Table of contents

Only star clusters that are sufficiently compact and massive survive largely unharmed beyond 10 . However, their compactness means a high stellar density, which can lead to strong gravitational interactions between the stars. As young stars are often initially surrounded by protoplanetary disks and later on potentially by planetary systems, the question arises to what degree these strong gravitational interactions influence planet formation and the properties of planetary systems. Here, we perform simulations of the evolution of compact high-mass clusters like Trumpler 14 and Westerlund 2 from the embedded to the gas-free phase and study the influence of stellar interactions. We concentrate on the development of the mean disk size in these environments. Our simulations show that in high-mass open clusters 80%–90% of all disks/planetary systems should be smaller than 50 just as a result of the strong stellar interactions in these environments. Already in the initial phases, three to four close flybys lead to typical disk sizes within the range of 18–27 . Afterward, the disk sizes are altered only to a small extent. Our findings agree with the recent observation that the disk sizes in the once dense environment of the Upper Scorpio OB association, NGC 2362, and h/χPersei are at least three times smaller in size than, for example, in Taurus. We conclude that the observed planetary systems in high-mass open clusters should also be on average smaller than those found around field stars; in particular, planets on wide orbits are expected to be extremely rare in such environments.

A new type of gamma-ray spectrum is predicted in a general hadronic framework by taking into account gluon condensation (GC) effects in proton. The result presents a power law with a sharp break in the gamma-ray spectra at the TeV band. We suggest probing this GC signature in Earth-limb gamma-ray spectra using the Dark Matter Particle Explorer and the Calorimetric Electron Telescope in orbit.

Starspots and flares are indicators of stellar magnetic activity and can both be studied in greater detail by utilizing the long-term, space-based archive of the Kepler satellite. Here, we aim to investigate a subset of the Kepler archive to reveal a connection between the starspots and the stellar flares, in order to provide insight into the overall stellar magnetic field. We use the flare-finding algorithm FLATW'RM in conjunction with a new suite of algorithms that aim to locate the local minima caused by starspot groups. We compare the phase difference between the time of maximum flux of a flare and the time of minimum stellar flux due to a starspot group. The strongest flares do not appear to be correlated to the largest starspot group present, but are also not uniformly distributed in phase with respect to the starspot group. The weaker flares, however, do show an increased occurrence close to the starspot groups.

We present a detailed study of the large-scale anisotropies of cosmic rays with energies above 4 EeV measured using the Pierre Auger Observatory. For the energy bins [4, 8] EeV and E ≥ 8 EeV, the most significant signal is a dipolar modulation in R.A. at energies above 8 EeV, as previously reported. In this paper we further scrutinize the highest-energy bin by splitting it into three energy ranges. We find that the amplitude of the dipole increases with energy above 4 EeV. The growth can be fitted with a power law with index β = 0.79 ± 0.19. The directions of the dipoles are consistent with an extragalactic origin of these anisotropies at all the energies considered. Additionally, we have estimated the quadrupolar components of the anisotropy: they are not statistically significant. We discuss the results in the context of the predictions from different models for the distribution of ultrahigh-energy sources and cosmic magnetic fields.

There has been considerable interest in sausage modes in photospheric waveguides such as pores and sunspots, and slow surface sausage modes (SSSMs) have been suggested to damp sufficiently rapidly to account for chromospheric heating. Working in the framework of linear resistive magnetohydrodynamics, we examine how efficient electric resistivity and resonant absorption in the cusp continuum can be for damping SSSMs in a photospheric waveguide with equilibrium parameters compatible with recent measurements of a photospheric pore. For SSSMs with the measured wavelength, we find that the damping rate due to the cusp resonance is substantially less strong than theoretically expected with the thin-boundary approximation. The damping-time-to-period ratio (τ/P) we derive for standing modes, equivalent to the damping-length-to-wavelength ratio for propagating modes given the extremely weak dispersion, can reach only ∼180. However, the accepted values for electric resistivity (η) correspond to a regime where both the cusp resonance and resistivity play a role. The values for τ/P attained at the largest allowed η may reach ∼30. We conclude that electric resistivity can be considerably more efficient than the cusp resonance for damping SSSMs in the pore in question, and needs to be incorporated into future studies on the damping of SSSMs in photospheric waveguides in general.

In this work we explore the possibility of using recurrence quantification analysis (RQA) in astronomical high-contrast imaging to statistically discriminate the signal of faint objects from speckle noise. To this end, we tested RQA on a sequence of high frame rate (1 kHz) images acquired with the SHARK-VIS forerunner at the Large Binocular Telescope. Our tests show promising results in terms of detection contrasts at angular separations as small as 50 mas, especially when RQA is applied to a very short sequence of data (2 s). These results are discussed in light of possible science applications and with respect to other techniques such as, for example, angular differential imaging and speckle-free imaging.

We present an analysis of gas densities in the central R = 300 pc of the Milky Way, focusing on three clouds: GCM –0.02–0.07 (the 50 km s⁻¹ cloud), GCM –0.13–0.08 (the 20 km s⁻¹ cloud), and GCM 0.25+0.01 (the "Brick"). Densities are determined using observations of the J = (3–2), (4–3), (5–4), (10–9), (18–17), (19–18), (21–20), and (24–23) transitions of the molecule HC₃N. We find evidence of at least two excitation regimes for HC₃N and constrain the low-excitation component to have a density less than 10⁴ cm⁻³ and the high-excitation component to have a density between 10⁵ and 10⁶ cm⁻³. This is much less than densities of 10⁷ cm⁻³ that are found in Sgr B2, the most actively star-forming cloud in the Galactic center. This is consistent with the requirement of a higher-density threshold for star formation in the Galactic center than is typical in the Galactic disk. We are also able to constrain the column density of each component in order to determine the mass fraction of "dense" (n > 10⁵ cm⁻³) gas for these clouds. We find that this is ∼15% for all three clouds. Applying the results of our models to ratios of the (10–9) and (3–2) line across the entire central R = 300 pc, we find that the fraction of dense (n > 10⁴ cm⁻³) gas increases inward of a radius of ∼140 pc, consistent with the predictions of recent models for the gas dynamics in this region. Our observations show that HC₃N is an excellent molecule for probing the density structure of clouds in the Galactic center.

The practical application of the harmonic summing technique in the power-spectrum analysis for searching pulsars has exhibited the technique's effectiveness. In this paper, theoretical verification of harmonic summing considering power's noise-signal probability distribution is given. With the top-hat and the modified von Mises pulse profile models, contours along which spectra total power is expected to exceed the 3σ detection threshold with 0.999 confidence corresponding to m = 1, 2, 4, 8, 16, or 32 harmonics summed are given with respect to the mean pulse amplitude and the pulse duty cycle. Optimized numbers of harmonics summed relative to the duty cycles are given. The routine presented builds a theoretical estimate of the minimum detectable mean flux density, i.e., sensitivity, under the power-spectrum searching method.

Recent high-sensitivity observations carried out with the Atacama Large Millimeter Array have revealed the presence of complex organic molecules (COMs) such as methyl cyanide (CH₃CN) and methanol (CH₃OH) in relatively evolved protoplanetary discs. The behavior and abundance of COMs in earlier phases of disk evolution remain unclear. Here, we combine a smoothed particle hydrodynamics simulation of a fragmenting, gravitationally unstable disk with a gas-grain chemical code. We use this to investigate the evolution of formamide (NH₂CHO), a prebiotic species, in both the disk and in the fragments that form within it. Our results show that formamide remains frozen onto grains in the majority of the disks where the temperatures are <100 K, with a predicted solid-phase abundance that matches those observed in comets. Formamide is present in the gas phase in three fragments as a result of the high temperatures (≥200 K), but remains in the solid phase in one colder (≤150 K) fragment. The timescale over which this occurs is comparable to the dust sedimentation timescales, suggesting that any rocky core that is formed would inherit their formamide content directly from the protosolar nebula.

We have carried out an extensive X-ray spectral analysis of a sample of galaxies exhibiting molecular outflows (MOX sample) to characterize the X-ray properties and investigate the effect of active galactic nuclei (AGNs) on the dynamical properties of the molecular outflows (MOs). We find that the X-ray bolometric correction (L_{2–10 keV}/L_AGN) of these sources ranges from ∼10^−4.5 to 10^−0.5, with ∼70% of the sources below 10⁻², implying a weak X-ray emission relative to the AGN bolometric luminosity (L_AGN). However, the upper limit on the 2–10 keV luminosity (${L}_{2-10\mathrm{keV},12\mu {\rm{m}}}$) obtained from 12 μm flux, following the correlation derived by Asmus et al., is ∼0.5–3 orders of magnitude larger than the L_{2–10 keV} values estimated using X-ray spectroscopy, implying a possibility that the MOX sources host normal AGNs (not X-ray weak), and their X-ray spectra are extremely obscured. We find that both L_{2–10 keV} and L_AGN correlate strongly with the MO velocity and the mass outflow rates (${\dot{M}}_{\mathrm{out}}$), implying that the central AGN plays an important role in driving these massive outflows. However, we also find statistically significant positive correlations between the starburst emission and MO mass outflow rate, ${L}_{\mathrm{Starburst}}$ versus ${\dot{M}}_{\mathrm{out}}$, and L_0.6–2keV versus ${\dot{M}}_{\mathrm{out}}$, which implies that starbursts can generate and drive the MOs. The correlations of MO velocity and ${\dot{M}}_{\mathrm{out}}$ with AGN luminosities are found to be stronger compared to those with the starburst luminosities. We conclude that both starbursts and AGNs play a crucial role in driving the large-scale MO.

Interesting among possible mechanisms responsible for X-ray emission from active galactic nuclei (AGNs) is ionized relativistic reflection (IRR). Since it arises close to the central black hole it can test strong gravity. Its characteristic features include a high energy Compton hump, a broad Fe Kα line, and a soft excess. Therefore, using the Swift-Burst Alert Telescope (BAT) catalog we looked for hard X-ray selected AGNs that may exhibit at least one of these characteristic features. Among the possibly interesting targets found is Seyfert I Galaxy LEDA 168563. We obtained a broadband 100 ks Suzaku observation of this source, and careful analysis of the data was carried out. The results support the presence of these IRR distinct features. Moreover, the comparison of the earlier combined XMM-Newton and Swift-BAT and more recent Suzaku data showed interesting long-term spectral variability. The soft excess decreased while higher energy features relatively increased and the power-law component became flatter—the behavior predicted by the IRR model.

