilities for discovering the remnants of disrupted satellite galaxies in the solar neighborhood. If the population of local accreted stars is correlated with the dark matter sourced by the same mergers, one can then map the dark matter distribution directly. Using two cosmological zoom-in hydrodynamic simulations of Milky-Way-mass galaxies from the Latte suite of the Fire-2 simulations, we find a strong correlation between the velocity distribution of stars and dark matter at the solar circle that were accreted from luminous satellites. This correspondence holds for dark matter that is either relaxed or in a kinematic substructure called debris flow, and is consistent between two simulated hosts with different merger histories. The correspondence is more problematic for streams because of possible spatial offsets between the dark matter and stars. We demonstrate how to reconstruct the dark matter velocity distribution from the observed properties of the accreted stellar population by properly accounting for the ratio of stars to dark matter contributed by individual mergers. This procedure does not account for the dark matter that originates from nonluminous satellites, which may constitute a nontrivial fraction of the local contribution. After validating this method using the Fire-2 simulations, we apply it to the Milky Way and use it to recover the dark matter velocity distribution associated with the recently discovered stellar debris field in the solar neighborhood. Based on results from Gaia, we estimate that ${42}_{-22}^{+26} \%$ of the local dark matter that is accreted from luminous mergers is in debris flow.

The carousel model of pulsar emission attributes the phenomenon of subpulse drifting to a set of discrete sparks located very near the stellar surface rotating around the magnetic axis. Here, we investigate the subpulse drifting behavior of PSR B0031−07 in the context of the carousel model. We show that B0031−07's three drift modes (A, B, and C) can be understood in terms of a single carousel rotation rate if the number of sparks is allowed to change by an integral number, and where the different drift rates are due to (first-order) aliasing effects. This also results in harmonically related values for P₃ (the time it takes a subpulse to reappear at the same pulse phase), which we confirm for B0031−07. A representative solution has [n_A, n_B, n_C] = [15, 14, 13] sparks and a carousel rotation period of P₄ = 16.4 P₁. We also investigate the frequency dependence of B0031−07's subpulse behavior. We extend the carousel model to include the dual effects of aberration and retardation, including the time it takes the information about the surface spark configuration to travel from the surface up to the emission point. Assuming these effects dominate at B0031−07's emission heights, we derive conservative emission height differences of ≲2000 km for mode A and ≲1000 km for modes B and C as seen between 185 and 610 MHz. This new method of measuring emission heights is independent of others that involve average profile components or the polarization position angle curve, and thus provides a potentially strong test of the carousel model.

We explore the possible dependencies of galaxy metal abundance and star formation rate (SFR) on local environment, focusing on the volume of space in and around the Boötes Void. Our sample of star-forming galaxies comes from the second catalog of the Hα-selected KPNO International Spectroscopic Survey (KISS), which overlaps the void. This sample represents a statistically complete, line-flux-limited ensemble of 820 star-forming galaxies, all of which possess metallicity and SFR estimates. We carry out two distinct analyses of the KISS galaxies: one that probes the properties of the entire sample as a function of local density, and a second that details the properties of 33 KISS star-forming galaxies located within the Boötes Void. In both cases, we find no evidence that either the metallicity of the KISS galaxies or their SFRs depend on the environments within which the galaxies are located. Our global analysis does show weak trends for decreasing stellar mass, decreasing metallicity, and decreasing SFRs with decreasing local densities. However, we argue that the metallicity and SFR trends are artifacts of the stellar mass—local density trend. In particular, the change in metallicity with density is precisely what one would predict from the mass–metallicity relation, given the observed drop in stellar mass with decreasing metallicity. Likewise, the SFR trend with density disappears when one instead considers the mass-normalized specific SFR. The KISS galaxies dwelling in the Boötes Void are found to have metallicity and SFR properties nearly identical to those of a matched comparison sample, despite the fact that the former are located in density environments that are, on average, more than 16 times lower.

We discover the significant (significance level of >99%) correlations between the fractional variation of the ionizing continuum and that of the C iv and/or Si iv broad absorption lines (BALs) in each of 21 BAL quasars that have at least five-epoch observations from the Sloan Digital Sky Survey-I/II/III. This result reveals that the fluctuation of the ionizing continuum is the driver of most of these BAL variations. Among them, 17 show negative correlations and the other 4 positive correlations, which agrees with the prediction of photoionization models that absorption line variability response to ionization changes is not monotonic. Eight quasars out of 21 examples have been observed at least 30 times on rest-frame timescales as short as a few days, which reveals that changes in the incident ionizing continuum can cause BAL variability even in such a short period of time. In addition, we find that most of the 21 quasars show larger variation amplitude in Si iv than C iv, which reveals the ubiquity of saturation in these BALs (at least for C iv BALs).

We report the discovery of six active galactic nuclei (AGNs) caught "turning on" during the first nine months of the Zwicky Transient Facility (ZTF) survey. The host galaxies were classified as low-ionization nuclear emission-line region galaxies (LINERs) by weak narrow forbidden line emission in their archival SDSS spectra, and detected by ZTF as nuclear transients. In five of the cases, we found via follow-up spectroscopy that they had transformed into broad-line AGNs, reminiscent of the changing-look LINER iPTF16bco. In one case, ZTF18aajupnt/AT2018dyk, follow-up Hubble Space Telescope ultraviolet and ground-based optical spectra revealed the transformation into a narrow-line Seyfert 1 with strong [Fe vii, x, xiv] and He iiλ 4686 coronal lines. Swift monitoring observations of this source reveal bright UV emission that tracks the optical flare, accompanied by a luminous soft X-ray flare that peaks ∼60 days later. Spitzer follow-up observations also detect a luminous mid-infrared flare, implying a large covering fraction of dust. Archival light curves of the entire sample from CRTS, ATLAS, and ASAS-SN constrain the onset of the optical nuclear flaring from a prolonged quiescent state. Here we present the systematic selection and follow-up of this new class of changing-look LINERs, compare their properties to previously reported changing-look Seyfert galaxies, and conclude that they are a unique class of transients well-suited to test the uncertain physical processes associated with the LINER accretion state.

On 2014 March 29, an intense solar flare classified as X1.0 occurred in active region 12017. Several associated phenomena accompanied this event, among them a fast-filament eruption, large-scale propagating disturbances in the corona and the chromosphere including a Moreton wave, and a coronal mass ejection. This flare was successfully detected in multiwavelength imaging in the Hα line by the Flare Monitoring Telescope (FMT) at Ica University, Peru. We present a detailed study of the Moreton wave associated with the flare in question. Special attention is paid to the Doppler characteristics inferred from the FMT wing (Hα ± 0.8 Å) observations, which are used to examine the downward/upward motion of the plasma in the chromosphere. Our findings reveal that the downward motion of the chromospheric material at the front of the Moreton wave attains a maximum velocity of 4 km s⁻¹, whereas the propagation speed ranges between 640 and 859 km s⁻¹. Furthermore, using the weak-shock approximation in conjunction with the velocity amplitude of the chromospheric motion induced by the Moreton wave, we derive the Mach number of the incident shock in the corona. We also performed the temperature-emission measure analysis of the coronal wave based on the Atmospheric Imaging Assembly observations, which allowed us to derive the compression ratio, and to estimate Alfvén and fast-mode Mach numbers on the order of 1.06–1.28 and 1.05–1.27. Considering these results and the magnetohydrodynamics linear theory, we discuss the characteristics of the shock front and the interaction with the chromospheric plasma.

The Fermi Large Area Telescope (LAT) has amassed a large data set of primary cosmic-ray protons throughout its mission. In fact, it is the largest set of identified cosmic-ray protons ever collected at this energy. The LAT's wide field of view and full-sky survey capabilities make it an excellent instrument for studying cosmic-ray anisotropy. As a space-based survey instrument, the LAT is sensitive to anisotropy in both R.A. and decl., while ground-based observations only measure the anisotropy in R.A. We present the results of the first-ever proton anisotropy search using Fermi LAT. The data set was collected over eight years and consists of approximately 179 million protons above 78 GeV, enabling it to probe dipole anisotropy below an amplitude of 10⁻³, resulting in the most stringent limits on the decl. dependence of the dipole to date. We measure a dipole amplitude δ = 3.9 ± 1.5 × 10⁻⁴ with a p-value of 0.01 (pretrials) for protons with energy greater than 78 GeV. We discuss various systematic effects that could give rise to a dipole excess and calculate upper limits on the dipole amplitude as a function of minimum energy. The 95% confidence level upper limit on the dipole amplitude is δ_UL = 1.3 × 10⁻³ for protons with energy greater than 78 GeV and δ_UL = 1.2 × 10⁻³ for protons with energy greater than 251 GeV.

It is well established that the chemical structure of the Milky Way exhibits a bimodality with respect to the α-enhancement of stars at a given [Fe/H]. This has been studied largely based on a bulk α abundance, computed as a summary of several individual α-elements. Inspired by the expected subtle differences in their nucleosynthetic origins, here we probe the higher level of granularity encoded in the inter-family [Mg/Si] abundance ratio. Using a large sample of stars with APOGEE abundance measurements, we first demonstrate that there is additional information in this ratio beyond what is already apparent in [α/Fe] and [Fe/H] alone. We then consider Gaia astrometry and stellar age estimates to empirically characterize the relationships between [Mg/Si] and various stellar properties. We find small but significant trends between this ratio and α-enhancement, age, [Fe/H], location in the Galaxy, and orbital actions. To connect these observed [Mg/Si] variations to a physical origin, we attempt to predict the Mg and Si abundances of stars with the galactic chemical evolution model Chempy. We find that we are unable to reproduce abundances for the stars that we fit, which highlights tensions between the yield tables, the chemical evolution model, and the data. We conclude that a more data-driven approach to nucleosynthetic yield tables and chemical evolution modeling is necessary to maximize insights from large spectroscopic surveys.

We study cylindrically symmetric steady-state accretion of polytropic test matter spiraling onto the symmetry axis in power-law and logarithmic potentials. The model allows one to qualitatively understand the accretion process in a symmetry different from that of the classical Bondi accretion. We study the integral curves as level lines of some Hamiltonian and also apply this method to Bondi accretion. The isothermal solutions in power-law potentials (as well as in any radius-dependent potential) can be expressed in exact form in terms of the Lambert W function, while in the case of logarithmic potential, exact solutions can be found for any polytropic exponent.

A mass of dark matter halo is commonly defined as the spherical overdensity (SO) mass with respect to a reference density, whereas the time evolution of an SO mass can be affected by the redshift evolution of the reference density as well as the physical mass accretion around halos. In this study, we directly measure the amount of pseudo evolution of the SO masses of cluster-sized halos by the changes in the reference density from a time series of N-body simulations for the first time. We find that the 52% ± 19% difference in the virial SO masses between z = 0 and 1 can be accounted for by the pseudo evolution of clusters with a virial mass of 10¹⁴h⁻¹M_⊙ at z = 0. The amount of pseudo evolution is found to be correlated with the age and density environment of a galaxy cluster. The stacked mass density profiles of cluster-sized halos with a greater amount of pseudo evolution in the SO mass shows the higher concentration and greater linear bias parameter that is a counterexample of the known secondary halo bias due to concentration on the scale of clusters. We discuss how more concentrated clusters can show larger clustering amplitudes than their less concentrated counterparts and argue that the presence of rich filamentary structures plays a critical role in determining the linear halo bias of galaxy clusters.

HD 100546 is a Herbig Ae/Be star surrounded by a disk with a large central region that is cleared of gas and dust (i.e., an inner hole). High-resolution near-infrared spectroscopy reveals a rich emission spectrum of fundamental rovibrational CO emission lines whose time variable properties point to the presence of an orbiting companion within the hole. The Doppler shift and spectroastrometric signal of the CO v = 1−0 P26 line, observed from 2003 to 2013, are consistent with a source of excess CO emission that orbits the star near the inner rim of the disk. The properties of the excess emission are consistent with those of a circumplanetary disk. In this paper, we report follow-up observations that confirm our earlier prediction that the orbiting source of excess emission would disappear behind the near side of the inner rim of the outer disk in 2017. We find that while the hot band CO lines remained unchanged in 2017, the v = 1−0 P26 line and its spectroastrometric signal returned to the profile observed in 2003. With these new observations, we further constrain the origin of the emission and discuss possible ways of confirming the presence of an orbiting planetary companion in the inner disk.

Particle acceleration in solar flares remains an outstanding problem in solar physics. It is currently unclear which of the acceleration mechanisms dominates and how exactly the excessive magnetic energy is transferred to nonthermal and other forms of energy. We emphasize that the ultimate acceleration mechanism must be capable of efficiently working in the most extreme conditions, such as the shortest detected timescales and the highest acceleration efficiency. Here we focus on a detailed multiwavelength analysis of the initial phase of the SOL2011-08-04 flare, which demonstrated prominent short subpeaks of nonthermal emission during filament eruption associated with the flare. We demonstrate that the three-dimensional configuration of the flare, combined with timing and spectral behavior of the rapidly varying component, put very stringent constraints on the acceleration regime. Specifically, the rapid subpeaks are generated by short injections of nonthermal electrons with a reasonably hard, single power-law spectrum and a relatively narrow spread of pitch-angles along the mean magnetic field. The acceleration site is a compact volume located near the top of the extended coronal loop(s). The electrons are promptly accelerated up to several hundreds of keV, with the characteristic acceleration time shorter than 50 ms. We show that these properties are difficult to reconcile with widely adopted stochastic acceleration models, while the data inescapably require acceleration by a super-Dreicer electric field, whether regular or random.

We report on X-ray and radio observations of the ultra-compact X-ray binary 4U 1543−624 taken in August 2017 during an enhanced accretion episode. We obtained Neutron Star Interior Composition Explorer (NICER) monitoring of the source over a ∼10 day period during which target-of-opportunity observations were also conducted with Swift, INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), and the Australia Telescope Compact Array. Emission lines were measured in the NICER X-ray spectrum at ∼0.64 keV and ∼6.4 keV that correspond to O and Fe, respectively. By modeling these line components, we are able to track changes in the accretion disk throughout this period. The innermost accretion flow appears to move inwards from hundreds of gravitational radii (R_g = GM/c²) at the beginning of the outburst to <8.7 R_g at peak intensity. We do not detect the source in radio, but are able to place a 3σ upper limit on the flux density at 27 μJy beam⁻¹. Comparing the radio and X-ray luminosities, we find that the source lies significantly away from the range typical of black holes in the ${L}_{r}$–${L}_{x}$ plane, suggesting a neutron star primary. This adds to the evidence that neutron stars (NSs) do not follow a single track in the ${L}_{r}$–${L}_{x}$ plane, limiting its use in distinguishing between different classes of NSs based on radio and X-ray observations alone.

Although there has recently been tremendous progress in studies of fast radio bursts (FRBs), the nature of their progenitors remains a mystery. We study the fluence and dispersion measure (DM) distributions of the ASKAP sample to better understand their energetics and statistics. We first consider a simplified model of a power-law volumetric rate per unit isotropic energy dN/dE ∝ E^−γ with a maximum energy E_max in a uniform Euclidean universe. This provides analytic insights for what can be learned from these distributions. We find that the observed cumulative DM distribution scales as N(>DM) ∝ DM^5−2γ (for γ > 1) until a maximum DM_max above which bursts near E_max fall below the fluence threshold of a given telescope. Comparing this model with the observed fluence and DM distributions, we find a reasonable fit for γ ∼ 1.7 and E_max ∼ 10³³ erg Hz⁻¹. We then carry out a full Bayesian analysis based on a Schechter rate function with cosmological factors. We find roughly consistent results with our analytical approach, although with large errors on the inferred parameters due to the small sample size. The power-law index and the maximum energy are constrained to be γ ≃ 1.6 ± 0.3 and $\mathrm{log}{E}_{\max }\,(\mathrm{erg}\,{\mathrm{Hz}}^{-1})\simeq {34.1}_{-0.7}^{+1.1}$ (68% confidence), respectively. From the survey exposure time, we further infer a cumulative local volumetric rate of $\mathrm{log}N(E\gt {10}^{32}\,\mathrm{erg}\,{\mathrm{Hz}}^{-1})({\mathrm{Gpc}}^{-3}\,{\mathrm{yr}}^{-1})\simeq 2.6\pm 0.4$ (68% confidence). The methods presented here will be useful for the much larger FRB samples expected in the near future to study their distributions, energetics, and rates.

To study motions and oscillations in the solar chromosphere and at the transition region level we analyze some extreme Doppler shifts observed off-limb with the Interface Region Imaging Spectrograph (IRIS). Raster scans and slit-jaw imaging observations performed in the near-ultraviolet channels were used. Large transverse oscillations are revealed by the far wings profiles after accurately removing the bulk average line profiles of each sequence. Different regions around the Sun are considered. Accordingly, the cool material of spicules is observed in Mg ii lines rather dispersed up to coronal heights. In the quiet Sun and especially in a polar coronal hole, we study dynamical properties of the dispersed spicules-material off-limb using high spectral, temporal, and spatial resolutions IRIS observations. We suggest that numerous small-scale jet-like spicules show rapid twisting and swaying motions evidenced by the large distortion and dispersion of the line profiles, including impressive periodic Doppler shifts. Most of these events repeatedly appear in red and blueshifts above the limb throughout the whole interval of the observation data sets, with an average swaying speed of order ±35 km s⁻¹ reaching a maximum value of 50 km s⁻¹ in the polar coronal hole region, well above the 2.2 Mm heights. We identified for the first time waves with a short period of order of 100 s, and less and transverse amplitudes of order of ±20–30 km s⁻¹ with the definite signature of Alfvén waves. No correlation exists between brightness and Doppler shift variations; the phase speed of the wave is very large and cannot definitely be determined from the spectral features seen along the quasi-radial features. Even shorter periods waves are evidenced, although their contrast is greatly attenuated by the overlapping effects along the line of sight.

The Low-Frequency Array radio telescope discovered the 707 Hz binary millisecond pulsar (MSP) J0952−0607 in a targeted radio pulsation search of an unidentified Fermi gamma-ray source. This source shows a weak energy flux of F_γ = 2.6 × 10⁻¹² erg cm⁻² s⁻¹ in the energy range between 100 MeV and 100 GeV. Here we report the detection of pulsed gamma-ray emission from PSR J0952−0607 in a very sensitive gamma-ray pulsation search. The pulsar's rotational, binary, and astrometric properties are measured over 7 years of Fermi-Large Area Telescope data. For this we take into account the uncertainty on the shape of the gamma-ray pulse profile. We present an updated radio-timing solution now spanning more than 2 years and show results from optical modeling of the black-widow-type companion based on new multiband photometric data taken with HiPERCAM on the Gran Telescopio Canarias on La Palma and ULTRACAM on the New Technology Telescope at ESO La Silla (based on observations collected at the European Southern Observatory, Chile; programme 0101.D-0925, PI: Clark, C. J.). PSR J0952−0607 is now the fastest-spinning pulsar for which the intrinsic spin-down rate has been reliably constrained (${\dot{P}}_{\mathrm{int}}\lesssim 4.6\times {10}^{-21}\,{\rm{s}}\,{{\rm{s}}}^{-1}$). The inferred surface magnetic field strength of ${B}_{\mathrm{surf}}\lesssim 8.2\times {10}^{7}\,{\rm{G}}$ is among the 10 lowest of all known pulsars. This discovery is another example of an extremely fast spinning black-widow pulsar hiding within an unidentified Fermi gamma-ray source. In the future such systems might help to pin down the maximum spin frequency and the minimum surface magnetic field strength of MSPs.

Coronal mass ejections (CMEs) often exhibit the classic three-part structure in a coronagraph, i.e., the bright front, dark cavity, and bright core, which are traditionally considered as the manifestations of coronal plasma pileup, magnetic flux rope (MFR), and filament, respectively. However, a recent survey based on 42 CMEs all possessing the three-part structure found that a large majority (69%) do not contain an eruptive filament at the Sun. Therefore, a challenging opinion is proposed and claims that the bright core can also correspond to the MFR, which is supported by the CME simulation. Then what is the nature of the CME core? In this paper, we address this issue through a CME associated with the eruption of a filament-hosting MFR on 2013 September 29. This CME exhibits the three-part morphology in multiple white-light coronagraphs from different perspectives. The new finding is that the bright core contains both a sharp and a fuzzy component. Through tracking the filament motion continuously from its source region to the outer corona, we conclude that the sharp component corresponds to the filament. The fuzzy component is suggested to result from the MFR that supports the filament against the gravity in the corona. Our study can shed more light on the nature of CME cores, and explain the core whether or not the filament is involved with a uniform scenario. The nature of the CME cavity is also discussed.

We discovered and studied an ultraluminous X-ray source (CXOU J203451.1+601043) that appeared in the spiral galaxy NGC 6946 at some point between 2008 February and 2012 May and has remained at luminosities ≈2–4 × 10³⁹ erg s⁻¹ in all observations since then. Our spectral modeling shows that the source is generally soft but with spectral variability from epoch to epoch. Using standard empirical categories of the ultraluminous regimes, we found that CXOU J203451.1+601043 was consistent with a broadened disk state in 2012 but was in a transitional state approaching the supersoft regime in 2016, with substantial down-scattering of the hard photons (similar, for example, to the ultraluminous X-ray source in NGC 55). It has since hardened again in 2018–2019 without any significant luminosity change. The most outstanding property of CXOU J203451.1+601043 is a strong emission line at an energy of of (0.66 ± 0.01) keV, with an equivalent width of ≈100 eV and de-absorbed line luminosity of ≈2 × 10³⁸ erg s⁻¹, seen when the continuum spectrum was softest. We identify the line as O viii Lyα (rest-frame energy of 0.654 keV); we interpret it as a strong indicator of a massive outflow. Our finding supports the connection between two independent observational signatures of the wind in super-Eddington sources: a lower temperature of the Comptonized component and the presence of emission lines in the soft X-ray band. We speculate that the donor star is oxygen-rich: a CO or O–Ne–Mg white dwarf in an ultracompact binary. If that is the case, the transient behavior of CXOU J203451.1+601043 raises intriguing theoretical questions.

Weak heating events are frequent and ubiquitous in the solar corona. They derive their energy from the local magnetic field and form a major source of local heating, signatures of which are seen in EUV and X-ray bands. Their radio emission arises from various plasma instabilities that lead to coherent radiation, making even a weak flare appear very bright. Hence, the radio observations probe nonequilibrium dynamics, providing complementary information on plasma evolution. However, a robust study of radio emission from a weak event among many simultaneous events requires high dynamic range imaging at subsecond and sub-MHz resolutions owing to its high spectrotemporal variability. Such observations were not possible until recently. This is among the first spatially resolved multiwaveband studies of active region loops hosting transient brightenings (ARTBs) and is dynamically linked to meter-wave type I noise storms. Observations at meter-wave, EUV, and X-ray bands are used, along with magnetogram data. We believe that this is the first spectroscopic radio imaging study of a type I source, the data for which were obtained using the Murchison Widefield Array. We report the discovery of 30 s quasi-periodic oscillations (QPOs) in the radio light curve riding on a coherent baseline flux. The strength of the QPOs and the baseline flux were enhanced during a microflare associated with the ARTB. Our observations suggest a scenario where magnetic stress builds up over an Alfvén timescale (30 s) across the typical magnetic field braiding scale and then dissipates via a cascade of weak reconnection events.

The macroturbulent atmospheric circulation of Earth-like planets mediates their equator-to-pole heat transport. For fast-rotating terrestrial planets, baroclinic instabilities in the mid-latitudes lead to turbulent eddies that act to transport heat poleward. In this work, we derive a scaling theory for the equator-to-pole temperature contrast and bulk lapse rate of terrestrial exoplanet atmospheres. This theory is built on the work of Jansen & Ferrari and determines how unstable the atmosphere is to baroclinic instability (the baroclinic "criticality") through a balance between the baroclinic eddy heat flux and radiative heating/cooling. We compare our scaling theory to General Circulation Model (GCM) simulations and find that the theoretical predictions for equator-to-pole temperature contrast and bulk lapse rate broadly agree with GCM experiments with varying rotation rate and surface pressure throughout the baroclincally unstable regime. Our theoretical results show that baroclinic instabilities are a strong control of heat transport in the atmospheres of Earth-like exoplanets, and our scalings can be used to estimate the equator-to-pole temperature contrast and bulk lapse rate of terrestrial exoplanets. These scalings can be tested by spectroscopic retrievals and full-phase light curves of terrestrial exoplanets with future space telescopes.

Circular-ribbon flares occur in a confined magnetic structure, but can also be associated with coronal mass ejections (CMEs) when a filament embedded under the fan erupts. Here we study an M8.7 circular-ribbon flare (SOL2014-12-17T04:51), which is accompanied by a CME yet without a clear indication of filament eruption. Using a nonlinear force-free field model, we find that the outer spine-like loops form a magnetic flux rope (FR1) rooted at the edge of the fan, and that there is another flux rope (FR2) at the main magnetic polarity inversion line (PIL) under a fan-like flux rope FR3. We divide the event evolution into two stages by combining modeling results with EUV observations. The onset stage is featured with bidirectional jets that occurred between a filament and FR1, immediately followed by an upward motion of the latter. During this first stage, the inner/outer spine-related ribbons and the circular ribbon begin to brighten up. After about 10 minutes, another ejection stems from the main PIL region. In this second stage, all ribbons are significantly enhanced, and the twist of FR2 footpoints is decreased. We discuss these results in favor of a scenario where the initial reconnection between the filament and FR1 activates the latter to reconnect with FR3 with opposite twist. This produces larger scale erupting loops and consequently causes a weakening of FR3, which induces another eruption of FR2 from below. This event thus represents a new type of eruptive circular-ribbon flare caused by unstable outer spine-like loops.