We compare 1D nonlocal turbulent convection models with 3D hydrodynamic numerical simulations. We study the validity of closure models and turbulent coefficients by varying the Prandtl number, the Péclet number, and the depth of the convection zone. Four closure models of the fourth-order moments are evaluated with the 3D simulation data. The performance of the closure models varies among different cases and different fourth-order moments. We solve the dynamic equations of moments together with equations of the thermal structure. Unfortunately, we cannot obtain steady-state solutions when these closure models of fourth-order moments are adopted. In contrast, the numerical solutions of the down-gradient approximations of the third-order moments are robust. We calibrate the coefficients of the 1D down-gradient model from the 3D simulation data. The calibrated coefficients are more robust in cases of deep convection zones. Finally, we have compared the 1D steady-state solutions with the 3D simulation results. The 1D model has captured many features that appear in the 3D simulations: (1) ∇ − ∇_a has a U-shape with a minimum value at the lower part of the convection zone; (2) there exists a bump for ∇ − ∇_a near the top of the convection zone when the Péclet number is large; and (3) the temperature gradient can be sub-adiabatic due to the nonlocal effect. However, aside from these similarities, the prediction on the kinetic energy flux is unsatisfactory.

Polarized models of relativistically hot astrophysical plasmas require transport coefficients as input: synchrotron absorption and emission coefficients in each of the four Stokes parameters, as well as three Faraday rotation coefficients. Approximations are known for all coefficients for a small set of electron distribution functions, such as the Maxwell–Jüttner relativistic thermal distribution, and a general procedure has been obtained by Huang & Shcherbakov for an isotropic distribution function. Here we provide an alternative general procedure, with a full derivation, for calculating absorption and rotation coefficients for an arbitrary isotropic distribution function. Our method involves the computation of the full plasma susceptibility tensor, which in addition to absorption and rotation coefficients may be used to determine plasma modes and the dispersion relation. We implement the scheme in a publicly available library (https://github.com/afd-illinois/symphony) with a simple interface, thus allowing for easy incorporation into radiation transport codes. We also provide a comprehensive survey of the literature and comparison with earlier results.

We have used narrowband [O iii] λλ4959, 5007 and Hα+[N ii] λλ6548, 84 Hubble Space Telescope (HST) images of nine luminous (L[O iii] > 10⁴² erg s⁻¹) type 2 QSOs with redshifts 0.1 < z < 0.5 in order to constrain the geometry of their extended narrow-line regions (ENLRs), as recent ground-based studies suggest that these regions become more spherical at high luminosities due to destruction of the torus. We instead find elongated ENLRs reaching 4–19 kpc from the nucleus and bipolar ionization cones in [O iii]/(Hα+[N ii]) excitation maps indicating that the torus survives these luminosities, allowing the escape of ≈10 times higher ionizing photon rates along the ionization axis than perpendicular to it. The exceptional HST angular resolution was key to our success in arriving at these conclusions. Combining our measurements with previous ones based on similar HST data, we have revisited the relation between the ENLR radius R_maj and L[O iii] over the range 39 < log(L[O iii]) < 43.5 (L in erg s⁻¹): log(R_maj) = (0.51 ± 0.03) log(L[O iii])−18.12 ± 0.98. The radius of the ENLR keeps increasing with L[O iii] in our data, implying that the ENLR can extend to distances beyond the limit of the galaxy if gas is present there—e.g., from active galactic nucleus (AGN) outflows or interactions, seen in six objects of our sample. We attribute the flattening previously seen in this relation to the fact that the ENLR is matter-bounded, meaning that ionizing photons usually escape to the intergalactic medium in luminous AGNs. Estimated ionized gas masses of the ENLRs range from 0.3 to 2 × 10⁸M_⊙, and estimated powers for associated outflows range from <0.1% to a few percent of the QSO luminosity.

We use optical and near-infrared spectroscopy to observe rest-UV emission lines and estimate the black hole mass of WISEA J224607.56−052634.9 (W2246−0526) at z = 4.601, the most luminous hot, dust-obscured galaxy yet discovered by WISE. From the broad component of the Mg ii 2799 Å emission line, we measure a black hole mass of log(M_BH/M_☉) = 9.6 ± 0.4. The broad C iv 1549 Å line is asymmetric and significantly blueshifted. The derived M_BH from the blueshift-corrected broad C iv line width agrees with the Mg ii result. From direct measurement using a well-sampled SED, the bolometric luminosity is 3.6 × 10¹⁴L_☉. The corresponding Eddington ratio for W2246−0526 is λ_Edd = L_AGN/L_Edd = 2.8. This high Eddington ratio may reach the level where the luminosity is saturating due to photon trapping in the accretion flow and may be insensitive to the mass accretion rate. In this case, the M_BH growth rate in W2246−0526 would exceed the apparent accretion rate derived from the observed luminosity.

We present spatially resolved emission diagnostics for eight z ∼ 0.9 galaxies that demonstrate extended low-ionization emission line regions over kpc scales. Eight candidates are selected based on their spatial extent and emission line fluxes from slitless spectroscopic observations with the Hubble Space Telescope/Wide Field Camera 3 G141 and G800L grisms in the well-studied Great Observatories Origins Deep Survey (GOODS) fields. Five of the candidates (62.5%) are matched to X-ray counterparts in the Chandra X-ray Observatory Deep Fields. We modify the traditional Baldwin–Philips–Terlevich (BPT) emission line diagnostic diagram to use [S ii]/(Hα + [N ii]) instead of [N ii]/Hα to overcome the blending of [N ii] and Hα + [N ii] in the low-resolution slitless grism spectra. We construct emission line ratio maps and place the individual pixels in the modified BPT. The extended low-ionization nuclear emission line regions (LINER)-like emission present in all of our candidates, coupled with X-ray properties consistent with star-forming galaxies and weak [O iii]λ5007 Å detections, is inconsistent with purely nuclear sources (LINERs) driven by active galactic nuclei (AGNs). While recent ground-based integral field unit spectroscopic surveys have revealed significant evidence for diffuse LINER-like emission in galaxies within the local universe (z ∼ 0.04), this work provides the first evidence for the non-AGN origin of LINER-like emission out to high redshifts.

If primordial black holes (PBH) with masses of ${10}^{25}\,{\rm{g}}\gtrsim m\gtrsim {10}^{17}\,{\rm{g}}$ constitute a non-negligible fraction of galactic dark-matter halos, their existence should have observable consequences: they necessarily collide with galactic neutron stars (NS), nest in their centers, and accrete the dense matter, eventually converting them to NS-mass black holes while releasing the NS magnetic field energy. Such processes may explain the fast radio bursts (FRB) phenomenology, in particular their millisecond durations, large luminosities ∼10⁴³ erg s⁻¹, high rate of occurrence $\gtrsim 1000\,{\mathrm{day}}^{-1}$, as well as high brightness temperatures, polarized emission, and Faraday rotation. Longer than the dynamical timescale of the Bondi-like accretion for light PBH allows for the repeating of FRB. This explanation follows naturally from the (assumed) existence of the dark-matter PBH and requires no additional unusual phenomena, in particular no unacceptably large magnetic fields of NS. In our model, the observed rate of FRB throughout the universe follows from the presently known number of NS in the Galaxy.

We investigate the 3D structure of kinematic oscillations of full halo coronal mass ejections (FHCMEs) using multi-spacecraft coronagraph data from two non-parallel lines of sight. For this, we consider 21 FHCMEs which are simultaneously observed by the Solar and Heliospheric Observatory and the Solar TErrestrial RElations Observatory A or B, from 2010 June to 2012 August when the spacecraft were roughly in quadrature. Using sequences of running difference images, we estimate the instantaneous projected speeds of the FHCMEs at 24 different azimuthal angles in the planes of the sky of those coronagraphs. We find that all these FHCMEs have experienced kinematic oscillations characterized by quasi-periodic variations of the instantaneous projected radial velocity with periods ranging from 24 to 48 min. The oscillations detected in the analyzed events are found to show distinct azimuthal wave modes. Thirteen events (about 62%) are found to oscillate with the azimuthal wave number m = 1. The oscillating directions of the nodes of the m = 1 mode for these FHCMEs are consistent with those of their position angles (or the direction of eruption), with a mean difference of about 23°. The oscillation amplitude is found to correlate well with the projected radial speed of the CME. An estimation of Lorentz accelerations shows that they are dominant over other forces, implying that the magnetic force is responsible for the kinematic oscillations of CMEs. However, we cannot rule out other possibilities: a global layer of enhanced current around the CMEs or the nonlinear nature of its driver, for example the effect of vortex shedding.

We investigate emission signatures of binary compact star gravitational wave (GW) sources consisting of strongly magnetized neutron stars (NSs) and/or white dwarfs (WDs) in their late-time inspiral phase. Because of electromagnetic interactions between the magnetospheres of the two compact stars, a substantial amount of energy will be extracted, and the resultant power is expected to be ∼10³⁸–10⁴⁴ erg s⁻¹ in the last few seconds before the two stars merge, when the binary system contains a NS with a surface magnetic field 10¹² G. The induced electric field in the process can accelerate charged particles up to the EeV energy range. Synchrotron radiation is emitted from energetic electrons, with radiative energies reaching the GeV energy for binary NSs and the MeV energy for NS–WD or double WD binaries. In addition, a blackbody component is also presented, and it peaks at several to hundreds keV for binary NSs and at several keV for NS–WD or double WD binaries. The strong angular dependence of the synchrotron radiation and the isotropic nature of the blackbody radiation lead to distinguishable modulation patterns between the two emission components. If coherent curvature radiation is presented, fast radio bursts could be produced. These components provide unique simultaneous electromagnetic signatures as precursors of GW events associated with magnetized compact star mergers and short gamma-ray bursts (e.g., GRB 100717).

The recent local measurement of the Hubble constant leads to a more than 3σ tension with Planck + ΛCDM. In this article we study the H₀ tension in non-flat QCDM cosmology, where Q stands for a minimally coupled and slowly or moderately rolling quintessence field ϕ with a smooth potential $V(\phi )$, and CDM refers to cold dark matter. By generalizing the QCDM one-parameter and three-parameter parameterizations in Huang et al. to a non-flat universe and using the latest cosmological data, we find that the H₀ tension remains above the 3.2σ level for this class of model.

Despite their cosmological utility, the progenitors of Type Ia supernovae (SNe Ia) are still unknown, with many efforts focused on whether accretion from a nondegenerate companion can grow a carbon–oxygen white dwarf to near the Chandrasekhar mass. The association of SNe Ia resembling SN 1991T ("91T-like") with circumstellar interaction may be evidence for this "single-degenerate" channel. However, the observed circumstellar medium (CSM) in these interacting systems is unlike a stellar wind—of particular interest, it is sometimes detached from the stellar surface, residing at ∼10¹⁶ cm. A Hubble Space Telescope (HST) program to discover detached CSM around 91T-like SNe Ia successfully discovered interaction nearly two years after explosion in SN 2015cp (Graham et al. 2018). In this work, we present radio and X-ray follow-up observations of SN 2015cp and analyze them in the framework of Harris et al. (2016) to limit the properties of a constant-density CSM shell in this system. Assuming the HST detection took place shortly after the shock crossed the CSM, we constrain the total CSM mass in this system to be <0.5 ${M}_{\odot }$. This limit is comparable to the CSM mass of supernova PTF11kx, but does not rule out lower masses predicted for recurrent novae. From lessons learned modeling PTF11kx and SN 2015cp, we suggest a strategy for future observations of these events to increase the sample of known interacting SNe Ia.

The effect of misalignment between the magnetic field ${\boldsymbol{B}}$ and the angular momentum ${{\boldsymbol{J}}}_{\mathrm{ang}}$ of molecular cloud cores on the angular momentum evolution during the gravitational collapse is investigated by ideal and non-ideal MHD simulations. For the non-ideal effect, we consider the ohmic and ambipolar diffusion. Previous studies that considered the misalignment reported qualitatively contradicting results. Magnetic braking was reported as being either strengthened or weakened by misalignment in different studies. We conducted simulations of cloud core collapse by varying the stability parameter α (the ratio of the thermal to gravitational energy of the core) with and without including magnetic diffusion. The non-ideal MHD simulations show the central angular momentum of the core, with θ = 0° (${{\boldsymbol{J}}}_{\mathrm{ang}}\parallel {\boldsymbol{B}}$) being always greater than that with θ = 90° (${{\boldsymbol{J}}}_{\mathrm{ang}}\perp {\boldsymbol{B}}$), independently of α, meaning that circumstellar disks form more easily in a core with θ = 0°. The ideal MHD simulations, in contrast, show the central angular momentum of the core with θ = 90° being greater than with θ = 0° for small α and smaller for large α. Inspection of the angular momentum evolution of the fluid elements reveals three mechanisms contributing to the evolution of the angular momentum: (i) magnetic braking in the isothermal collapse phase, (ii) selective accretion of the rapidly (for θ = 90°) or slowly (for θ = 0°) rotating fluid elements to the central region, and (iii) magnetic braking in the first core and the disk. The difference between the ideal and non-ideal simulations arises from the different efficiencies of (iii).

Here we revisit the derivation of the instability of dense shocked layers, originally developed by Vishniac and Ryu. Our motivation is that density profiles found in actual astrophysical and laboratory systems often do not match the assumptions in that paper. In order to identify the anticipated theoretical growth rates for various circumstances, one must first revisit the derivation and allow for the possibility that the density scale length differs, in magnitude and/or in sign, from the isothermal scale height. This analysis leads us to find regimes of purely convective instability and also of Vishniac stabilization of this instability, in addition to some new regimes of Vishniac behavior. We also identify a typographical error in the original paper that matters for quantitative evaluation of growth rates.

We have derived a new thermonuclear rate with an associated uncertainty for the ¹⁰B(α,p)¹³C reaction by evaluating the available experimental data for the first time. We provide this rate with a much smaller uncertainty than that estimated in the literature. Our rate differs significantly from the theoretical rates adopted in the current reaction rate libraries. Utilizing this new rate, we have investigated its astrophysical implications on the heavy-element (especially, p-nuclei) production in the νp-process in a stellar model of the neutrino-driven wind of type II core-collapse supernova. It shows that our rate with a much smaller uncertainty strongly constrains the nucleosynthetic results of the light p-nulcei with A ∼ 80–100. In addition, it shows that the difference between observed and predicted abundances for light p-nuclei is quite large, implying either that the present stellar model still needs modification or that additional astrophysical sources are required to account for the origin of some p-nuclei, such as ⁹²Mo and ⁹⁴Mo.

We investigate the nature of nearby (10–15 kpc) high-speed stars in the Gaia DR2 archive identified on the basis of parallax, proper motion and radial velocity. Together with a consideration of their kinematic, orbital, and photometric properties, we develop a novel strategy for evaluating whether high-speed stars are statistical outliers of the bound population or unbound stars capable of escaping the Galaxy. Out of roughly 1.5 million stars with radial velocities, proper motions, and 5σ parallaxes, we identify just over 100 high-speed stars. Of these, only two have a nearly 100% chance of being unbound, with an indication that they are not just bound outliers; both are likely hyper-runaway stars. The rest of the high-speed stars are likely statistical outliers. We use the sample of high-speed stars to demonstrate that radial velocity alone provides a poor discriminant of nearby, unbound stars. However, these stars are efficiently identified from the tangential velocity, using just parallax and proper motion. Within the full Gaia DR2 archive of stars with 5σ parallax and proper motion but no radial velocity, we identify a sample of 19 with speeds significantly larger than the local escape speed of the Milky Way based on tangential motion alone.

The Epoch of Reionization (EoR) is an uncharted era in our universe's history during which the birth of the first stars and galaxies led to the ionization of neutral hydrogen in the intergalactic medium. There are many experiments investigating the EoR by tracing the 21 cm line of neutral hydrogen. Because this signal is very faint and difficult to isolate, it is crucial to develop analysis techniques that maximize sensitivity and suppress contaminants in data. It is also imperative to understand the trade-offs between different analysis methods and their effects on power spectrum estimates. Specifically, with a statistical power spectrum detection in HERA's foreseeable future, it has become increasingly important to understand how certain analysis choices can lead to the loss of the EoR signal. In this paper, we focus on signal loss associated with power spectrum estimation. We describe the origin of this loss using both toy models and data taken by the 64-element configuration of the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER). In particular, we highlight how detailed investigations of signal loss have led to a revised, higher 21 cm power spectrum upper limit from PAPER-64. Additionally, we summarize errors associated with power spectrum error estimation that were previously unaccounted for. We focus on a subset of PAPER-64 data in this paper; revised power spectrum limits from the PAPER experiment are presented in a forthcoming paper by Kolopanis et al. and supersede results from previously published PAPER analyses.

The streaming instability is a promising mechanism to drive the formation of planetesimals in protoplanetary disks. To trigger this process, it has been argued that sedimentation of solids onto the mid-plane needs to be efficient, and therefore that a quiescent gaseous environment is required. It is often suggested that dead-zone or disk-wind structure created by non-ideal magnetohydrodynamical (MHD) effects meets this requirement. However, simulations have shown that the mid-plane of a dead zone is not completely quiescent. In order to examine the concentration of solids in such an environment, we use the local-shearing-box approximation to simulate a particle-gas system with an Ohmic dead zone including mutual drag force between the gas and the solids. We systematically compare the evolution of the system with ideal or non-ideal MHD, with or without backreaction drag force from particles on gas, and with varying solid abundances. Similar to previous investigations of dead-zone dynamics, we find that particles of dimensionless stopping time ${\tau }_{s}=0.1$ do not sediment appreciably more than those in ideal magnetorotational turbulence, resulting in a vertical scale height an order of magnitude larger than in a laminar disk. Contrary to the expectation that this should curb the formation of planetesimals, we nevertheless find that strong clumping of solids still occurs in the dead zone when solid abundances are similar to the critical value for a laminar environment. This can be explained by the weak radial diffusion of particles near the mid-plane. The results imply that the sedimentation of particles to the mid-plane is not a necessary criterion for the formation of planetesimals by the streaming instability.

Magnetohydrodynamic (MHD) and photoevaporative winds are thought to play an important role in the evolution and dispersal of planet-forming disks. We report the first high-resolution (Δv ∼ 6 km s⁻¹) analysis of [S ii] λ4068, [O i] λ5577, and [O i] λ6300 lines from a sample of 48 T Tauri stars. Following Simon et al. we decompose them into three kinematic components: a high-velocity component (HVC) associated with jets, and low-velocity narrow (LVC-NC) and broad (LVC-BC) components. We confirm previous findings that many LVCs are blueshifted by more than 1.5 km s⁻¹ and thus most likely trace a slow disk wind. We further show that the profiles of individual components are similar in the three lines. We find that most LVC-NC and LVC-BC line ratios are explained by thermally excited gas with temperatures between 5000 and 10,000 K and electron densities of ∼10⁷–10⁸ cm⁻³. The HVC ratios are better reproduced by shock models with a pre-shock H number density of ∼10⁶–10⁷ cm⁻³. Using these physical properties, we estimate ${\dot{M}}_{\mathrm{wind}}/{\dot{M}}_{\mathrm{acc}}$ for the LVC and ${\dot{M}}_{\mathrm{jet}}/{\dot{M}}_{\mathrm{acc}}$ for the HVC. In agreement with previous work, the mass carried out in jets is modest compared to the accretion rate. With the likely assumption that the LVC-NC wind height is larger than the LVC-BC, the LVC-BC ${\dot{M}}_{\mathrm{wind}}/{\dot{M}}_{\mathrm{acc}}$ is found to be higher than the LVC-NC. These results suggest that most of the mass loss occurs close to the central star, within a few au, through an MHD-driven wind. Depending on the wind height, MHD winds might play a major role in the evolution of the disk mass.