We report our identification of the optical afterglow and host galaxy of the short-duration gamma-ray burst sGRB 160821B. The spectroscopic redshift of the host is z = 0.162, making it one of the lowest redshift short-duration gamma-ray bursts (sGRBs) identified by Swift. Our intensive follow-up campaign using a range of ground-based facilities as well as Hubble Space Telescope, XMM-Newton, and Swift, shows evidence for a late-time excess of optical and near-infrared emission in addition to a complex afterglow. The afterglow light curve at X-ray frequencies reveals a narrow jet, ${\theta }_{j}\sim {1.9}_{-0.03}^{+0.10}$ deg, that is refreshed at >1 day post-burst by a slower outflow with significantly more energy than the initial outflow that produced the main GRB. Observations of the 5 GHz radio afterglow shows a reverse shock into a mildly magnetized shell. The optical and near-infrared excess is fainter than AT2017gfo associated with GW170817, and is well explained by a kilonova with dynamic ejecta mass M_dyn = (1.0 ± 0.6) × 10⁻³M_⊙ and a secular (post-merger) ejecta mass with M_pm = (1.0 ± 0.6) × 10⁻²M_⊙, consistent with a binary neutron star merger resulting in a short-lived massive neutron star. This optical and near-infrared data set provides the best-sampled kilonova light curve without a gravitational wave trigger to date.

We present numerical modeling of particle acceleration at coronal shocks propagating through a streamer-like magnetic field by solving the Parker transport equation with spatial diffusion both along and across the magnetic field. We show that the location on the shock where the high-energy particle intensity is the largest, depends on the energy of the particles and on time. The acceleration of particles to more than 100 MeV mainly occurs in the shock-streamer interaction region, due to perpendicular shock geometry and the trapping effect of closed magnetic fields. A comparison of the particle spectra to that in a radial magnetic field shows that the intensity at 100 MeV (200 MeV) is enhanced by more than one order (two orders) of magnitude. This indicates that the streamer-like magnetic field can be an important factor in producing large solar energetic particle events. We also show that the energy spectrum integrated over the simulation domain consists of two different power laws. Further analysis suggests that it may be a mixture of two distinct populations accelerated in the streamer and open field regions, where the acceleration rate differs substantially. Our calculations also show that the particle spectra are affected considerably by a number of parameters, such as the streamer tilt angle, particle spatial diffusion coefficient, and shock compression ratio. While the low-energy spectra agree well with standard diffusive shock acceleration theory, the break energy ranges from ∼1 MeV to ∼90 MeV and the high-energy spectra can extend to ∼1 GeV with a slope of ∼2–3.

Merger simulations predict that tidally induced gas inflows can trigger kiloparsec-scale dual active galactic nuclei (dAGN) in heavily obscured environments. Previously, with the Very Large Array, we have confirmed four dAGN with redshifts between 0.04 < z < 0.22 and projected separations between 4.3 and 9.2 kpc in the Sloan Digital Sky Survey Stripe 82 field. Here, we present Chandra X-ray observations that spatially resolve these dAGN and compare their multiwavelength properties to those of single AGN from the literature. We detect X-ray emission from six of the individual merger components and obtain upper limits for the remaining two. Combined with previous radio and optical observations, we find that our dAGN have properties similar to nearby low-luminosity AGN, and they agree with the black hole fundamental plane relation well. There are three AGN-dominated X-ray sources, whose X-ray hardness-ratio derived column densities show that two are unobscured and one is obscured. The low obscured fraction suggests these dAGN are no more obscured than single AGN, in contrast to the predictions from simulations. These three sources show an apparent X-ray deficit compared to their mid-infrared continuum and optical [O iii] line luminosities, suggesting higher levels of obscuration, in tension with the hardness-ratio derived column densities. Enhanced mid-infrared and [O iii] luminosities from star formation may explain this deficit. There is ambiguity in the level of obscuration for the remaining five components because their hardness ratios may be affected by nonnuclear X-ray emissions, or are undetected altogether. They require further observations to be fully characterized.

We present EVR-CB-001, the discovery of a compact binary with an extremely low-mass (0.21 ± 0.05M_⊙) helium core white dwarf progenitor (pre-He WD) and an unseen low-mass (0.32 ± 0.06M_⊙) helium white dwarf (He WD) companion. He WDs are thought to evolve from the remnant helium-rich core of a main-sequence star stripped during the giant phase by a close companion. Low-mass He WDs are exotic objects (only about 0.2% of WDs are thought to be less than 0.3 M_⊙), and are expected to be found in compact binaries. Pre-He WDs are even rarer, and occupy the intermediate phase after the core is stripped, but before the star becomes a fully degenerate WD and with a larger radius (≈0.2R_⊙) than a typical WD. The primary component of EVR-CB-001 (the pre-He WD) was originally thought to be a hot subdwarf (sdB) star from its blue color and under-luminous magnitude, characteristic of sdBs. The mass, temperature (T_eff = 18,500 ± 500 K), and surface gravity ($\mathrm{log}(g)=4.96\pm 0.04$) solutions from this work are lower than values for typical hot subdwarfs. The primary is likely to be a post-red-giant branch, pre-He WD contracting into a He WD, and at a stage that places it nearest to sdBs on color–magnitude and T_eff–log(g) diagrams. EVR-CB-001 is expected to evolve into a fully double degenerate, compact system that should spin down and potentially evolve into a single hot subdwarf star. Single hot subdwarfs are observed, but progenitor systems have been elusive.

We report on bidirectional coronal reconnection outflows reaching ±200 km s⁻¹ as observed in an active region with the Si iv and C ii spectra of the Interface Region Imaging Spectrograph (IRIS). The evolution of the active region with an emerging flux, a failed filament eruption, and a jet is followed in Solar Dynamical Observatory (SDO)/Atmospheric Imaging Assembly (AIA) filters from 304 to 94 Å, IRIS slit jaw images, and SDO/Helioseismic and Magnetic Imager movies. The bidirectional outflow reconnection is located at a bright point visible in multiwavelength AIA filters above an arch filament system. This suggests that the reconnection occurs between rising loops above the emergence of magnetic bipoles and the longer, twisted magnetic field lines remnant of the failed filament eruption one hour before. The reconnection occurs continuously in the corona between quasi-parallel magnetic field lines, which is possible in a 3D configuration. The reconnection also triggers a jet with transverse velocities around 60 km s⁻¹. Blueshifts and redshifts along its axis confirm the existence of a twist along the jet, which could have been transferred from the filament flux rope. The jet finally blows up the material of the filament before coming back during the second phase. In the Hα Dopplergrams provided by the MSDP spectrograph, we see more redshift than blueshift, indicating the return of the jet and filament plasma.

The zodiacal dust complex, a population of dust and small particles that pervades the solar system, provides important insight into the formation and dynamics of planets, comets, asteroids, and other bodies. We present a new set of data obtained from direct measurements of momentum transfer to a spacecraft from individual particle impacts. This technique is made possible by the extreme precision of the instruments flown on the LISA Pathfinder spacecraft, a technology demonstrator for a future space-based gravitational wave observatory. Pathfinder employed a technique known as drag-free control that achieved rejection of external disturbances, including particle impacts, using a micropropulsion system. Using a simple model of the impacts and knowledge of the control system, we show that it is possible to detect impacts and measure properties such as the transferred momentum, direction of travel, and location of impact on the spacecraft. In this paper, we present the results of a systematic search for impacts during 4348 hr of Pathfinder data. We report a total of 54 candidates with transferred momenta ranging from 0.2 to 230 μNs. We furthermore make a comparison of these candidates with models of micrometeoroid populations in the inner solar system, including those resulting from Jupiter-family comets (JFCs), Oort Cloud comets, Halley-type comets, and asteroids. We find that our measured population is consistent with a population dominated by JFCs, with some evidence for a smaller contribution from Halley-type comets, in agreement with consensus models of the zodiacal dust complex in the momentum range sampled by LISA Pathfinder.

The molecular gas in the central molecular zone (CMZ) of the Galaxy has been studied using infrared absorption spectra of H₃⁺ lines at 3.5–4.0 μm and CO lines near 2.34 μm. In addition to the previously reported spectra of these lines toward eight stars located within 30 pc of Sgr A*, there are now spectra toward ∼30 bright stars located from 140 pc west to 120 pc east of Sgr A*. The spectra show the presence of warm (T ∼ 200 K) and diffuse (n < 100 cm⁻³) gas with N(H₃⁺) ∼ 3 × 10¹⁵ cm⁻² on the majority of sight lines. Instead of our previous analysis, in which only electrons from photoionization of carbon atoms were considered, we have developed a simple model calculation in which the cosmic-ray ionization of H₂ and H is also taken into account. We conclude the following: (1) Warm and diffuse gas dominates the volume of the CMZ. The volume filling factor of dense gas must be much less than 0.1, and the CMZ is not as opaque as previously considered. The X-ray-emitting ultrahot 10⁸ K plasma, which some thought to dominate the CMZ, does not exist over extended regions. (2) The cosmic-ray ionization rate is ζ ∼ 2 × 10⁻¹⁴ s⁻¹, higher than in Galactic dense clouds and diffuse clouds by factors of ∼1000 and ∼100, respectively. If the equipartition law stands, this suggests a pervading magnetic field on the order of ∼100 μG.

The magnetic field in the corona is important for understanding solar activity. Linear polarization measurements in forbidden lines in the visible/IR provide information about coronal magnetic direction and topology. However, these measurements do not provide a constraint on coronal magnetic field strength. The unsaturated, or critical regime of the magnetic Hanle effect is potentially observable in permitted lines for example in the UV, and would provide an important new constraint on the coronal magnetic field. In this paper we present the first side-by-side comparison of forbidden versus permitted linear polarization signatures, examining the transition from the unsaturated to the saturated regime. In addition, we use an analytic 3D flux rope model to demonstrate the Hanle effect for the line-of-sight versus plane-of-sky (POS) components of the magnetic field. As expected, the linear polarization in the unsaturated regime will vary monotonically with increasing magnetic field strength for regions where the magnetic field is along the observer's line of sight. The POS component of the field produces a linear polarization signature that varies with both the field strength and direction in the unsaturated regime. Once the magnetic field is strong enough that the effect is saturated, the resulting linear polarization signal is essentially the same for the forbidden and permitted lines. We consider how such observations might be used together in the future to diagnose the coronal magnetic field.

Using the photometric data from the Next Generation Fornax Survey, we find a significant radial alignment signal among the Fornax dwarf galaxies. For the first time, we report that the radial alignment signal of nucleated dwarfs is stronger than that of non-nucleated ones at the 2.4σ confidence level, and the dwarfs located in the outer region (R > R_vir/3; R_vir is the Fornax virial radius) show a slightly stronger radial alignment signal than those in the inner region (R < R_vir/3) at the 1.5σ level. We also find that the significance of the radial alignment signal is independent of the luminosities or sizes of the dwarfs.

The Interface Region Imaging Spectrograph has routinely observed the flaring Mg ii near-ultraviolet (NUV) spectrum, offering excellent diagnostic potential and a window into the location of energy deposition. A number of studies have forward-modeled both the general properties of these lines and specific flare observations. Generally these have forward-modeled radiation via post-processing of snapshots from hydrodynamic flare simulations through radiation transfer codes. There has, however, not been a study of how the physics included in these radiation transport codes affects the solution. A baseline setup for forward-modeling Mg ii in flares is presented and contrasted with approaches that add or remove complexity. It is shown for Mg ii that (1) partial frequency distribution (PRD) is still required during flare simulations despite the increased densities; (2) using full angle-dependent PRD affects the solution but takes significantly longer to process a snapshot; (3) including Mg i in non-LTE (NLTE) results in negligible differences to the Mg ii lines but does affect the NUV quasi-continuum; (4) only hydrogen and Mg ii need to be included in NLTE; (5) ideally the nonequilibrium hydrogen populations, with nonthermal collisional rates, should be used rather than the statistical equilibrium populations; (6) an atom consisting of only the ground state, h and k upper levels, and continuum level is insufficient to model the resonance lines; and (7) irradiation from a hot, dense flaring transition region can affect the formation of Mg ii. We discuss modifications to the RH code allowing straightforward inclusion of the transition region and coronal irradiation in flares.

The far side of the Milky Way's disk is one of the most concealed parts of the known universe due to extremely high interstellar extinction and point-source density toward low Galactic latitudes. Large time-domain photometric surveys operating in the near-infrared hold great potential for the exploration of these vast uncharted areas of our Galaxy. We conducted a census of distant classical and type II Cepheids along the southern Galactic midplane using near-infrared photometry from the VISTA Variables in the Vía Láctea survey. We performed a machine-learned classification of the Cepheids based on their infrared light curves using a convolutional neural network. We have discovered 640 distant classical Cepheids with up to ∼40 mag of visual extinction and over 500 type II Cepheids, most of them located in the inner bulge. Intrinsic color indices of individual Cepheids were predicted from sparse photometric data using a neural network, allowing their use as accurate reddening tracers. They revealed a steep, spatially varying near-infrared extinction curve toward the inner bulge. Type II Cepheids in the Galactic bulge were also employed to measure robust mean selective-to-absolute extinction ratios. They trace a centrally concentrated spatial distribution of the old bulge population with a slight elongation, consistent with earlier results from RR Lyrae stars. Likewise, the classical Cepheids were utilized to trace the Galactic warp and various substructures of the Galactic disk and uncover significant vertical and radial age gradients of the thin disk population at the far side of the Milky Way.