The detection of gravitational waves (GWs) provides a direct way to measure the luminosity distance, which enables us to probe cosmology. In this paper, we continue to expand the application of GW standard sirens in cosmology, and propose that the spatial curvature can be estimated in a model-independent way by comparing the distances from future GW sources and current cosmic-chronometer observations. We expect an electromagnetic counterpart of the GW event to give the source redshift, and simulate hundreds of GW data from the coalescence of double neutron stars and black hole–neutron star binaries using the Einstein Telescope as a reference. Our simulations show that, from 100 simulated GW events and 31 current cosmic-chronometer measurements, the error of the curvature parameter Ω_K is expected to be constrained at the level of ∼0.125. If 1000 GW events were observed, the uncertainty of Ω_K would be further reduced to ∼0.040. We also find that adding 50 mock H(z) data points (consisting of 81 cosmic-chronometer data points and 1000 simulated GW events) could result in a much tighter constraint on the zero cosmic curvature, for which Ω_K = −0.002 ± 0.028. Compared to some actual model-independent curvature tests involving distances from other cosmic probes, this method using GW data achieves constraints with much higher precision.

We present the analysis of a peculiar W Virginis (pWVir) type II Cepheid, OGLE-LMC-T2CEP-211 (P_puls = 9.393 days), in a double-lined binary system (P_orb = 242 days), which shed light on virtually unknown evolutionary status and structure of pWVir stars. The dynamical mass of the Cepheid (first ever for a type II Cepheid) is 0.64 ± 0.02 M_⊙, and the radius R = 25.1 ± 0.3 R_⊙. The companion is a massive (5.67 M_⊙) main-sequence star obscured by a disk. Such a configuration suggests a mass transfer in the system history. We found that originally the system (${P}_{\mathrm{orb}}^{\mathrm{init}}$ = 12 days) was composed of 3.5 and 2.8 M_⊙ stars, with the current Cepheid being more massive. The system age is now ∼200 Myr, and the Cepheid is almost completely stripped of hydrogen, with helium mass of ∼92% of the total mass. It finished transferring the mass 2.5 Myr ago and is evolving toward lower temperatures passing through the instability strip. Comparison with observations indicates a reasonable 2.7 × 10⁻⁸M_⊙ yr⁻¹ mass loss from the Cepheid. The companion is most probably a Be main-sequence star with T = 22,000 K and R = 2.5 R_⊙. Our results yield a good agreement with a pulsation theory model for a hydrogen-deficient pulsator, confirming the described evolutionary scenario. We detected a two-ring disk (R_disk ∼ 116 R_⊙) and a shell (R_shell ∼ 9 R_⊙) around the companion, which is probably a combination of the matter from the past mass transfer, the mass being lost by the Cepheid owing to wind and pulsations, and a decretion disk around a rapidly rotating secondary. Our study, together with observational properties of pWVir stars, suggests that the majority of them are products of a similar binary evolution interaction.

The extremely high brightness temperatures of pulsars and fast radio bursts (FRBs) require their radiation mechanisms to be coherent. Coherent curvature radiation from bunches has been long discussed as the mechanism for radio pulsars and recently for FRBs. Assuming that bunches are already generated in pulsar magnetospheres, we calculate the spectrum of coherent curvature radiation under a three-dimensional magnetic field geometry. Different from previous works assuming parallel trajectories and a monoenergetic energy distribution of electrons, we consider a bunch characterized by its length, curvature radius of the trajectory family, bunch opening angle, and electron energy distribution. We find that the curvature radiation spectra of the bunches are characterized by a multisegment broken power law, with the break frequencies depending on bunch properties and trajectory configuration. We also emphasize that in a pulsar magnetosphere, only the fluctuation of net charges with respect to the background (Goldreich–Julian) outflow can make a contribution to coherent radiation. We apply this model to constrain the observed spectra of pulsars and FRBs. For a typical pulsar (${B}_{p}={10}^{12}\,{\rm{G}}$, P = 0.1 s), a small fluctuation of the net charge δn_GJ ∼ 0.1n_GJ can provide the observable flux. For FRBs, the fluctuating net charge may be larger due to its abrupt nature. For δn_GJ ∼ n_GJ, a neutron star with a strong magnetic field and fast rotation is required to power an FRB in the spindown-powered model. The requirement is less stringent in the cosmic comb model thanks to the larger cross section and compressed charge density of the bunch made by the external astrophysical stream that combs the magnetosphere.

Scorpius–Centaurus is the nearest OB association, and its hundreds of members are divided into subgroups, including the Lower Centaurus Crux (LCC). Here we study the dynamics of the LCC area. We report the revelation of a large moving group containing more than 1800 intermediate- and low-mass young stellar objects and brown dwarfs that escaped identification until Gaia DR2 allowed a kinematic and photometric selection to be performed. We investigate the stellar and substellar content of this moving group using the Gaia DR2 astrometric and photometric measurements. The median distance of the members is 114.5 pc, and 80% lie between 102 and 135 pc from the Sun. Our new members cover a mass range of 0.02–5 M_⊙ and add up to a total mass of about 700 M_⊙. The present-day mass function follows a log-normal law with m_c = 0.22 M_⊙ and σ = 0.64. We find more than 200 brown dwarfs in our sample. The star formation rate had its maximum of $8\times {10}^{-5}\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$ about 9 Myr ago. We grouped the new members into four denser subgroups, which have increasing age from 7 to 10 Myr, surrounded by "free-floating" young stars with mixed ages. Our isochronal ages, now based on accurate parallaxes, are compatible with several earlier studies of the region. The whole complex is presently expanding, and the expansion started between 8 and 10 Myr ago. Two hundred members show infrared excess compatible with circumstellar disks from full to debris disks. This discovery provides a large sample of nearby young stellar and substellar objects for disk and exoplanet studies.

Observations of low-frequency gravitational waves (GWs) will require the highest possible timing precision from an array of the most spin-stable pulsars. We can improve the sensitivity of a pulsar timing array (PTA) to different GW sources by observing pulsars with low timing noise over years to decades and distributed across the sky. We discuss observing strategies for a PTA focused on a stochastic GW background such as from unresolved supermassive black hole binaries as well as focused on single continuous-wave sources. First, we describe the method to calculate a PTA's sensitivity to different GW-source classes. We then apply our method to the 45 pulsars presented in the North American Nanohertz Observatory for the GW 11 year data set. For expected amplitudes of the stochastic background, we find that all pulsars contribute significantly over the timescale of decades; the exception is for very pessimistic values of the stochastic-background amplitude. For individual single sources, we find that a number of pulsars contribute to the sensitivity of a given source, but that which pulsars contribute is different depending on the source, or versus an all-sky metric. Our results seem robust to the presence of red noise in pulsar arrival times. It is critical to obtain more robust pulsar-noise parameters as they heavily affect our results. Our results show that it is also imperative to locate and time as many high-precision pulsars as possible, as quickly as possible, to maximize the sensitivity of next-generation PTA detectors.

We analyzed seven presolar SiC grains of supernova origin (average diameter: 1–2 μm) with transmission electron microscopy. Five grains are polycrystalline, whereas two grains are single crystals. Individual crystal domains of polycrystalline grains are in epitaxial relationship, with two grains consisting almost entirely of twinned crystal domains. Most grains are free of inclusions (only one TiC inclusion and one iron- and nickel-rich inclusion were found in two separate grains). Almost all crystals have cubic symmetry (3C polytype), but we found hexagonal SiC (6H polytype) in two grains. The large range of crystal domain sizes (average diameter: 50–970 nm), as well as the larger fraction of noncubic SiC polytypes in supernova grains relative to SiC grains that crystallized in the winds of asymptotic giant branch (AGB) stars, suggest that SiC condensation in supernova ejecta occurs at a larger range of chemical and physical conditions, including supersaturation, than in the winds of AGB stars. Modeling condensation of SiC struggles to produce SiC grains as large as, or bigger than, observed here, if condensation of large (i.e., several μm in diameter) graphite grains is to precede that of SiC, which is suggested by the presolar grain record and published equilibrium condensation models. We propose that future models of graphite and SiC condensation in SN ejecta explore higher ejecta densities than before, as well as gas compositions that are more silicon- and carbon-rich. Furthermore, we infer that some supernova SiC grains may have formed without prior condensation of graphite from their parent gas.

Space environment forecasts are based on ab initio modeling of the solar wind (SW) wherein solar magnetic fields and plasmas are propagated from an initial/boundary model source surface in the lower solar corona out to 1 au. Testing of these space environment forecasts relative to in situ measurements at 1 au typically shows uncertainties in the arrival times of fast SW streams of the order of a day, with broad distributions, means/medians of the order of half a day, and large variations between models but no definite winner. Here the effect of flow-line (FL) wandering due to the higher frequency velocity fluctuations within the turbulent SW on the arrival-time statistics of parcels of SW plasma transiting through the inner heliosphere out to 1 au is evaluated for a range of cutoff timescales in the velocity fluctuations. Used for this evaluation are in situ SW velocity measurements onboard Wind at 1 au, detailed spectral analysis of these measurements, WKB extrapolations to the inner heliosphere and simple application of a newly extended theoretical calculation of the mean SWFL cross-flow and "flow-aligned" displacements from the measured spectra. It is found that the velocity fluctuations near 1 au have little effect on the arrival times. The effect of the velocity fluctuations increases sunward, however, to a level sufficient to explain the large and broadly distributed "uncertainties" found in the testing of the forecasts.

As a fundamental astrophysical process, the scattering of particles by turbulent magnetic fields has its physical foundation laid by the magnetohydrodynamic (MHD) turbulence theory. In the framework of the modern theory of MHD turbulence, we derive a generalized broadened resonance function by taking into account both the magnetic fluctuations and nonlinear decorrelation of turbulent magnetic fields arising in MHD turbulence, and we specify the energy range of particles for the dominance of different broadening mechanisms. The broadened resonance allows for scattering of particles beyond the energy threshold of the linear resonance. By analytically determining the pitch-angle diffusion coefficients for transit time damping (TTD) with slow and fast modes, we demonstrate that the turbulence anisotropy of slow modes suppresses their scattering efficiency. Furthermore, we quantify the dependence of the relative importance between slow and fast modes in TTD scattering on (i) particle energy, (ii) plasma β (the ratio of gas pressure to magnetic pressure), and (iii) damping of MHD turbulence, and we also provide the parameter space for the dominance of slow modes. To exemplify its applications, we find that among typical partially ionized interstellar phases, in the warm neutral medium slow and fast modes have comparable efficiencies in TTD scattering of cosmic rays. For low-energy particles, e.g., sub-Alfvénic charged grains, we show that slow modes always dominate TTD scattering.