On 2017 April 1 and 3, two large eruptions on the western solar limb, which were associated with M4.4- and M5.8-class flares, respectively, were observed with the Sun Watcher with Active Pixels and Image Processing (SWAP) Extreme Ultraviolet (EUV) solar telescope on board the Project for On Board Autonomy 2 (PROBA2) spacecraft. The large field-of-view (FOV) of SWAP, combined with an advantageous off-point, allows us to study the eruptions up to approximately 2 solar radii (Rs), where space-based coronagraph observations begin. These measurements provide us with some of the highest EUV observations of an eruption, giving crucial additional data points to track the early evolution of Coronal Mass Ejections. In SWAP observations, we track the evolution of off-limb erupting features as well as associated on-disk EUV waves, and the kinematics of both are calculated. The first eruption shows a clear deceleration throughout the lower corona into coronagraph observations, whereas the second eruption, which had a lower initial velocity, shows no obvious acceleration or deceleration profile. This paper presents a unique set of observations, allowing features observed in EUV to be traced to greater heights in the solar atmosphere, helping to bridge the gap to the FOV of white-light coronagraphs. Even with these favorable data sets, it remains a challenging task to associate features observed in EUV with those observed in white light, highlighting our urgent need for single-instrument observations of the combined lower and middle corona.

Low sonic Mach number shocks form in the intracluster medium (ICM) during the formation of the large-scale structure of the universe. Nonthermal cosmic-ray (CR) protons are expected to be accelerated via diffusive shock acceleration (DSA) in those ICM shocks, although observational evidence for the γ-ray emission of hadronic origin from galaxy clusters has yet to be established. Considering the results obtained from recent plasma simulations, we improve the analytic test-particle DSA model for weak quasi-parallel (${Q}_{\parallel }$) shocks, previously suggested by Kang & Ryu. In the model CR spectrum, the transition from the postshock thermal to CR populations occurs at the injection momentum, p_inj, above which protons can undergo the full DSA process. As the shock energy is transferred to CR protons, the postshock gas temperature should decrease accordingly and the subshock strength weakens due to the dynamical feed of the CR pressure to the shock structure. This results in the reduction of the injection fraction, although the postshock CR pressure approaches an asymptotic value when the CR spectrum extends to the relativistic regime. Our new DSA model self-consistently accounts for such behaviors and adopts better estimations for p_inj. With our model DSA spectrum, the CR acceleration efficiency ranges from η ∼ 10⁻³–0.01 for supercritical, ${Q}_{\parallel }$-shocks with sonic Mach number 2.25 ≲ M_s ≲ 5 in the ICM. Based on Ha et al., on the other hand, we argue that proton acceleration would be negligible in subcritical shocks with M_s < 2.25.

We report for the first time below 1.5 keV, the detection of a secondary peak in an Eddington-limited thermonuclear X-ray burst observed by the Neutron Star Interior Composition Explorer (NICER) from the low-mass X-ray binary 4U 1608–52. Our time-resolved spectroscopy of the burst is consistent with a model consisting of a varying-temperature blackbody, and an evolving persistent flux contribution, likely attributed to the accretion process. The dip in the burst intensity before the secondary peak is also visible in the bolometric flux. Prior to the dip, the blackbody temperature reached a maximum of ≈3 keV. Our analysis suggests that the dip and secondary peak are not related to photospheric expansion, varying circumstellar absorption, or scattering. Instead, we discuss the observation in the context of hydrodynamical instabilities, thermonuclear flame spreading models, and reburning in the cooling tail of the burst.

New high-precision Mo isotopic data were obtained for 10 iron meteorites and two carbonaceous, five ordinary, and two rumuruti chondrites. A clear isotopic dichotomy is observed in μⁱMo−μ⁹⁴Mo diagrams between the CC meteorites (carbonaceous chondrites and IVB irons) and other noncarbonaceous (NC) meteorites. The Mo isotope variabilities within the CC meteorites can indicate either s-process matter distributed heterogeneously throughout various chondritic components in the different outer solar system materials or that generated by a local parent-body processing. In contrast, the presence of two end-member components for the Mo isotope composition, that is, NC-A and NC-B, was suggested in the NC reservoir. The NC-B component represents the remaining counterpart of the gaseous source reservoir for type B calcium-aluminum-rich inclusions, which was presumably formed via thermal processing that destroyed r-process-rich carriers. Two models were proposed to consider the observed Mo isotope variability among the NCs. In model 1, the NC-A reservoir was formed closer to the Sun than the NC-B reservoir by another thermal processing that destroyed s-process-depleted phases. The Mo isotopic composition of the NC region changed via outward motion of particles from the two reservoirs, resulting in a gradual change from NC-A- to NC-B-like components as a function of the heliocentric distance. In model 2, the Mo isotopic composition in individual NCs is controlled by the amount of metal and matrix-like material that is removed from and added to the NC-B reservoir. Such a fractionation process most likely occurred locally in time and/or space in the inner solar system.

Classification of intermediate redshift (z = 0.3–0.8) emission line galaxies as star-forming galaxies, composite galaxies, active galactic nuclei (AGNs), or low-ionization nuclear emission regions (LINERs) using optical spectra alone was impossible because the lines used for standard optical diagnostic diagrams: [N ii], Hα, and [S ii] are redshifted out of the observed wavelength range. In this work, we address this problem using four supervised machine-learning classification algorithms: k-nearest neighbors (KNN), support vector classifier (SVC), random forest (RF), and a multilayer perceptron (MLP) neural network. For input features, we use properties that can be measured from optical galaxy spectra out to z < 0.8—[O iii]/Hβ, [O ii]/Hβ, [O iii] line width, and stellar velocity dispersion—and four colors (u − g, g − r, r − i, and i − z) corrected to z = 0.1. The labels for the low redshift emission line galaxy training set are determined using standard optical diagnostic diagrams. RF has the best area under curve score for classifying all four galaxy types, meaning the highest distinguishing power. Both the AUC scores and accuracies of the other algorithms are ordered as MLP > SVC > KNN. The classification accuracies with all eight features (and the four spectroscopically determined features only) are 93.4% (92.3%) for star-forming galaxies, 69.4% (63.7%) for composite galaxies, 71.8% (67.3%) for AGNs, and 65.7% (60.8%) for LINERs. The stacked spectrum of galaxies of the same type as determined by optical diagnostic diagrams at low redshift and RF at intermediate redshift are broadly consistent. Our publicly available code (https://github.com/zkdtc/MLC_ELGs) and trained models will be instrumental for classifying emission line galaxies in upcoming wide-field spectroscopic surveys.

We study the coherence of the near-infrared and X-ray background fluctuations and the X-ray spectral properties of the sources producing it. We use data from multiple Spitzer and Chandra surveys, including the UDS/SXDF surveys, the Hubble Deep Field North, the EGS/AEGIS field, the Chandra Deep Field South, and the COSMOS surveys, comprising ∼2275 Spitzer/IRAC hours and ∼16 Ms of Chandra data collected over a total area of ∼1 deg². We report an overall ∼5σ detection of a cross-power signal on large angular scales >20'' between the 3.6 and 4.5 μm and the X-ray bands, with the IR versus [1–2] keV signal detected at 5.2σ. The [0.5–1] and [2–4] keV bands are correlated with the infrared wavelengths at a ∼1–3σ significance level. The hardest X-ray band ([4–7] keV) alone is not significantly correlated with any infrared wavelengths due to poor photon and sampling statistics. We study the X-ray spectral energy distribution of the cross-power signal. We find that its shape is consistent with a variety of source populations of accreting compact objects, such as local unabsorbed active galactic nuclei or high-z absorbed sources. We cannot exclude that the excess fluctuations are produced by more than one population. Because of poor statistics, the current relatively broad photometric bands employed here do not allow distinguishing the exact nature of these compact objects or if a fraction of the fluctuations have instead a local origin.

In the Sun, the properties of acoustic modes are sensitive to changes in the magnetic activity. In particular, mode frequencies are observed to increase with increasing activity level. Thanks to CoRoT and Kepler, such variations have been found in other solar-type stars and encode information on the activity-related changes in their interiors. Thus, the unprecedented long-term Kepler photometric observations provide a unique opportunity to study stellar activity through asteroseismology. The goal of this work is to investigate the dependencies of the observed mode frequency variations on the stellar parameters and whether those are consistent with an activity-related origin. We select the solar-type oscillators with highest signal-to-noise ratio, in total, 75 targets. Using the temporal frequency variations determined in Santos et al., we study the relation between those variations and the fundamental stellar properties. We also compare the observed frequency shifts with chromospheric and photometric activity indexes, which are only available for a subset of the sample. We find that frequency shifts increase with increasing chromospheric activity, which is consistent with an activity-related origin of the observed frequency shifts. Frequency shifts are also found to increase with effective temperature, which is in agreement with the theoretical predictions for the activity-related frequency shifts by Metcalfe et al. Frequency shifts are largest for fast rotating and young stars, which is consistent with those being more active than slower rotators and older stars. Finally, we find evidence for frequency shifts increasing with stellar metallicity.

Inverse Compton-pair cascades are initiated when gamma-rays are absorbed on an ambient soft photon field to produce relativistic pairs, which in turn up-scatter the same soft photons to produce more gamma-rays. If the Compton scatterings take place in the deep Klein–Nishina regime, then triplet pair production ($e{\gamma }_{b}\to {{ee}}^{+}{e}^{-}$) becomes relevant and may even regulate the development of the cascade. We investigate the properties of pair-Compton cascades with triplet pair production in accelerating gaps, i.e., regions with an unscreened electric field. Using the method of transport equations for the particle evolution, we compute the growth rate of the pair cascade as a function of the accelerating electric field in the presence of blackbody and power-law ambient photon fields. Informed by the numerical results, we derive simple analytical expressions for the peak growth rate and the corresponding electric field. We show that for certain parameters, which can be realized in the vicinity of accreting supermassive black holes at the centers of active galactic nuclei, the pair cascade may well be regulated by inverse Compton scattering in the deep Klein–Nishina regime and triplet pair production. We present indicative examples of the escaping gamma-ray radiation from the gap, and discuss our results in application to the TeV observations of radio galaxy M87.

The Sun and Sun-like stars lose angular momentum to their magnetized stellar winds. This braking torque is coupled to the stellar magnetic field, such that changes in the strength and/or geometry of the field modifies the efficiency of this process. Since the space age, we have been able to directly measure solar wind properties using in situ spacecraft. Furthermore, indirect proxies such as sunspot number, geomagnetic indices, and cosmogenic radionuclides, constrain the variation of solar wind properties on centennial and millennial timescales. We use near-Earth measurements of the solar wind plasma and magnetic field to calculate the torque on the Sun throughout the space age. Then, reconstructions of the solar open magnetic flux are used to estimate the time-varying braking torque during the last nine millennia. We assume a relationship for the solar mass-loss rate based on observations during the space age which, due to the weak dependence of the torque on mass-loss rate, does not strongly affect our predicted torque. The average torque during the last nine millennia is found to be 2.2 × 10³⁰ erg, which is comparable to the average value from the last two decades. Our data set includes grand minima (such as the Maunder Minimum), and maxima in solar activity, where the torque varies from ∼1 to 5 × 10³⁰ erg (averaged on decadal timescales), respectively. We find no evidence for any secular variation of the torque on timescales of less than 9000 yr.

Many debris disks seen in scattered light have shapes that imply their dust grains trace highly eccentric, apsidally aligned orbits. Apsidal alignment is surprising, especially for dust. Even when born from an apse-aligned ring of parent bodies, dust grains have their periastra dispersed in all directions by stellar radiation pressure. The periastra cannot be reoriented by planets within the short dust lifetimes at the bottom of the collisional cascade. We propose that what realigns dust orbits is drag exerted by second-generation gas. Gas is largely immune to radiation pressure, and when released by photodesorption or collisions within an eccentric ring of parent bodies should occupy a similarly eccentric, apse-aligned ring. Dust grains launched onto misaligned orbits cross the eccentric gas ring supersonically and can become dragged into alignment within collisional lifetimes. The resultant dust configurations, viewed nearly but not exactly edge-on, with periastra pointing away from the observer, appear moth-like, with kinked wings and even doubled pairs of wings, explaining otherwise mysterious features in HD 61005 ("The Moth") and HD 32297, including their central bulbs when we account for strong forward scattering from irregularly shaped particles. Around these systems we predict gas at Kuiper-belt-like distances to move on highly elliptical streamlines that owe their elongation, ultimately, to highly eccentric planets. Unresolved issues and an alternative explanation for apsidal alignment are outlined.