We analyze the stellar age indicators (D_n4000 and EW(Hδ)) and sizes of 467 quiescent galaxies with M_* ≥ 10¹⁰M_⊙ at z ∼ 0.7 drawn from DR2 of the LEGA-C survey. Interpreting index variations in terms of equivalent single stellar population age, we find that the median stellar population is younger for larger galaxies at fixed stellar mass. The effect is significant, yet small; the ages of the larger and smaller subsets differ by only <500 Myr, much less than the age variation among individual galaxies (∼1.5 Gyr). At the same time, post-starburst galaxies—those that experienced recent and rapid quenching events—are much smaller than expected based on the global correlation between age and size of normal quiescent galaxies. These coexisting trends unify seemingly contradictory results in the literature; the complex correlations between size and age indicators revealed by our large sample of galaxies with high-quality spectra suggest that there are multiple evolutionary pathways to quiescence. Regardless of the specific physical mechanisms responsible for the cessation of star formation in massive galaxies, the large scatter in D_n4000 and EW(Hδ) immediately implies that galaxies follow a large variety of evolutionary pathways. On the one hand, we see evidence for a process that slowly shuts off star formation and transforms star-forming galaxies to quiescent galaxies without necessarily changing their structures. On the other hand, there is likely a mechanism that rapidly quenches galaxies, an event that coincides with dramatic structural changes, producing post-starburst galaxies that can be smaller than their progenitors.

We present a method that enables wide-field ground-based telescopes to scan the sky for subsecond stellar variability. The method has operational and image processing components. The operational component takes star trail images. Each trail serves as a light curve for its corresponding source and facilitates subexposure photometry. We train a deep neural network to identify stellar variability in wide-field star trail images. We use the Large Synoptic Survey Telescope Photon Simulator to generate simulated star trail images and include transient bursts as a proxy for variability. The network identifies transient bursts on timescales down to 10 ms. We argue that there are multiple fields of astrophysics that can be advanced by the unique combination of time resolution and observing throughput that our method offers.

In this paper we explore the effects of self-obscuration in protostellar disks with a radially decreasing temperature gradient and a colder midplane. We are motivated by recent reports of resolved dark lanes ("hamburgers") and (sub)millimeter spectral indices systematically below the ISM value for optically thin dust, α_ISM = 3.7. We explore several model grids, scaling disk mass and varying inclination angle i and observing frequency ν from the VLA Ka band (∼37 GHz) to ALMA Band 8 (∼405 GHz). We also consider the effects of decreasing the index of the (sub-)millimeter dust opacity power-law β from 1.7 to 1. We find that a distribution of disk masses in the range M_disk = 0.01–2 M_⊙ is needed to reproduce the observed distribution of spectral indices, and that assuming a fixed β = 1.7 gives better results than β = 1. A wide distribution of disk masses is also needed to produce some cases with α < 2, as reported for some sources in the literature. Such extremely low spectral indices arise naturally when the selected observing frequencies sample the appropriate change in the temperature structure of the optically thick model disk. Our results show that protostellar disk masses could often be underestimated by > ×10, and are consistent with recent hydrodynamical simulations. Although we do not rule out the possibility of some grain growth occurring within the short protostellar timescales, we conclude that self-obscuration needs to be taken into account.

Cosmic rays are the primary initiators of interstellar chemistry, and getting a better understanding of the varying impact they have on the chemistry of interstellar clouds throughout the Milky Way will not only expand our understanding of interstellar medium chemistry in our own galaxy, but also aid in extra-galactic studies. This work uses the ALCHEMIC astrochemical modeling code to perform numerical simulations of chemistry for a range of ionization rates. We study the impact of variations in the cosmic-ray ionization rate on molecular abundances under idealized conditions given by constant temperatures and a fixed density of 10⁴ cm⁻³. As part of this study we examine whether observations of molecular abundances can be used to infer the cosmic-ray ionization rate in such a simplified case. We find that intense cosmic-ray ionization results in molecules, in particular the large and complex ones, being largely dissociated, and the medium becoming increasingly atomic. Individual species have limitations in their use as probes of the cosmic-ray ionization rate. At early times (<1 Myr) ions such as ${{\rm{N}}}_{2}{{\rm{H}}}^{+}$ and HOC⁺ make the best probes, while at later times, neutral species such as HNCO and SO stand out, in particular due to their large abundance variations. It is, however, by combining species into pairs that we find the best probes. Molecular ions such as ${{\rm{N}}}_{2}{{\rm{H}}}^{+}$ combined with different neutral species can provide probe candidates that outmatch individual species, in particular ${{\rm{N}}}_{2}{{\rm{H}}}^{+}/{{\rm{C}}}_{4}{\rm{H}}$, ${{\rm{N}}}_{2}{{\rm{H}}}^{+}/{{\rm{C}}}_{2}{\rm{H}}$, HOC⁺/O, and HOC⁺/HNCO. These still have limitations to their functional range, but are more functional as probes than as individual species.

The emission-line spectra of cyanoacetylene and methanol reveal chemical and physical heterogeneity on very small (<0.1 pc) scales toward the peak in cyanopolyyne emission in the Taurus Molecular Cloud, TMC-1 CP. We generate grids of homogeneous chemical models using a three-phase rate equation approach to obtain all time-dependent abundances spanning the physical conditions determined from molecular tracers of compact and extended regions of emission along this line of sight. Each time-dependent abundance is characterized by one of four features: a maximum/minimum, a monotonic increase/decrease, oscillatory behavior, or inertness. We similarly classify the time-dependent agreement between modeled and observed abundances by calculating both the rms logarithm difference and rms deviation between the modeled and observed abundances at every point in our grid models for three groups of molecules: (i) a composite group of all species present in both the observations and our chemical network G, (ii) the cyanopolyynes C = {HC₃N, HC₅N, HC₇N, HC₉N}, and (iii) the oxygen-containing organic species methanol and acetaldehyde S = {CH₃OH, CH₃CHO}. We discuss how the Bayesian uncertainties in the observed abundances constrain solutions within the grids of chemical models. The calculated best-fit times at each grid point for each group are tabulated to reveal the minimum solution space of the grid models and the effects the Bayesian uncertainties have on the grid model solutions. The results of this approach separate the effects different physical conditions and model-free parameters have on reproducing accurately the abundances of different groups of observed molecular species.

We report lifetimes, branching fractions, and the resulting oscillator strengths for transitions within the P ii multiplet (3s²3p²³P–3s3p³³P^o) at 1308 Å. These comprehensive beam-foil measurements, which are the most precise set currently available experimentally, resolve discrepancies involving earlier experimental and theoretical results. Interstellar phosphorus abundances derived from λ1308 can now be interpreted with greater confidence. In the course of our measurements, we also obtained an experimental lifetime for the 3p4s${}^{3}{P}_{0}^{{\rm{o}}}$ level of P iv. This lifetime agrees well with the available theoretical calculation.

The Disk Detective citizen science project aims to find new stars with excess 22 μm emission from circumstellar dust in the AllWISE data release from the Wide-field Infrared Survey Explorer. We evaluated 261 Disk Detective objects of interest with imaging with the Robo-AO adaptive optics instrument on the 1.5 m telescope at Palomar Observatory and with RetroCam on the 2.5 m du Pont Telescope at Las Campanas Observatory to search for background objects at 015–12'' separations from each target. Our analysis of these data leads us to reject 7% of targets. Combining this result with statistics from our online image classification efforts implies that at most 7.9% ± 0.2% of AllWISE-selected infrared excesses are good disk candidates. Applying our false-positive rates to other surveys, we find that the infrared excess searches of McDonald et al. and Marton et al. all have false-positive rates >70%. Moreover, we find that all 13 disk candidates in Theissen & West with W4 signal-to-noise ratio >3 are false positives. We present 244 disk candidates that have survived vetting by follow-up imaging. Of these, 213 are newly identified disk systems. Twelve of these are candidate members of comoving pairs based on Gaia astrometry, supporting the hypothesis that warm dust is associated with binary systems. We also note the discovery of 22 μm excess around two known members of the Scorpius–Centaurus association, and we identify known disk host WISEA J164540.79-310226.6 as a likely Sco-Cen member. Thirty of these disk candidates are closer than ∼125 pc (including 26 debris disks), making them good targets for both direct-imaging exoplanet searches.

We report on the characterization of a nearby ($d={11.20}_{-0.08}^{+0.09}$ pc) ultracool L dwarf (WISE J192512.78+070038.8; hereafter W1925) identified as a faint (G = 20.038 ± 0.009) object with high proper motion (219.834 ± 1.843 mas yr⁻¹)in the Gaia Data Releases 1 and 2. A Palomar/TripleSpec near-infrared spectrum of W1925 confirms a photometric L7 spectral type previously estimated by Scholz & Bell, and its infrared colors and absolute magnitudes are consistent with a single object of this type. We constructed a spectral energy distribution using the Gaia parallax, literature photometry, and near-infrared spectrum and find a luminosity log(L_bol/L_⊙) = −4.443 ± 0.008. Applying evolutionary models, we infer that W1925 is likely a 53 ± 18 M_Jup brown dwarf with T_eff = 1404 ± 71 K and log g = 5.1 ± 0.4 dex (cgs). While W1925 was detected in both the 2MASS and WISE infrared sky surveys, it was not detected in photographic plate sky surveys. Its combination of extreme optical–infrared colors, high proper motion, and location near the crowded Galactic plane (b = −42) likely contributed to its having evaded detection in pre-Gaia surveys.

We analyzed 200 ks of Chandra ACIS observations of the merging galaxy cluster A2142 to examine its prominent cold fronts in detail. We find that the southern cold front exhibits well-developed Kelvin–Helmholtz (KH) eddies seen in the sky plane. Comparing their wavelength and amplitude with those in hydrodynamic simulations of cold fronts in viscous gas, and estimating the gas tangential velocity from centripetal acceleration, we constrain the effective viscosity to be at most 1/5 of Spitzer isotropic viscosity, but consistent with full Braginskii anisotropic viscosity for magnetized plasma. While the northwestern front does not show obvious eddies, its shape and the structure of its brightness profile suggest KH eddies seen in projection. The southern cold front continues in a spiral to the center of the cluster, ending with another cold front only 12 kpc from the gas density peak. The cool peak itself is displaced ∼30 kpc from the brightest cluster galaxy (BCG) (the biggest such offset among centrally peaked clusters), while the X-ray emission on a larger scale is still centered on the BCG, indicating that the BCG is at the center of the gravitational potential and the cool gas is sloshing in it. The specific entropy index of the gas in the peak (K ≈ 49 keV cm²) makes A2142 a rare "warm core"; apparently the large displacement of the cool peak by sloshing is the reason. Finally, we find a subtle narrow, straight channel with a 10% drop in X-ray brightness, aligned with the southern cold front—possibly a plasma depletion layer in projection.

We present a study of the influence of galaxy mergers on star formation at 0.3 < z < 2.5. Major mergers are selected from the CANDELS/3D-HST catalog using a peak-finding algorithm. Mergers have projected galaxy nucleus separation of their members between 3 and 15 kpc. We compare the star formation activity in merging and nonmerging galaxies and find no significant differences. We find that only 12% of the galaxies in major mergers (in which both galaxies have $\mathrm{log}\,({M}_{\star }/{M}_{\odot })\geqslant 10$) are starbursting (i.e., with star formation rate (SFR) above the main sequence of star-forming galaxies by >0.5 dex). Merging galaxies, which include galaxies with lower masses, show a higher fraction of starbursting galaxies (20%). The low fraction of starbursting merging galaxies in this sample suggests that at galaxy nucleus separations of 3–15 kpc merging galaxies are still in an early stage and are yet to reach the maximum level of star formation activity. Furthermore, the level of star formation enhancement and its duration could be arguably reduced compared to local mergers, as shown by simulations of high-z mergers, and might also depend on the physical properties (such as stellar mass and gas fraction) of the merging galaxies. Finally, we compare the specific SFR between merging galaxies. Our results suggest that, as the mass of the merging galaxies increases, the star formation activity in the less massive member in the merger suffers a more dramatic impact than its companion galaxy.

The Swift Small Magellanic Cloud (SMC) Survey, S-CUBED, is a high-cadence shallow X-ray survey of the SMC. The survey consists of 142 tiled pointings covering the optical extent of the SMC, which is performed weekly by NASA's Neil GehrelsSwiftObservatory, with an exposure per tile of 60 s. The survey is focused on discovery and monitoring of X-ray outbursts from the large known and unknown population of Be/X-ray binaries (BeXRBs) in the SMC. Given the very low background of Swift's X-ray telescope, even with a short exposure per tile, S-CUBED is typically sensitive to outbursts in the SMC at >1%–2% Eddington luminosity for a typical 1.4 M_⊙ neutron star compact object. This sensitivity, combined with the high cadence and the fact that the survey can be performed all year round, makes it a powerful discovery tool for outbursting accreting X-ray pulsars in the SMC. In this paper we describe results from the first year of observations of S-CUBED, which includes the 1SCUBEDX catalog of 265 X-ray sources, 160 of which are not identified with any previously cataloged X-ray source. We report on bulk properties of sources in the 1SCUBEDX catalog. Finally, we focus on results of S-CUBED observations of several interesting sources, which includes discovery of three Type II outbursts from BeXRBs and the detection of Type I outbursts and orbital periods in six BeXRB systems.

When a planet forms a deep gap in a protoplanetary disk, dust grains cannot pass through the gap. As a consequence, the density of the dust grains can increase up to the same level of the density of the gas at the outer edge. The feedback on the gas from the drifting dust grains is not negligible in such a dusty region. We carried out two-dimensional two-fluid (gas and dust) hydrodynamic simulations. We found that when the radial flow of the dust grains across the gap is halted, a broad ring of dust grains can be formed because of the dust feedback and the diffusion of the dust grains. The minimum mass of the planet needed to form the broad dust ring is consistent with the pebble-isolation mass in the parameter range of our simulations. The broad ring of dust grains is a good environment for the formation of the protoplanetary solid core. If the ring is formed in the disk around a Sun-like star at ∼2 au, a massive, solid core (∼50 M_⊕) can be formed within the ring, which may be connected to the formation of hot Jupiters holding a massive, solid core, such as HD 149026b. In the disk of a dwarf star, a number of Earth-sized planets can be formed within the dust ring around ∼0.5 au, a phenomenon that potentially explains a planet system made of multiple Earth-sized planets around a dwarf star such as TRAPPIST-1.

Following the derivation of a more accurate model of the evolution of the solar Lyα line with the changing solar activity by Kowalska-Leszczynska et al. (IKL18) than the formerly used model by Tarnopolski & Bzowski (ST09), we investigate the potential consequences that adoption of the resulting refined model of radiation pressure has for the model distribution of interstellar neutral (ISN) H in the inner heliosphere and on the interpretation of selected observations. We simulated the ISN H densities using the two alternative radiation pressure models and identical models of all other factors affecting the ISN H distribution. We found that during most of the solar cycle, the IKL18 model predicts larger densities of ISN H and pickup ions than ST09 in the inner heliosphere, especially in the downwind hemisphere. However, the density of ISN H at the termination shock estimated by Bzowski et al. obtained using ST09 does not need revision, and the detection of ISN D by IBEX is supported. However, we point out the existence of a considerable absorption of a portion of the solar Lyα spectral flux inside the heliosphere. Therefore, the model of radiation pressure for ISN H is still likely to need revision, and hence the available models of ISN H are not self-consistent.

Numerical simulations of star formation have found that a power-law mass function can develop at high masses. In a previous paper, we employed isothermal simulations that created large numbers of sinks over a large range in masses to show that the power-law exponent of the mass function, ${dN}/d\mathrm{log}M\propto {M}^{{\rm{\Gamma }}}$, asymptotically and accurately approaches Γ = −1. Simple analytic models show that such a power law can develop if the mass accretion rate $\dot{M}\propto {M}^{2}$, as in Bondi–Hoyle accretion; however, the sink mass accretion rates in the simulations show significant departures from this relation. In this paper, we show that the expected accretion rate dependence is more closely realized provided the gravitating mass is taken to be the sum of the sink mass and the mass in the near environment. This reconciles the observed mass functions with the accretion rate dependencies, and demonstrates that power-law upper mass functions are essentially the result of gravitational focusing, a mechanism present in, for example, the competitive accretion model.

A model of magnetic field structure is presented to help test the prevalence of flux freezing in star-forming clouds of various shapes, orientations, and degrees of central concentration, and to estimate their magnetic field strength. The model is based on weak-field flux freezing in centrally condensed Plummer spheres and spheroids of oblate and prolate shape. For a spheroid of given density contrast, aspect ratio, and inclination, the model estimates the local field strength and direction, and the global field pattern of hourglass shape. Comparisons with a polarization simulation indicate typical angle agreement within 1°–10°. Scalable analytic expressions are given to match observed polarization patterns and to provide inputs to radiative transfer codes for more accurate predictions. The model may apply to polarization observations of dense cores, elongated filamentary clouds, and magnetized circumstellar disks.

Filaments and coronal holes, two principal features observed in the solar corona, are sources of space weather variations. Filament formation is closely associated with polarity inversion lines (PILs) on the solar photosphere which separate positive and negative polarities of the surface magnetic field. The origin of coronal holes is governed by large-scale unipolar magnetic patches on the photosphere from where open magnetic field lines extend to the heliosphere. We study the properties of filaments, PILs, and coronal holes in solar cycles 20, 21, 22, and 23 utilizing the McIntosh archive. We detect a prominent cyclic behavior of filament length, PIL length, and coronal hole area with significant correspondence with the solar magnetic cycle. The spatio-temporal evolution of the geometric centers of filaments shows a butterfly-like structure and distinguishable poleward migration of long filaments during cycle maxima. We identify this rush to the poles of filaments to be co-temporal with the initiation of polar field reversal as gleaned from Mount Wilson and Wilcox Solar Observatory polar field observations, and quantitatively establish their temporal correspondence. We analyze the filament tilt angle distribution to constrain their possible origins. The majority of the filaments exhibit negative and positive tilt angles in the northern and the southern hemispheres, respectively, strongly suggesting that their formation is governed by the overall large-scale magnetic field distribution on the solar photosphere and not by the small-scale intra-active region magnetic field configurations. We also investigate the hemispheric asymmetry in filaments, PILs, and coronal holes. We find that the hemispheric asymmetry in filaments and PILs is positively correlated with sunspot area asymmetry, whereas coronal hole asymmetry is uncorrelated.

We present a timing analysis of the eclipsing post-common envelope binary (PCEB) DE CVn. Based on new CCD photometric observations and published data, we found that the orbital period in DE CVn has a cyclic period oscillation with an amplitude of 28.08 s and a period of 11.22 years plus a rapid period decrease at a rate of $\dot{P}=-3.35\times {10}^{-11}{{ss}}^{-1}$. According to the evolutionary theory, secular period decreases in PCEBs arise from angular momentum losses (AMLs) driven by gravitational radiation (GR) and magnetic braking (MB). However, the observed orbital decay is too fast to be produced by AMLs via GR and MB, indicating that there could be another AML mechanism. We suggest that a circumbinary disk around DE CVn may be responsible for the additional AML. The disk mass was derived as a few ×10⁻⁴–10⁻³M_⊙ , which is in agreement with that inferred from previous studies in the order of magnitude. The cyclic change is most likely the result of the gravitational perturbation by a circumbinary object due to the Applegate's mechanism failing to explain such a large period oscillation. The mass of the potential third body is calculated as ${M}_{3}\sin i^{\prime} =0.011(\pm 0.003)\,{M}_{\odot }$. Supposing the circumbinary companion and the eclipsing binary are coplanar, its mass would correspond to a giant planet. This hypothetical giant planet is moving in a circular orbit of a radius of ∼5.75(±2.02) au around its host star.