We performed numerical simulations of general relativistic magnetohydrodynamics with uniform resistivity to investigate the occurrence of magnetic reconnection in a split-monopole magnetic field around a Schwarzschild black hole. We found that magnetic reconnection happens near the black hole at its equatorial plane. The magnetic reconnection has a point-like reconnection region and slow shock waves, as in the Petschek reconnection model. The magnetic reconnection rate decreases as the resistivity becomes smaller. When the global magnetic Reynolds number is 10⁴ or larger, the magnetic reconnection rate increases linearly with time from 2τ_S to ∼10τ_S (τ_S = r_S/c, r_S is the Schwarzschild radius and c is the speed of light). The linear increase of the reconnection rate agrees with the magnetic reconnection in the Rutherford regime of the tearing mode instability.

We introduce a simple model to explain the time-reversed and stretched residuals in gamma-ray burst (GRB) pulse light curves. In this model an impactor wave in an expanding GRB jet accelerates from subluminal to superluminal velocities, or decelerates from superluminal to subluminal velocities. The impactor wave interacts with the surrounding medium to produce Cerenkov and/or other collisional radiation when traveling faster than the speed of light in this medium, and other mechanisms (such as thermalized Compton or synchrotron shock radiation) when traveling slower than the speed of light. These transitions create both a time-forward and a time-reversed set of light-curve features through the process of relativistic image doubling. The model can account for a variety of unexplained yet observed GRB pulse behaviors, including the amount of stretching observed in time-reversed GRB pulse residuals and the relationship between stretching factor and pulse asymmetry. The model is applicable to all GRB classes since similar pulse behaviors are observed in long/intermediate GRBs, short GRBs, and X-ray flares. The free model parameters are the impactor's Lorentz factor when moving subluminally, its Lorentz factor when moving superluminally, and the speed of light in the impacted medium.

Understanding planet formation requires one to discern how dust grows in protoplanetary disks. An important parameter to measure in disks is the maximum dust grain size present. This is usually estimated through measurements of the dust opacity at different millimeter wavelengths assuming optically thin emission and dust opacity dominated by absorption. However, Atacama Large Millimeter/submillimeter Array (ALMA) observations have shown that these assumptions might not be correct in the case of protoplanetary disks, leading to overestimation of particle sizes and to underestimation of the disk's mass. Here, we present an analysis of high-quality ALMA and Very Large Array images of the HL Tau protoplanetary disk, covering a wide range of wavelengths, from 0.8 mm to 1 cm, and with a physical resolution of ∼7.35 au. We describe a procedure to analyze a set of millimeter images without any assumption about the optical depth of the emission, and including the effects of absorption and scattering in the dust opacity. This procedure allows us to obtain the dust temperature, the dust surface density, and the maximum particle size at each radius. In the HL Tau disk, we found that particles have already grown to a few millimeters in size. We detect differences in the dust properties between dark and bright rings, with dark rings containing low dust density and small dust particles. Different features in the HL Tau disk seem to have different origins. Planet–disk interactions can explain substructure in the external half of the disk, but the internal rings seem to be associated with the presence of snow lines of several molecules.

The velocity oscillations observed in the chromosphere of sunspot umbrae resemble a resonance in that their power spectra are sharply peaked around a period of about three minutes. In order to describe the resonance that leads to the observed 3-minute oscillations, we propose the photospheric resonator model of acoustic waves in the solar atmosphere. The acoustic waves are driven by the motion of a piston at the lower boundary, and propagate in a nonisothermal atmosphere that consists of the lower layer (photosphere), where temperature rapidly decreases with height, and the upper layer (chromosphere), where temperature slowly increases with height. We have obtained the following results: (1) The lower layer (photosphere) acts as a leaky resonator of acoustic waves. The bottom end is established by the piston, and the top end by the reflection at the interface between the two layers. (2) The temperature minimum region partially reflects and partially transmits acoustic waves of frequencies around the acoustic cutoff frequency at the temperature minimum. (3) The resonance occurs in the photospheric layer at one frequency around this cutoff frequency. (4) The waves escaping the photospheric layer appear as upward-propagating waves in the chromosphere. The power spectrum of the velocity oscillation observed in the chromosphere can be fairly well reproduced by this model. The photospheric resonator model was compared with the chromospheric resonator model and the propagating wave model.

In this work, we analyze the long-term cosmic-ray modulation observed by the Hermanus neutron monitor, which is the detector with the longest cosmic-ray record, from 1957 July. For our study we use the force-field approximation to the cosmic-ray transport equation, and the newest results on the mean free paths from the scattering theory. We compare the modulation parameter (ϕ) with different rigidity (P) dependences: P, P², and P^2/3. We correlate them with solar and interplanetary parameters. We found that (1) these rigidity dependences properly describe the modulation, (2) long-term cosmic-ray variations are better correlated with the magnitude of the heliospheric magnetic field (HMF) than the sunspot number, solar wind speed, and tilt angle of the HMF, and (3) the theoretical dependence of the parallel mean free path on the magnetic field variance is in agreement with the modulation parameter and therefore with the neutron monitor record. We also found that the force-field approximation is not able to take into account the effects of three-dimensional particle transport, showing a poor correlation with the perpendicular mean free path.

In this paper, we present an analysis of a pseudostreamer embedding a filament cavity, observed on 2015 April 18 on the solar southwest limb. We use the flux-rope insertion method to construct nonlinear force-free field (NLFFF) models constrained by observed Solar Dynamics Observatory (SDO)/AIA coronal structures and the SDO/Helioseismic Magnetic Imager photospheric magnetogram. The resulting magnetic field models are forward-modeled to produce synthetic data directly comparable to Mauna Loa Solar Observatory/Coronal Multichannel Polarimeter (CoMP) observations of the intensity and linear polarization of the Fe xiii 1074.7 nm infrared coronal emission line using FORWARD. In addition, we determine the location of quasi-separatrix layers in the magnetic models, producing a Q-map from which the signatures of magnetic null points and separatrices can be identified. An apparent magnetic null observed in linear polarization by CoMP is reproduced by the model and appears in the region of the 2D-projected magnetic null in the Q-map. Further, we find that the height of the CoMP null is better reproduced by our NLFFF model than by the synthetic data we produce with potential-field source-surface models, implying the presence of a flux rope in the northern lobe of the pseudostreamer.

The cosmic infrared background (CIB) is a powerful probe of large-scale structure across a very large redshift range, and consists of unresolved redshifted infrared emission from dusty galaxies. It can be used to study the astrophysics of galaxies, the star formation history of the universe, and the connection between dark and luminous matter. It can furthermore be used as a tracer of the large-scale structure and thus assist in de-lensing of the cosmic microwave background. The major difficulty in its use lies in obtaining accurate and unbiased large-scale CIB images that are cleaned of the contamination by Galactic dust. We used data on neutral atomic hydrogen from the recently released HI4PI Survey to create template maps of Galactic dust, allowing us to remove this component from the Planck intensity maps from 353 to 857 GHz for approximately 25% of the sky. This allows us to constrain the CIB power spectrum down to ℓ ≳ 70. We present these CIB maps and the various processing and validation steps that we have performed to ensure their quality, as well as a comparison with previous studies. All our data products are made publicly available,⁴ thereby enabling the community to investigate a wide range of questions related to the universe's large-scale structure.

Despite their factor of ∼10⁸ difference in black hole mass, several lines of evidence suggest possible similarities between black hole accretion flows in active galactic nuclei (AGN) and Galactic X-ray binaries. However, it is still unclear whether the geometry of the disk–corona system in X-ray binaries directly scales up to AGN and whether this analogy still holds in different accretion states. We test this AGN/X-ray binary analogy by comparing the observed correlations between the UV–to–X-ray spectral index (α_OX) and Eddington ratio in AGN to those predicted from observations of X-ray binary outbursts. This approach probes the geometry of their disk–corona systems as they transition between different accretion states. We use new Chandra X-ray and ground-based rest-UV observations of faded "changing-look" quasars to extend this comparison to lower Eddington ratios of <10⁻², where observations of X-ray binaries predict a softening of α_OX in AGN. We find that the observed correlations between the α_OX and Eddington ratio of AGN displays a remarkable similarity to accretion state transitions in prototypical X-ray binary outbursts, including an inversion of this correlation at a critical Eddington ratio of ∼10⁻². Our results suggest that the structures of black hole accretion flows directly scale across a factor of ∼10⁸ in black hole mass and across different accretion states, enabling us to apply theoretical models of X-ray binaries to explain AGN phenomenology.

The net radial flow velocity of gas is an important parameter for understanding galaxy evolution. It is difficult to measure in the presence of the elliptical orbits of an oval distortion because the mathematical model describing the observed velocity is degenerate in the unknown velocity components. A method is developed in this paper that breaks the degeneracy using additional information about the angular frequency of the oval distortion. The method is applied to the neutral hydrogen in the oval distortion of NGC 4736. The neutral hydrogen is flowing inward at a mean rate of −6.1 ± 1.9 km s⁻¹. At this rate, it takes 400 Myr, or 1.7 rotations of the oval distortion, for the neutral hydrogen to travel the 2.5 kpc from the end to the beginning of the oval distortion. The mean mass flow rate of the neutral hydrogen in this region is −0.25 ± 0.11 M_⊙ yr⁻¹, which is similar to estimates for the star formation rate reported in the literature.

We investigate the geometric distribution of gas metallicities in the circumgalactic medium (CGM) around 47, z < 0.7 galaxies from the "Multiphase Galaxy Halos" Survey. Using a combination of quasar spectra from Hubble Space Telescope (HST)/COS and from Keck/HIRES or Very Large Telescope/UVES, we measure column densities of, or determine limits on, CGM absorption lines. We then use a Markov Chain Monte Carlo approach with Cloudy to estimate the metallicity of cool (T ∼ 10⁴ K) CGM gas. We also use HST images to determine host-galaxy inclination and quasar-galaxy azimuthal angles. Our sample spans a H i column density range of 13.8 cm⁻² < $\mathrm{log}{N}_{{\rm{H}}{\rm{i}}}$ < 19.9 cm⁻². We find (1) while the metallicity distribution appears bimodal, a Hartigan dip test cannot rule out a unimodal distribution (0.4σ). (2) CGM metallicities are independent of halo mass, spanning three orders of magnitude at a fixed halo mass. (3) The CGM metallicity does not depend on the galaxy azimuthal and inclination angles regardless of H i column density, impact parameter, and galaxy color. (4) The ionization parameter does not depend on azimuthal angle. We suggest that the partial Lyman limit metallicity bimodality is not driven by a spatial azimuthal bimodality. Our results are consistent with simulations where the CGM is complex and outflowing, accreting, and recycled gas are well-homogenized at z < 0.7. The presence of low-metallicity gas at all orientations suggests that cold streams of accreting filaments are not necessarily aligned with the galaxy plane at low redshifts or intergalactic transfer may dominate. Finally, our results support simulations showing that strong metal absorption can mask the presence of low-metallicity gas in integrated line-of-sight CGM metallicities.

Terrestrial planets have been found orbiting Sun-like stars with extremely short periods—some as short as 4 hr. These "ultra-short-period planets" or "hot Earths" are so strongly irradiated that any initial H/He atmosphere has probably been lost to photoevaporation. As such, the sample of hot Earths may give us a glimpse at the rocky cores that are often enshrouded by thick H/He envelopes on wider-orbiting planets. However, the mass and radius measurements of hot Earths have been derived from a hodgepodge of different modeling approaches, and include several cases of contradictory results. Here, we perform a homogeneous analysis of the complete sample of 11 known hot Earths with an insolation exceeding 650 times that of the Earth. We combine all available data for each planet, incorporate parallax information from Gaia to improve the stellar and planetary parameters, and use Gaussian process regression to account for correlated noise in the radial-velocity data. The homogeneous analysis leads to a smaller dispersion in the apparent composition of hot Earths, although there does still appear to be some intrinsic dispersion. Most of the planets are consistent with an Earth-like composition (35% iron and 65% rock), but two planets (K2-141b and K2-229b) show evidence for a higher iron fraction, and one planet (55 Cnc e) has either a very low iron fraction or an envelope of low-density volatiles. All of the planets are less massive than 8 M_⊕, despite the selection bias toward more massive planets, suggesting that 8 M_⊕ is the critical mass for runaway accretion.

Neutrino fast pairwise conversions have been postulated to occur in the dense core of a core-collapse supernova (SN), possibly having dramatic consequences on the SN mechanism and the observable neutrino signal. One crucial condition favoring pairwise conversions is the presence of crossings between the electron neutrino and antineutrino angular distributions (i.e., electron neutrino lepton number crossings, ELN crossings). A stationary and spherically symmetric SN toy model is constructed to reproduce the development of the neutrino angular distributions in the dense SN core in the absence of perturbations induced by hydrodynamical instabilities. By iteratively solving the neutrino Boltzmann equations including the collisional term, our model predicts that ELN crossings can develop only in the proximity of the decoupling region and for a sharp radial evolution of the baryon density, when the electron neutrino and antineutrino number densities are comparable. Such conditions are likely to occur only in the late SN stages. Interestingly, flavor instabilities induced by spatial or temporal perturbations are unlikely to generate ELN crossings dynamically within our simplified setup.