We report X-ray, optical, and near-infrared monitoring of the new X-ray transient MAXI J1820+070 discovered with MAXI on 2018 March 11. Its X-ray intensity reached ∼2 crab at 2–20 keV at the end of March, and then gradually decreased until the middle of June. In this period, the X-ray spectrum was described by Comptonization of the disk emission, with a photon index of ∼1.5 and an electron temperature of ∼50 keV, which is consistent with a black hole X-ray binary in the low/hard state. The electron temperature was slightly decreased, and the photon index increased, with increasing flux. The source showed significant X-ray flux variation on a timescale of seconds. This short-term variation was found to be associated with changes in the spectral shape, and the photon index became slightly harder at higher fluxes. This suggests that the variation was produced by a change in the properties of the hot electron cloud responsible for the strong Comptonization. Modeling a multi-wavelength spectral energy distribution around the X-ray flux peak at the end of March, covering the near-infrared to X-ray bands, we found that the optical and near-infrared fluxes were likely contributed substantially by the jet emission. Before this outburst, the source was never detected in the X-ray band with MAXI (with a 3σ upper limit of ∼0.2 mcrab at 4–10 keV, obtained from seven years of data from 2009 to 2016), whereas weak optical and infrared activity was found at flux levels ∼3 orders of magnitude lower than the peak fluxes in the outburst.

The Pan-Andromeda Archaeological Survey is a survey of >400 square degrees centered on the Andromeda (M31) and Triangulum (M33) galaxies that has provided the most extensive panorama of an L_⋆ galaxy group to large projected galactocentric radii. Here, we collate and summarize the current status of our knowledge of the substructures in the stellar halo of M31, and discuss connections between these features. We estimate that the 13 most distinctive substructures were produced by at least 5 different accretion events, all in the last 3 or 4 Gyr. We suggest that a few of the substructures farthest from M31 may be shells from a single accretion event. We calculate the luminosities of some prominent substructures for which previous estimates were not available, and we estimate the stellar mass budget of the outer halo of M31. We revisit the problem of quantifying the properties of a highly structured data set; specifically, we use the OPTICS clustering algorithm to quantify the hierarchical structure of M31's stellar halo and identify three new faint structures. M31's halo, in projection, appears to be dominated by two "mega-structures," which can be considered as the two most significant branches of a merger tree produced by breaking M31's stellar halo into increasingly smaller structures based on the stellar spatial clustering. We conclude that OPTICS is a powerful algorithm that could be used in any astronomical application involving the hierarchical clustering of points. The publication of this article coincides with the public release of all PAndAS data products.

X-ray emissions in protostars play an important role in the chemistry of protostellar disks and in constraining the physics of jet formation. We have experimentally investigated the mechanism of X-ray emission in protostellar jets and modeled their interaction with the surrounding medium. The simulated supersonic jet is generated by intense laser beams irradiating a K-shaped target and then impacts an obstacle. We have successfully observed X-ray emission from the obstacle surface, and we find that it comes from the outflow material and not completely from the ambient medium heated by shock.

We present a comprehensive stellar atmosphere analysis of 329 O- and B-type stars in the Small Magellanic Cloud (SMC) from the RIOTS4 survey. Using spectroscopically derived effective temperature T_eff and surface gravities, we find that classical Be stars appear misplaced to low T_eff and high luminosity in the spectroscopic Hertzsprung–Russell diagram (sHRD). Together with the most luminous stars in our sample, the stellar masses derived from the sHRD for these objects are systematically larger than those obtained from the conventional Hertzsprung–Russell diagram. This suggests that the well-known, spectroscopic mass-discrepancy problem may be linked to the fact that both groups of stars have outer envelopes that are nearly gravitationally unbound. The non-emission-line stars in our sample mainly appear on the main sequence, allowing a first estimate of the terminal-age main sequence (TAMS) in the SMC, which matches the predicted TAMS between 12 and 40 M_⊙ at SMC metallicity. We further find a large underabundance of stars above ∼25 M_⊙ near the zero-age main sequence, reminiscent of such earlier findings in the Milky Way and Large Magellanic Cloud.

Observations have shown that the UV/optical variation amplitude of quasars depends on several physical parameters including luminosity, Eddington ratio, and possibly black hole mass. Identifying new factors which correlate with the variation is essential to probing the underlying physical processes. Combining around 10 years of quasar light curves from SDSS stripe 82 and X-ray data from Stripe 82X, we build a sample of X-ray-detected quasars to investigate the relation between UV/optical variation amplitude (${\sigma }_{\mathrm{rms}}$) and X-ray loudness. We find that quasars with more intense X-ray radiation (compared to bolometric luminosity) are more variable in the UV/optical. This correlation remains highly significant after excluding the effect of other parameters including luminosity, black hole mass, Eddington ratio, redshift, and rest frame wavelength (i.e., through partial correlation analyses). We further find that the intrinsic link between X-ray loudness and UV/optical variation is gradually more prominent on longer timescales (up to 10 yr in the observed frame), but tends to disappear at timescales <100 days. This suggests a slow and long-term underlying physical process. The X-ray reprocessing paradigm, in which the UV/optical variation is produced by variable central X-ray emission illuminating the accretion disk, is thus disfavored. This discovery points to an interesting scenario in which both the X-ray coronal heating and UV/optical variation in quasars are closely associated with magnetic disc turbulence, and the innermost disc turbulence (where coronal heating occurs) correlates with slow turbulence at larger radii (where UV/optical emission is produced).

We investigate the formation and magnetic topology of four flare/coronal mass ejection events with filament-sigmoid systems, in which the sigmoidal hot channels are located above the filaments and appear in pairs before eruption. The formation of hot channels usually takes several to dozens of hours, during which two J-shaped sheared arcades gradually evolve into sigmoidal hot channels and then keep stable for tens of minutes or hours and erupt, while the low-lying filaments show no significant change. We construct a series of magnetic field models and find that the best-fit preflare models contain magnetic flux ropes with hyperbolic flux tubes (HFTs). The field lines above the HFT correspond to the high-lying hot channel, whereas those below the HFT surround the underlying filaments. In particular, the continuous and long field lines representing the flux rope located above the HFT match the observed hot channels well in three events. However, for the SOL2014-04-18 event, the flux bundle that mimics the observed hot channel is located above the flux rope. The flux rope axis lies in a height range of 19.8 and 46 Mm above the photosphere for the four events, among which the flux rope axis in the SOL2012-07-12 event has a maximum height, which probably explains why it is often considered as a double-decker structure. Our modeling suggests that the high-lying hot channel may be formed by magnetic reconnections between sheared field lines occurring above the filament before eruption.

One of the most important problems in the context of cataclysmic variables (CVs) is the lack of observations of systems with periods between 2 and 3.12 hr, known as the period gap. The orbital evolution of CVs with periods shorter than those in the gap is dominated by gravitational radiation, while for periods exceeding those of the gap it is dominated by magnetic braking of the secondary star. Spruit & Ritter showed that as periods approach 3 hr and secondary stars become fully convective a sharp decline in magnetic dynamo and braking efficiency would result in such a gap. Recent X-ray observations finding coronal magnetic energy dissipation is similar in fully convective and partly radiative M dwarfs cast this theory into doubt. In this work, we use Zeeman–Doppler imaging observations culled from the literature to show that the complexity of the surface magnetic fields of rapidly rotating M dwarfs increases with decreasing rotation period. Garraffo et al. have shown that the efficiency of angular momentum loss of cool stars declines strongly with increasing complexity of their surface magnetic field. We explore the idea of Taam & Spruit that magnetic complexity might then explain the period gap. By generating synthetic CV populations using a schematic CV evolutionary approach, we show that the CV period gap can naturally arise as a consequence of a rise in secondary star magnetic complexity near the long-period edge of the gap that renders a sharp decline in their angular-momentum-loss rate.

We present the results from the application of a two-dimensional emission line detection method, EMission-line two-Dimensional (EM2D), to the near-infrared G102 grism observations obtained with the Wide-Field Camera 3 (WFC3) as part of the Cycle 22 Hubble Space Telescope Treasury Program: the Faint Infrared Grism Survey (FIGS). Using the EM2D method, we have assembled a catalog of emission line galaxies (ELGs) with resolved star formation from each of the four FIGS fields. Not only can one better assess the global properties of ELGs, but the EM2D method allows for the analysis and improved study of the individual emission-line region within each galaxy. This paper includes a description of the methodology, advantages, and the first results of the EM2D method applied to ELGs in FIGS. The advantage of 2D emission line measurements includes significant improvement of galaxy redshift measurements, approaching the level of accuracy seen in high-spectral-resolution data, but with greater efficiency; and the ability to identify and measure the properties of multiple sites of star formation and over scales of ∼1 kpc within individual galaxies out to z ∼ 4. The EM2D method also significantly improves the reliability of high-redshift (z ∼ 7) Lyα detections. Coupled with the wide field of view and high efficiency of space-based grism observations, EM2D provides a noteworthy improvement on the physical parameters that can be extracted from grism observations.

Motivated by recent observations suggesting that core-collapse supernovae may on average produce ∼0.3 M_⊙ of dust, we explore a simple dust production scenario that applies to star-forming galaxies in the local environment (the Magellanic Clouds and possibly the Milky Way) as well as to high-redshift (submillimeter, QSO, Lyman-break) galaxies. We assume that the net dust destruction (due to supernova reverse shock, shocks in the interstellar medium, or astration) is negligible on a timescale of 1 Gyr, in which case the dust mass can be estimated as 0.004 times the star formation rate (for a Chabrier initial mass function) multiplied by the duration of the star formation episode. The model can account for observed dust masses over four orders of magnitude and across the redshift range 0–8.4, with dust production rates spanning five orders of magnitude. This suggests that star-forming galaxies may be seen as maximally dusty, in the sense that a dominant fraction of the dust-forming elements forged in a supernova eventually will go into the solid phase. In turn, this indicates little destruction of supernova dust or almost complete replenishment, on a short timescale, of any dust that is destroyed.

We estimate the 21 cm radio background from accretion onto the first intermediate-mass black holes between z ≈ 30 and z ≈ 16. Combining potentially optimistic, but plausible, scenarios for black hole formation and growth with empirical correlations between luminosity and radio emission observed in low-redshift active galactic nuclei, we find that a model of black holes forming in molecular cooling halos is able to produce a 21 cm background that exceeds the cosmic microwave background (CMB) at z ≈ 17, though models involving larger halo masses are not entirely excluded. Such a background could explain the surprisingly large amplitude of the 21 cm absorption feature recently reported by the EDGES collaboration. Such black holes would also produce significant X-ray emission and contribute to the 0.5–2 keV soft X-ray background at the level of ≈10⁻¹³–10⁻¹² erg s⁻¹ cm⁻² deg⁻², consistent with existing constraints. In order to avoid heating the intergalactic medium (IGM) over the EDGES trough, these black holes would need to be obscured by hydrogen column depths of N_H ∼ 5 × 10²³ cm⁻². Such black holes would avoid violating constraints on the CMB optical depth from Planck if their UV photon escape fractions were below f_esc ≲ 0.1, which would be a natural result of N_H ∼ 5 × 10²³ cm⁻² being imposed by an unheated IGM.