We present high spatial resolution imaging of the CO(1–0) line from the Karl G. Jansky Very Large Array of COSMOS 27289, a massive, compact star-forming galaxy (SFG) at z = 2.234. This galaxy was selected because of its structural similarity to z ∼ 2 passive galaxies. Our previous observations showed that it is very gas poor with respect to typical SFGs at these redshifts, consistent with a rapid transition to quiescence as the molecular gas is depleted. The new data show that both the molecular gas fraction, ${f}_{{{\rm{H}}}_{2}}\equiv {M}_{{{\rm{H}}}_{2}}/{M}_{\mathrm{star}}$, and the molecular gas depletion time, ${t}_{\mathrm{dep}}\equiv {M}_{{{\rm{H}}}_{2}}$/SFR, are lower in the central 1–2 kpc of the galaxy and rise at larger radii ∼2–4 kpc. These observations are consistent with a scenario in which COSMOS 27289 will imminently cease star formation in the inner regions before the outskirts, i.e., inside-out quenching, the first time this phenomenon has been seen via observations of molecular gas in the high-redshift universe. We find good qualitative and quantitative agreement with a hydrodynamical simulation of galaxy quenching, in which the central suppression of molecular gas arises due to rapid gas consumption and outflows that evacuate the central regions of gas. Our results provide independent evidence for inside-out quenching of star formation as a plausible formation mechanism for z ∼ 2 quiescent galaxies.

We present a method of fitting optical spectra of galaxies using a basis set of six vectors obtained from principal-component analysis of a library of synthetic spectra of 40,000 star formation histories (SFHs). Using this library, we provide estimates of the resolved effective stellar mass-to-light ratio (${{\rm{\Upsilon }}}^{* }$) for thousands of galaxies from the SDSS-IV/MaNGA integral-field spectroscopic survey. Using a testing framework built on additional synthetic SFHs, we show that the estimates of $\mathrm{log}{{\rm{\Upsilon }}}_{i}^{* }$ are reliable (as are their uncertainties) at a variety of signal-to-noise ratios, stellar metallicities, and dust attenuation conditions. Finally, we describe the future release of the resolved stellar mass-to-light ratios as an SDSS-IV/MaNGA Value-Added Catalog and provide a link to the software used to conduct this analysis. (The software can be found at https://github.com/zpace/pcay.)

A galaxy's stellar mass is one of its most fundamental properties, but it remains challenging to measure reliably. With the advent of very large optical spectroscopic surveys, efficient methods that can make use of low signal-to-noise spectra are needed. With this in mind, we created a new software package for estimating effective stellar mass-to-light ratios ${{\rm{\Upsilon }}}^{* }$ that uses a principal component analysis (PCA) basis set to optimize the comparison between observed spectra and a large library of stellar population synthesis models. In Paper I, we showed that with a set of six PCA basis vectors we could faithfully represent most optical spectra from the Mapping Nearby Galaxies at APO (MaNGA) survey, and we tested the accuracy of our M/L estimates using synthetic spectra. Here, we explore sources of systematic error in our mass measurements by comparing our new measurements to data from the literature. We compare our stellar mass surface density estimates to kinematics-derived dynamical mass surface density measurements from the DiskMass Survey and find some tension between the two that could be resolved if the disk scale heights used in the kinematic analysis were overestimated by a factor of ∼1.5. We formulate an aperture-corrected stellar mass catalog for the MaNGA survey, and compare to previous stellar mass estimates based on multiband optical photometry, finding typical discrepancies of 0.1 dex. Using the spatially resolved MaNGA data, we evaluate the impact of estimating total stellar masses from spatially unresolved spectra, and we explore how the biases that result from unresolved spectra depend upon the galaxy's dust extinction and star formation rate. Finally, we describe an SDSS Value-Added Catalog that will include both spatially resolved and total (aperture-corrected) stellar masses for MaNGA galaxies.

We observe two metal-poor main-sequence stars that are members of the recently discovered Sylgr stellar stream. We present radial velocities, stellar parameters, and abundances for 13 elements derived from high-resolution optical spectra collected using the Magellan Inamori Kyocera Echelle spectrograph. The two stars have identical compositions (within 0.13 dex or 1.2σ) among all elements detected. Both stars are very metal-poor ([Fe/H] = −2.92 ± 0.06). Neither star is highly enhanced in C ([C/Fe] < +1.0). Both stars are enhanced in the α elements Mg, Si, and Ca ([α/Fe] = +0.32 ± 0.06), and the ratios among Na, Al, and all Fe-group elements are typical for other stars in the halo and ultra-faint and dwarf spheroidal galaxies at this metallicity. Sr is mildly enhanced ([Sr/Fe] = +0.22 ± 0.11), but Ba is not enhanced ([Ba/Fe] < −0.4), indicating that these stars do not contain high levels of neutron-capture elements. The Li abundances match those found in metal-poor unevolved field stars and globular clusters (GCs) (log (Li) = 2.05 ± 0.07), which implies that environment is not a dominant factor in determining the Li content of metal-poor stars. The chemical compositions of these two stars cannot distinguish whether the progenitor of the Sylgr stream was a dwarf galaxy or a GC. If the progenitor was a dwarf galaxy, the stream may originate from a dense region such as a nuclear star cluster. If the progenitor was a GC, it would be the most metal-poor GC known.

We present a combined experimental and theoretical study of the mutual neutralization (MN) process in collisions of lithium ions (Li⁺) with deuterium anions (D⁻) at collision energies below 1 eV. We employ a merged-beam apparatus to determine total and state-to-state MN cross sections. We perform nuclear dynamics calculations using the multichannel Landau–Zener model based on accurate ab initio molecular data. We obtain an excellent agreement between the experimental and theoretical results over the energy range covered in this work. We show that the basis sets used in the ab initio calculations have a limited influence on the total cross section, but strongly impacts the results obtained for the partial cross sections or the reaction branching ratios. This demonstrates the important role of high-precision measurements to validate the theoretical approaches used to study gas-phase reactive processes. Finally, we compute MN rate coefficients for Li⁺ + H⁻ and Li⁺ + D⁻, and discuss their significance for astrochemistry models.

Due to the small amount of hydrogen (≤0.1 M_⊙) remaining on the surface of their progenitors, SNe IIb are sensitive probes of the mass-loss processes of massive stars toward the ends of their lives, including the role of binarity. We report late-time Hubble Space Telescope observations of SN 2011dh in M51, and a brief period of rebrightening and plateau in the photometric light curve, from 1.8 to 6.2 yr after the explosion. These observations exclude the role of circumstellar interaction, however, a slow rotating magnetar, a significant quantity of radioactive elements, or a light echo could be responsible for the late-time luminosity observed at t > 1000 days. If the late-time light curve is powered by the decay of radioactive elements, SN 2011dh is required to have produced ∼2.6 × 10⁻³M_⊙ of ⁴⁴Ti, which is significantly in excess of the amount inferred from earlier nebular spectra of SN 2011dh itself or measured in the Cas A SN remnant. The evolution of the brightness and the color of the late-time light curve also supports the role of a light echo originating from dust with a preferred geometry of a disk of extent ∼1.8 to ∼2.7 pc from the SN, consistent with a wind-blown bubble. Accounting for the long-term photometric evolution due to a light echo, the flux contribution from a surviving binary companion at ultraviolet wavelengths can be isolated and corresponds to a star of ∼9–10 M_⊙.

The morphology of thin stellar streams can be used to test the nature of dark matter. It is therefore crucial to extend searches for globular cluster (GC) streams to other galaxies than the Milky Way. In this paper, we investigate the current and future prospects of detecting GC streams in external galaxies in resolved stars (e.g., with Wide Field InfraRed Survey Telescope (WFIRST)) and using integrated light (e.g., with Hyper Suprime-cam (HSC), the Large Synoptic Survey Telescope (LSST), and Euclid). In particular, we inject mock streams to data from the PAndAS M31 survey and produce simulated M31 backgrounds mimicking what WFIRST will observe in M31. Additionally, we estimate the distance limit to which GC streams will be observable. Our results demonstrate that for a 1 hr (1000 s) exposure, using conservative estimates, WFIRST should detect GC streams in resolved stars in galaxies out to distances of ∼3.5 Mpc (∼2 Mpc). This volume contains 199 (122) galaxies, of which >90% are dwarfs. With integrated light, thin streams can be resolved out to ∼100 Mpc with HSC and LSST and to ∼600 Mpc with WFIRST and Euclid. The low surface brightness of the streams (typically >30 mag arcsec⁻²), however, will make them difficult to detect, unless the streams originate from very young clusters. We emphasize that if the external galaxies do not host spiral arms or galactic bars, gaps in their stellar streams provide an ideal test case for evidence of interactions with dark matter subhalos. Furthermore, obtaining a large samples of thin stellar streams can help constrain the orbital structure and hence the potentials of external halos.

We report on a population of short-duration near-ultraviolet (NUV) flares in stars observed by the Kepler and Galaxy Evolution Explorer (GALEX) missions. We analyzed the NUV light curves of 34,276 stars observed from 2009 to 2013 by both the GALEX (NUV) and Kepler (optical) space missions with the eventual goal of investigating multiwavelength flares. From the GALEX data, we constructed light curves with a 10 s cadence, and we ultimately detected 1904 short-duration flares on 1021 stars. The vast majority (94.5%) of these flares have durations less than 5 minutes, with flare flux enhancements above the quiescent flux level ranging from 1.5 to 1700. The flaring stars are primarily solar-like, with T_eff ranging from 3000 to 11,000 K and radii between 0.5 and 15 R_⊙. This set of flaring stars is almost entirely distinct from that of previous flare surveys of Kepler data and indicates a previously undetected collection of small flares contained within the Kepler sample. The range in flare energies spans 1.8 × 10³²–8.9 × 10³⁷ erg, with associated relative errors spanning 2%–87%. The flare frequency distribution by energy follows a power law with index α = 1.72 ± 0.05, consistent with results of other solar and stellar flare studies at a range of wavelengths. This supports the idea that the NUV flares we observed are governed by the same physical processes present in solar and optical flares. The relationship between flare duration and associated flare energy extends results found for solar and stellar white-light flares, and suggests that these flares originate in regions with magnetic field strengths of several hundred Gauss, and length scales of the order of 10¹⁰ cm.

The mechanism of mass loss in late evolutionary stages of low- and intermediate-mass stars is not yet well understood. Therefore, it is crucial to study the dynamics of the region within a few R_⋆, where the wind acceleration is considered to take place. We present a three-dimensional diagnosis of the atmospheric dynamics of the closest asymptotic giant branch star R Dor from the low photospheric layers to the extended outer atmosphere, for the first time for a star other than the Sun. The images reconstructed with a spatial resolution of 6.8 mas—seven times finer than the star's angular diameter of 51.2 mas in the continuum—using the AMBER instrument at the Very Large Telescope Interferometer show a large, bright region over the surface of the star and an extended atmosphere. The velocity-field maps over the star's surface and atmosphere obtained from the Mg and H₂O lines near 2.3 μm forming at atmospheric heights below ∼1.5 R_⋆ show little systematic motion beyond the measurement uncertainty of 1.7 km s⁻¹. In marked contrast, the velocity-field map obtained from the CO first overtone lines reveals systematic outward motion at 7–15 km s⁻¹ in the extended outer atmosphere at a height of ∼1.8 R_⋆. Given the detection of dust formation at ∼1.5 R_⋆, the strong acceleration of material between ∼1.5 and 1.8 R_⋆ may be caused by the radiation pressure on dust grains. However, we cannot yet exclude the possibility that the outward motion may be intermittent, caused by ballistic motion due to convection and/or pulsation.

Using archival Chandra observations with a total effective exposure of 734 ks, we derive an updated catalog of point sources in the massive globular cluster (GC) Terzan 5. Our catalog covers an area of 58.1 arcmin² (R ≤ 43) with 489 X-ray sources, and more than 75% of these sources are first detected in this cluster. We find significant dips in the radial distribution profiles of X-ray sources in Terzan 5, with the projected distance and width of the distribution dips for bright (L_X ≳ 9.5 × 10³⁰ erg s⁻¹) X-ray sources larger than those of the faint (L_X ≲ 9.5 × 10³⁰ erg s⁻¹) sources. By fitting the radial distribution of the X-ray sources with a "generalized King model," we estimated an average mass of 1.48 ± 0.11 and 1.27 ± 0.13 M_⊙ for the bright and faint X-ray sources, respectively. These results are in agreement with that observed in 47 Tuc, which may suggest a universal mass segregation effect for X-ray sources in GCs. Compared with 47 Tuc, we show that the two-body relaxation timescale of Terzan 5 is much smaller, but its dynamical age is significantly younger than 47 Tuc. These features suggest that the evolution of Terzan 5 is not purely driven by two-body relaxation, and the tidal stripping effect also plays an important role in accelerating the dynamical evolution of this cluster.

Association of solar flares and coronal mass ejections (CMEs) with ground-level enhancement (GLE) is a recognized fact, but questions arise when a similar association is observed for non-GLEs. In this respect, we carry out a detailed study of the relation between flare fluences (ϕ J m⁻²) and CME speeds (V_cme km s⁻¹) during some selected GLEs and non-GLEs. As we found, most of the data points of ϕ (J m⁻²) and V_cme (km s⁻¹) of GLEs follow a near-linear trend, with the ϕ (J m⁻²) increasing as the V_cme (km s⁻¹) increases, resulting in a strong positive correlation (r ≥ 0.82), while the correlation (r ≤ 0.47) remains weak for non-GLEs. For any exceptional GLE, the ϕ (J m⁻²) and V_cme (km s⁻¹) that do not maintain a near-linear trend over the whole flare phase do maintain at least a minimum rational proportionality over the flare rise phase, whereas this characteristic was not generally observed for non-GLEs. Although the ϕ (J m⁻²) and V_cme (km s⁻¹) of some non-GLEs show a trend similar to those of GLEs, they indeed originated over the flare impulsive phases concomitant with coronal shock manifested in m type II bursts, while GLEs originated over the flare initial phase before the m type II. Flare peak fluences (ϕ_pk J m⁻²) and V_cme (km s⁻¹) maintain weak correlation for both GLEs and non-GLEs, likely because the CME main acceleration ceases around the flare peak. However, though the ϕ_pk (J m⁻²) governs the flare total fluence, it does not blur the correlation between the fluence over the flare rise phase (ϕ_r J m⁻²) and V_cme (km s⁻¹), indicating that the flare peak/strength does not control the GLE occurrence.

We present a study of the gas kinematics of star-forming galaxies associated with protocluster 4C 23.56 at z = 2.49 using 04 resolution CO (4–3) data taken with ALMA. Eleven Hα emitters (HAEs) are detected in CO (4–3), including six HAEs that were previously detected in CO (3–2) at a coarser angular resolution. The detections in both CO lines are broadly consistent in the line widths and the redshifts, confirming both detections. With an increase in the number of spectroscopic redshifts, we confirm that the protocluster is composed of two merging groups with a total halo mass of log (M_cl/M_⊙) = 13.4–13.6, suggesting that the protocluster would evolve into a Virgo-like cluster (>10¹⁴M_⊙). We compare the CO line widths and the CO luminosities with other (proto)clusters (n_gal = 91) and general field (n_gal = 80) galaxies from other studies. The 4C 23.56 protocluster galaxies have CO line widths and luminosities comparable to other protocluster galaxies on average. On the other hand, the CO line widths are on average broader by ≈50% compared to field galaxies, while the median CO luminosities are similar. The broader line widths can be attributed to both effects of unresolved gas-rich mergers and/or compact gas distribution, which is supported by our limited but decent angular resolution observations and the size estimate of three galaxies. Based on these results, we argue that gas-rich mergers may play a role in the retention of the specific angular momentum to a value similar to that of field populations during cluster assembly, though we need to verify this with a larger number of samples.

We use helioseismic data from ground- and space-based instruments to analyze how solar rotation has changed since the beginning of solar Cycle 23 with emphasis on studying the differences between Cycles 23 and 24. We find that the nature of solar rotation is indeed different for the two cycles. While the changes in the latitudinally independent component follows solar-cycle indices, some of the other components have a more complicated behavior. There is a substantial change in the behavior of the solar zonal flows and their spatial gradients too. While the zonal flows in Cycle 24 are weaker in general than those in Cycle 23, there are clear signs of the emergence of Cycle 25. We have also investigated the properties of the solar tachocline, in particular, its position, width, and the change (or jump) in the rotation rate across it. We find significant temporal variation in the change of the rotation rate across the tachocline. We also find that the changes in solar Cycle 24 were very different from those of Cycle 23. We do not find any statistically significant change in the position or the width of the tachocline.

We study the sudden optical and ultraviolet (UV) brightening of 1ES 1927+654, which until now was known as a narrow-line active galactic nucleus (AGN). 1ES 1927+654 was part of the small and peculiar class of "true Type-2" AGNs that lack broad emission lines and line-of-sight obscuration. Our high-cadence spectroscopic monitoring captures the appearance of a blue, featureless continuum, followed several weeks later by the appearance of broad Balmer emission lines. This timescale is generally consistent with the expected light travel time between the central engine and the broadline emission region in (persistent) broadline AGN. Hubble Space Telescope spectroscopy reveals no evidence for broad UV emission lines (e.g., C ivλ1549, C iii] λ1909, Mg iiλ2798), probably owing to dust in the broadline emission region. To the best of our knowledge, this is the first case where the lag between the change in continuum and in broadline emission of a "changing look" AGN has been temporally resolved. The nature and timescales of the photometric and spectral evolution disfavor both a change in line-of-sight obscuration and a change of the overall rate of gas inflow as driving the drastic spectral transformations seen in this AGN. Although the peak luminosity and timescales are consistent with those of tidal disruption events seen in inactive galaxies, the spectral properties are not. The X-ray emission displays a markedly different behavior, with frequent flares on timescales of hours to days, and will be presented in a companion publication.

We present the B-fields mapped in IRDC G34.43+0.24 using 850 μm polarized dust emission observed with the POL-2 instrument at the James Clerk Maxwell telescope. We examine the magnetic field geometries and strengths in the northern, central, and southern regions of the filament. The overall field geometry is ordered and aligned closely perpendicular to the filament's main axis, particularly in regions containing the central clumps MM1 and MM2, whereas MM3 in the north has field orientations aligned with its major axis. The overall field orientations are uniform at large (POL-2 at 14'' and SHARP at 10'') to small scales (TADPOL at 25 and SMA at 15) in the MM1 and MM2 regions. SHARP/CSO observations in MM3 at 350 μm from Tang et al. show a similar trend as seen in our POL-2 observations. TADPOL observations demonstrate a well-defined field geometry in MM1/MM2 consistent with MHD simulations of accreting filaments. We obtained a plane-of-sky magnetic field strength of 470 ± 190 μG, 100 ± 40 μG, and 60 ± 34 μG in the central, northern, and southern regions of G34, respectively, using the updated Davis–Chandrasekhar–Fermi relation. The estimated value of field strength, combined with column density and velocity dispersion values available in the literature, suggests G34 to be marginally critical with criticality parameter λ values 0.8 ± 0.4, 1.1 ± 0.8, and 0.9 ± 0.5 in the central, northern, and southern regions, respectively. The turbulent motions in G34 are sub-Alfvénic with Alfvénic Mach numbers of 0.34 ± 0.13, 0.53 ± 0.30, and 0.49 ± 0.26 in the three regions. The observed aligned B-fields in G34.43+0.24 are consistent with theoretical models suggesting that B-fields play an important role in guiding the contraction of the cloud driven by gravity.

We analyze the imaging observations of an M-class eruptive flare of 2015 November 4. The pre-eruptive Hα filament was modeled by the nonlinear force-free field model, which showed that it consisted of two helical systems. Tether-cutting reconnection involving these two systems led to the formation of a hot sigmoidal loop structure rooted in a small hook that formed at the end of the flare ribbon. Subsequently, the hot loops started to slip away from the small hook until it disappeared. The loops continued slipping and the ribbon elongated itself by several tens of arcseconds. A new and larger hook then appeared at the end of the elongated ribbon with hot and twisted loops rooted there. After the eruption of these hot loops, the ribbon hook expanded and later contracted. We interpret these observations in the framework of the recent three-dimensional (3D) extensions to the standard solar flare model predicting the drift of the flux rope footpoints. The hot sigmoidal loop is interpreted as the flux rope, whose footpoints drift during the eruption. While the deformation and drift of the new hook can be described by the model, the displacement of the flux rope footpoint from the filament to that of the erupting flux rope indicate that the hook evolution can be more complex than those captured by the model.

Based on the early-year observations from Neil Gehrels Swift Observatory, Liang et al. performed a systematic analysis for the shallow decay component of gamma-ray bursts (GRBs) X-ray afterglow, in order to explore its physical origin. Here we revisit the analysis with an updated sample (with Swift/XRT GRBs between 2004 February and 2017 July). We find that with a larger sample, (1) the distributions of the characteristic properties of the shallow decay phase (e.g., t_b, S_X, Γ_X,1, and α_X,1) still accord with normal or lognormal distribution; (2) Γ_X,1 and Γ_γ still show no correlation, but the tentative correlations of durations, energy fluences, and isotropic energies between the gamma-ray and X-ray phases still exist; (3) for most GRBs, there is no significant spectral evolution between the shallow decay segment and its follow-up segment, and the latter is usually consistent with the external-shock models; (4) assuming that the central engine has a power-law luminosity release history as $L\left(t\right)={L}_{0}{\left(\tfrac{t}{{t}_{0}}\right)}^{-q}$, we find that the value q is mainly distributed between −0.5 and 0.5, with an average value of 0.16 ± 0.12; (5) the tentative correlation between ${E}_{\mathrm{iso},{\rm{X}}}$ and ${t}_{b}^{{\prime} }$ disappears, so that the global three-parameter correlation (${E}_{\mathrm{iso},{\rm{X}}}-{E}_{p}^{{\prime} }-{t}_{b}^{{\prime} }$) becomes less significant; (6) the anticorrelation between L_X and ${t}_{b}^{{\prime} }$ and the three-parameter correlation (${E}_{\mathrm{iso},\gamma }-{L}_{{\rm{X}}}-{t}_{b}$) indeed exist with a high confidence level. Overall, our results are generally consistent with Liang et al., confirming their suggestion that the shallow decay segment in most bursts is consistent with an external forward shock origin, probably due to a continuous energy injection from a long-lived central engine.

CO is the most widely used gas tracer of protoplanetary disks. Its abundance is usually assumed to be an interstellar ratio throughout the warm molecular layer of the disk. But recent observations of low CO gas abundance in many protoplanetary disks challenge our understanding of physical and chemical evolutions in disks. Here we investigate the CO abundance structures in four well-studied disks and compare their structures with predictions of chemical processing of CO and transport of CO ice-coated dust grains in disks. We use spatially resolved CO isotopologue line observations and detailed thermo-chemical models to derive CO abundance structures. We find that the CO abundance varies with radius by an order of magnitude in these disks. We show that although chemical processes can efficiently reduce the total column of CO gas within 1 Myr under an ISM level of cosmic-ray ionization rate, the depletion mostly occurs at the deep region of a disk. Without sufficient vertical mixing, the surface layer is not depleted enough to reproduce the weak CO emissions observed. The radial profiles of CO depletion in three disks are qualitatively consistent with predictions of pebble formation, settling, and drifting in disks. But the dust evolution alone cannot fully explain the high depletion observed in some disks. These results suggest that dust evolution may play a significant role in transporting volatile materials and a coupled chemical–dynamical study is necessary to understand what raw materials are available for planet formation at different distances from the central star.

We present 16 new ultrabright H_AB ≲ 25 galaxy candidates at z ∼ 8 identified over the COSMOS/UltraVISTA field. The new search takes advantage of the deepest-available ground-based optical and near-infrared observations, including the DR3 release of UltraVISTA and full-depth Spitzer/IRAC observations from the SMUVS and SPLASH programs. Candidates are selected using Lyman-break color criteria, combined with strict optical non-detection and SED-fitting criteria, designed to minimize contamination by low-redshift galaxies and low-mass stars. HST/WFC3 coverage from the DASH program reveals that one source evident in our ground-based near-IR data has significant substructure and may actually correspond to 3 separate z ∼ 8 objects, resulting in a total sample of 18 galaxies, 10 of which seem to be fairly robust (with a >97% probability of being at z > 7). The UV-continuum slope β for the bright z ∼ 8 sample is β = −2.2 ± 0.6, bluer but still consistent with that of similarly bright galaxies at z ∼ 6 (β = −1.55 ± 0.17) and z ∼ 7 (β = −1.75 ± 0.18). Their typical stellar masses are ${10}^{{9.1}_{-0.4}^{+0.5}}$M_⊙, with the SFRs of ${32}_{-32}^{+44}{M}_{\odot }$ yr⁻¹, specific SFR of ${4}_{-4}^{+8}$ Gyr⁻¹, stellar ages of $\sim {22}_{-22}^{+69}$ Myr, and low dust content ${A}_{V}={0.15}_{-0.15}^{+0.30}$ mag. Using this sample we constrain the bright end of the z ∼ 8 UV luminosity function. When combined with recent empty field luminosity function estimates at similar redshifts, the resulting z ∼ 8 luminosity function can be equally well represented by either a Schechter or a double-power-law form. Assuming a Schechter parameterization, the best-fit characteristic magnitude is ${M}^{* }=-{20.95}_{-0.35}^{+0.30}$ mag with a very steep faint-end slope $\alpha =-{2.15}_{-0.19}^{+0.20}$. These new candidates include some of the brightest objects found at these redshifts, 0.5–1.0 magnitude brighter than those found over CANDELS, and providing excellent targets for spectroscopic and longer-wavelength follow-up studies.

We present new 0.6–4 μm imaging of the SR 21 transition disk from Keck/NIRC2 and Magellan/MagAO. The protoplanetary disk around SR 21 has a large (∼30–40 au) clearing first inferred from its spectral energy distribution and later detected in submillimeter imaging. Both the gas and small dust grains are known to have a different morphology, with an inner truncation in CO at ∼7 au, and micron-sized dust detected within the millimeter clearing. Previous near-infrared imaging could not distinguish between an inner dust disk with a truncation at ∼7 au or one that extended to the sublimation radius. The imaging data presented here require an inner dust disk radius of a few au, and complex structure such as a warp or spiral. We present a parametric warped disk model that can reproduce the observations. Reconciling the images with the spectral energy distribution gathered from the literature suggests grain growth to ≳2–5 μm within the submillimeter clearing. The complex disk structure and possible grain growth can be connected to dynamical shaping by a giant-planet-mass companion, a scenario supported by previous observational and theoretical studies.

In mid-2012, a global merged interaction region (GMIR) observed by Voyager 2 crossed through the heliosheath and collided with the heliopause, generating a pressure pulse that propagated into the very local interstellar medium. The effects of the transmitted wave were seen by Voyager 1 just 93 days after its own heliopause crossing. The passage of the transient was accompanied by long-lasting decreases in Galactic cosmic ray intensities that occurred from ∼2012.55 to ∼2013.35 and ∼2012.91 to ∼2013.70 at Voyager 2 and Voyager 1, respectively. Omnidirectional (≳20 MeV) proton-dominated measurements from each spacecraft's Cosmic Ray Subsystem reveal a remarkable similarity between these causally related events, with a correlation coefficient of 91.2% and a time lag of 130 days. Knowing the locations of the two spacecraft, we use the observed time delay to calculate the GMIR's average speed through the heliosheath (inside the heliopause) as a function of temperature in the very local interstellar medium. This, combined with particle, field, and plasma observations, enables us to infer previously unmeasured properties of the heliosheath, including a range of sound speeds and total effective pressures. For a nominal temperature of ∼20,000 K just outside the heliopause, we find a sound speed of 314 ± 32 km s⁻¹ and total effective pressure of 267 ± 55 fPa inside the heliopause. We compare these results with the Interstellar Boundary Explorer's data-driven models of heliosheath pressures derived from energetic neutral atom fluxes (the globally distributed flux) and present them as additional evidence that the heliosheath's dynamics are driven by suprathermal energetic processes.

Using a suite of radiation hydrodynamic simulations of star cluster formation in turbulent clouds, we study the escape fraction of ionizing (Lyman continuum) and non-ionizing (FUV) radiation for a wide range of cloud masses and sizes. The escape fraction increases as H ii regions evolve and reaches unity within a few dynamical times. The cumulative escape fraction before the onset of the first supernova explosion is in the range 0.05–0.58; this is lower for higher initial cloud surface density, and higher for less massive and more compact clouds due to rapid destruction. Once H ii regions break out of their local environment, both ionizing and non-ionizing photons escape from clouds through fully ionized, low-density sight lines. Consequently, dust becomes the dominant absorber of ionizing radiation at late times, and the escape fraction of non-ionizing radiation is only slightly larger than that of ionizing radiation. The escape fraction is determined primarily by the mean $\langle \tau \rangle$ and width σ of the optical-depth distribution in the large-scale cloud, increasing for smaller $\langle \tau \rangle$ and/or larger σ. The escape fraction exceeds (sometimes by three orders of magnitude) the naive estimate ${e}^{-\langle \tau \rangle }$ due to the nonzero σ induced by turbulence. We present two simple methods to estimate, within ∼20%, the escape fraction of non-ionizing radiation using the observed dust optical depth in clouds projected on the plane of sky. We discuss implications of our results for observations, including inference of star formation rates in individual molecular clouds and accounting for diffuse ionized gas on galactic scales.

Orbital properties of stars, computed from their six-dimensional phase-space measurements and an assumed Galactic potential, are used to understand the structure and evolution of the Galaxy. Stellar actions, computed from orbits, have the attractive quality of being invariant under certain assumptions and are therefore used as quantitative labels of a star's orbit. We report a subtle but important systematic error that is induced in the actions as a consequence of local midplane variations expected for the Milky Way. This error is difficult to model because it is non-Gaussian and bimodal, with neither mode peaking on the null value. An offset in the vertical position of the Galactic midplane of ∼15 pc for a thin disk-like orbit or ∼120 pc for a thick disk-like orbit induces a 25% systematic error in the vertical action J_z. In Feedback in Realistic Environments simulations of Milky Way-mass galaxies, these variations are on the order of ∼100 pc at the solar circle. From observations of the mean vertical velocity variation of ∼5–10 km s⁻¹ with radius, we estimate that the Milky Way midplane variations are ∼60–170 pc, consistent with three-dimensional dust maps. Action calculations and orbit integrations, which assume the global and local midplanes are identical, are likely to include this induced error, depending on the volume considered. Variation in the local standard of rest or distance to the Galactic center causes similar issues. The variation of the midplane must be taken into account when performing dynamical analysis across the large regions of the disk accessible to Gaia and future missions.

Ubiquitous solar jets or jet-like activities are generally regarded as an important source of energy and mass input to the upper solar atmosphere and the solar wind. However, their triggering and driving mechanisms are not completely understood. By taking advantage of stereoscopic observations with high temporal and spatial resolutions taken by the Solar Dynamic Observatory (SDO) and the Solar Terrestrial Relations Observatory (STEREO), we report an intriguing two-sided-loop jet that occurred on 2013 June 02, and was dynamically associated with the eruption of a mini-filament below an overlying large filament. Additionally, two distinct reconnection processes are identified during the formation stage. The SDO observations reveal that the two-sided-loop jet showed a concave shape with a projection speed of about 80–136 km s⁻¹. From the other view angle, the STEREO observations clearly showed that the trajectories of the two arms of the two-sided-loop were along the cavity magnetic field lines hosting the large filament. Contrary to the well-accepted theoretical model, the present observation sheds new light on our understanding of the formation mechanism of two-sided-loop jets. Moreover, the eruption of the two-sided-loop jet not only supplied mass to the overlying large filament, but also provided a rare opportunity to diagnose the magnetic structure of the overlying large filament via the method of three-dimensional reconstruction.

The empirical mass–luminosity relation in the Hyades cluster rests on dynamical mass determinations for five binary systems, of which one is eclipsing and the other four are visual or interferometric binaries. The last one was identified and first measured more than 20 yr ago. Here we present dynamical mass measurements for a new binary system in the cluster, 80 Tau, which is also a visual pair with a much longer orbital period of about 170 yr. Although we lack the radial-velocity information that has enabled the individual mass determinations in all of the previous binaries, we show that it is still possible to derive the component masses for 80 Tau using only astrometric observations. This is enabled by the accurate proper motion measurements from the Hipparcos and Gaia missions, which constrain the orbital acceleration in the plane of the sky. Separate proper motion values from Gaia for the primary and secondary provide a direct constraint on the mass ratio. Our mass measurements, ${M}_{{\rm{A}}}={1.63}_{-0.13}^{+0.30}\,{M}_{\odot }$ and ${M}_{{\rm{B}}}={1.11}_{-0.14}^{+0.21}\,{M}_{\odot }$, are consistent with the mass–luminosity relation defined by the five previously known systems, which in turn is in good agreement with current models of stellar evolution.

Weak magnetic fields have recently been detected in a number of A-type stars, including Vega and Sirius. At the same time, space photometry observations of A and late B-type stars from Kepler and TESS have highlighted the existence of rotational modulation of surface features akin to stellar spots. Here we explore the possibility that surface magnetic spots might be caused by the presence of small envelope convective layers at or just below the stellar surface, caused by recombination of H and He. Using 1D stellar evolution calculations and assuming an equipartition dynamo, we make simple estimates of field strength at the photosphere. For most models, the largest effects are caused by a convective layer driven by second helium ionization. While it is difficult to predict the geometry of the magnetic field, we conclude that the majority of intermediate-mass stars should have dynamo-generated magnetic fields of order a few Gauss at the surface. These magnetic fields can appear at the surface as bright spots and cause photometric variability via rotational modulation, which could also be widespread in A-stars. The amplitude of surface magnetic fields and their associated photometric variability are expected to decrease with increasing stellar mass and surface temperature, so that magnetic spots and their observational effects should be much harder to detect in late B-type stars.

Modern theories of galaxy formation predict that the Galactic stellar halo was hierarchically assembled from the accretion and disruption of smaller systems. This hierarchical assembly is expected to produce a high degree of structure in the combined phase and chemistry space; this structure should provide a relatively direct probe of the accretion history of our Galaxy. Revealing this structure requires precise 3D positions (including distances), 3D velocities, and chemistry for large samples of stars. The Gaia satellite is delivering proper motions and parallaxes for >1 billion stars to G ≈ 20. However, radial velocities and metallicities will only be available to G ≈ 15, which is insufficient to probe the outer stellar halo (≳10 kpc). Moreover, parallaxes will not be precise enough to deliver high-quality distances for stars beyond ∼10 kpc. Identifying accreted systems throughout the stellar halo therefore requires a large ground-based spectroscopic survey to complement Gaia. Here we provide an overview of the H3 Stellar Spectroscopic Survey, which will deliver precise stellar parameters and spectrophotometric distances for ≈200,000 stars to r = 18. Spectra are obtained with the Hectochelle instrument at the MMT, which is configured for the H3 Survey to deliver resolution R ≈ 23,000 spectra covering the wavelength range 5150–5300 Å. The survey is optimized for stellar halo science and therefore focuses on high Galactic latitude fields ($| b| \gt 30^\circ$), sparsely sampling 15,000 sq. degrees. Targets are selected on the basis of Gaia parallaxes, enabling very efficient selection of bona fide halo stars. The survey began in the fall of 2017 and has collected 88,000 spectra to-date. All of the data, including the derived stellar parameters, will eventually be made publicly available via the survey website: h3survey.rc.fas.harvard.edu.

We collect new and archival optical observations of nine "black-widow" millisecond pulsar binaries. New measurements include direct imaging with the Keck, Gemini-S, MDM, and Las Cumbres Observatory 2 m telescopes. This is supplemented by synthesized colors from Keck long-slit spectra. Four black-widow optical companions are presented here for the first time. Together these data provide multicolor photometry covering a large fraction of the orbital phase. We fit these light curves with a direct (photon) heating model using a version of the ICARUS light-curve modeling code. The fits provide distance and fill-factor estimates, inclinations, and heating powers. We compare the heating powers with the observed GeV luminosities, noting that the ratio is sensitive to pulsar distance and to the gamma-ray beaming. We make a specific correction for "outer gap" model beams, but even then some sources are substantially discrepant, suggesting imperfect beaming corrections and/or errors in the fit distance. The fits prefer large metal abundance for half of the targets, a reasonable result for these wind-stripped secondaries. The companion radii indicate substantial Roche-lobe filling, f_c ≈ 0.7−1 except for PSR J0952−0607, which with f_c < 0.5 has a companion density ρ ≈ 10 g cm⁻³, suggesting unusual evolution. We note that the direct-heating fits imply large heating powers and rather small inclinations, and we speculate that unmodeled effects can introduce such bias.

Detecting anions in space has relied on a strong collaboration between theoretical and laboratory analyses to measure rotational spectra and spectroscopic constants to high accuracy. The advent of improved quantum chemical tools operating at higher accuracy and reduced computational cost is a crucial solution for the fundamental characterization of astrophysically relevant anions and their detection in the interstellar medium (ISM) and planetary atmospheres. In this context, we have turned toward the quantum chemical analysis of the penta-atomic dicyanoamine anion NCNCN⁻ (${{\rm{C}}}_{2}{{{\rm{N}}}_{3}}^{-}$), a structurally bent and polar compound. We have performed high-level coupled cluster theory quartic force field computations of ${{\rm{C}}}_{2}{{{\rm{N}}}_{3}}^{-}$ satisfying both computational cost and accuracy conditions. We provide for the first time accurate spectroscopic constants and vibrational frequencies for this ion. In addition to exhibiting various Fermi resonances, ${{\rm{C}}}_{2}{{{\rm{N}}}_{3}}^{-}$ displays a bright ν₂ (2130.9 cm⁻¹) and a less intense ν₁ (2190.7 cm⁻¹) fundamental vibrational frequency, making for strong markers for future infrared observations <5 μm. We have also determined near-IR overtone and combination bands of the bright fundamentals for which the 2ν₂ at 4312.1 cm⁻¹ (2.319 μm) is the best candidate. ${{\rm{C}}}_{2}{{{\rm{N}}}_{3}}^{-}$ could potentially exist and be detected in nitrogen-rich environments of the ISM such as IRC +10216 and other carbon-rich circumstellar envelopes, or in the atmosphere of Saturn's moon Titan, where advanced N-based reactions may lead to its formation.