Here we report on the total-intensity 610 MHz Giant Meterwave Radio Telescope (GMRT) observations of the peculiar hybrid blazar SBS B1646+499, which merges the properties of BL Lacertae objects and flat-spectrum radio quasars. The complex radio structure of SBS B1646+499, emerging from the archival radio data and our new GMRT observations, consists of the megaparsec-scale elongated halo, the unilateral kiloparsec-scale jet, and the nuclear jet extending up to ∼20 pc from the compact core. The giant halo is characterized by a steep radio spectrum, indicative of the advanced aging of the electron population within the lobes. For the large-scale jet, we detected a spectral gradient along and across the outflow, and in particular spectral flattening of the radio continuum toward the jet edges, suggestive of the spine-boundary shear layer morphology. The nuclear jet displays superluminal knots emerging from the self-absorbed and variable radio core. We interpret all these findings in the framework of the model of an episodic jet activity with a precessing jet axis.

Radioactive energies from unstable nuclei made in the ejecta of neutron star mergers play principal roles in powering kilonovae. In previous studies, power-law-type heating rates (e.g., $\propto {t}^{-1.3}$) have frequently been used, which may be inadequate if the ejecta are dominated by nuclei other than the A ∼ 130 region. We consider, therefore, two reference abundance distributions that match the r-process residuals to the solar abundances for A ≥ 69 (light trans-iron plus r-process elements) and A ≥ 90 (r-process elements). Nucleosynthetic abundances are obtained by using free-expansion models with three parameters: expansion velocity, entropy, and electron fraction. Radioactive energies are calculated as an ensemble of weighted free-expansion models that reproduce the reference abundance patterns. The results are compared with the bolometric luminosity (> a few days since merger) of the kilonova associated with GW170817. We find that the former case (fitted for A ≥ 69) with an ejecta mass 0.06 M_⊙ reproduces the light curve remarkably well, including its steepening at ≳7 days, in which the mass of r-process elements is ≈0.01 M_⊙. Two β-decay chains are identified: ⁶⁶Ni $\,\to \,$⁶⁶Cu $\,\to \,$⁶⁶Zn and ⁷²Zn $\,\to \,$⁷²Ga $\,\to \,$⁷²Ge with similar halflives of parent isotopes (≈2 days), which leads to an exponential-like evolution of heating rates during 1–15 days. The light curve at late times (>40 days) is consistent with additional contributions from the spontaneous fission of ²⁵⁴Cf and a few Fm isotopes. If this is the case, the GW170817 event is best explained by the production of both light trans-iron and r-process elements that originate from dynamical ejecta and subsequent disk outflows from the neutron star merger.

The magnetic/current helicity of the coronal field is closely associated with the presence of a nonpotential axial component directed along the photospheric polarity inversion line (PIL), which is also the source of the axial/toroidal field in flux ropes and coronal mass ejections (CMEs). To better understand the role of this axial component in the evolution of coronal helicity, we use Fe xii 19.3 nm images and longitudinal magnetograms from the Solar Dynamics Observatory to track active regions (ARs) and their filament channels as they decay due to flux transport processes. We find that the Fe xii loop legs or "stalks," initially oriented almost perpendicular to the PIL, become closely aligned with it after ∼1–4 rotations; this alignment is attributed to the progressive cancellation of the transverse field component at the PIL. As the AR flux continues to decay, the PIL becomes ever more distorted and the directions of the stalks are increasingly randomized. These observations suggest that most of the original axial field in ARs is not expelled in CMEs, but instead pinches off after the eruptions and becomes concentrated at the PIL. Because the twist of the field decreases, however, the helicity itself decreases, with CMEs removing a significant fraction of it in the form of disconnected flux ropes. Like most of the AR flux, the bulk of the axial field is eventually canceled/resubmerged, brought to the equator by the subsurface meridional flow, and annihilated (along with the remaining helicity) by merging with its opposite-handed counterpart from the other hemisphere.

We analyze optical data cubes of the nuclear regions of two late-type galaxies, NGC 908 and NGC 1187, obtained with the Integral Field Unit of the Gemini Multi-Object Spectrograph. Both data cubes show stellar structures consistent with double nuclei. The morphology of the line-emitting areas in the central region of NGC 1187 is also of a double nucleus, while the spatial morphology of the line-emitting areas in the data cube of NGC 908 is consistent with a circumnuclear asymmetric ring. The emission-line ratios of the nuclear spectra (and, actually, along the entire field of view) of both galaxies are characteristic of H ii regions. In particular, based on its emission-line properties, the circumnuclear ring in NGC 908 can be identified as a star-forming ring. The observed spatial morphology of the stellar emission and also the differences in the properties of the stellar populations detected in the stellar nuclei of NGC 908 suggest that the most likely scenario to explain the double stellar nucleus in this object involves a minor merger, probably with a high mass ratio. On the other hand, considering the similar properties of the stellar populations in the stellar nuclei of NGC 1187, together with the stellar and gas kinematic properties, we conclude that the most likely scenario to explain the double stellar and gas nucleus in this galaxy involves the stellar and gas kinematics, in the form of a circular nuclear disk subject to perturbations.

We compute the evolution of a quasi-spherical, slowly rotating accretion flow around a black hole, whose mass and spin evolve adequately to transfer of mass and energy through the horizon. Our model is relevant for a central engine driving a long gamma-ray burst (GRB) that originates from the collapse of a massive star. The computations of a GRB engine in a dynamically evolving spacetime metric are important specifically due to the transient nature of the event, in which a huge amount of mass is accreted and changes the fundamental black hole parameters—its mass and spin—during the process. We discuss the results in the context of the angular momentum magnitude of the collapsing star. We also study the possible formation and evolution of shocks in the envelope, which may temporarily affect accretion. Our results are important for the limitations on the mass and spin range of black holes detected independently by electromagnetic observations of GRBs and gravitational waves. We speculate on the possible constraints for the final masses and spins of these astrophysical black holes. It is shown that the most massive black holes are not formed in a powerful GRB explosion if the cores of their progenitors were only weakly rotating.

The transverse peculiar velocities caused by the mass distribution of large-scale structure (LSS) provide a test of the theoretical matter power spectrum and the cosmological parameters that contribute to its shape. Typically, the matter density distribution of the nearby universe is measured through redshift or line-of-sight peculiar velocity surveys. However, both methods require model-dependent distance measures to place the galaxies or to differentiate peculiar velocity from the Hubble expansion. In this paper, we use the correlated proper motions of galaxy pairs from the VLBA Extragalactic Proper Motion Catalog to place limits on the transverse peculiar velocity of galaxy pairs with comoving separations <1500 Mpc without a reliance on precise distance measurements. The relative proper motions of galaxy pairs across the line of sight can be directly translated into relative peculiar velocities because no proper motion will occur in a homogeneous expansion. We place a 3σ limit on the relative proper motion of pairs with comoving separations <100 Mpc of −17.4 μas yr${}^{-1}\lt \dot{\theta }/\sin \theta \,\lt 19.8$μas yr⁻¹. We also confirm that large-separation objects (>200 Mpc) are consistent with pure Hubble expansion to within ∼5.3 μas yr⁻¹ (1σ). Finally, we predict that Gaia end-of-mission proper motions will be able to significantly detect the mass distribution of LSS on length scales <25 Mpc. This future detection will allow a test of the shape of the theoretical mass power spectrum without a reliance on precise distance measurements.

We selected 4593 hot subdwarf candidates from the Gaia DR2 Hertzsprung–Russell (HR) diagram. By combining the sample with Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) DR5, we identified 294 hot subdwarf stars, including 169 sdB, 63 sdOB, 31 He-sdOB, 22 sdO, 7 He-sdO, and 2 He-sdB stars. The atmospheric parameters (e.g., T_eff, log g, log(nHe/nH)) are obtained by fitting the hydrogen (H) and helium (He) line profiles with synthetic spectra. Two distinct He sequences of hot subdwarf stars are clearly presented in the T_eff–log g diagram. We found that the He-rich sequence consists of the bulk of sdB and sdOB stars, as well as all of the He-sdB, He-sdO, and He-sdOB stars in our samples, while all the stars in the He-weak sequence belong to the sdO spectral type, combined with a few sdB and sdOB stars. We demonstrated that the combination of Gaia DR2 and LAMOST DR5 allows one to uncover a huge number of new hot subdwarf stars in our Galaxy.

We present a spectral-timing analysis of observations taken in fall 2017 of the newly detected X-ray transient MAXI J1535–571. We included 38 Swift/XRT window timing mode observations, 3 XMM-Newton observations, and 31 Neutron star Interior Composition Explorer observations in our study. We computed the fundamental diagrams commonly used to study black hole transients, and fitted power density and energy spectra to study the evolution of spectral and timing parameters. The observed properties are consistent with a bright black hole X-ray binary (${F}_{0.6-10\,\mathrm{keV}}^{\max }=3.71\pm 0.02$$\times {10}^{-7}$ erg cm⁻² s⁻¹) that evolves from the low-hard-state to the high-soft state and back to the low-hard-state. In some observations the power density spectra showed type-C quasiperiodic oscillations, giving additional evidence that MAXI J1535–571 is in a hard state during these observations. The duration of the soft state with less than 10 days is unusually short and observations taken in spring 2018 show that MAXI J1535–571 entered a second (and longer) soft state.

Observations of dust-enshrouded evolved stars selected from infrared catalogs requiring high positional accuracy, like infrared spectroscopy or long baseline radio interferometric observations, often require preparational observational steps determining a position with an accuracy much better than 1''. Using phase-referencing observations with the Very Large Array at its highest resolution, we have compared the positions of SiO 43 GHz masers in evolved stars, assumed to originate in their infrared detected circumstellar shells, with the positions listed in the MSX, WISE, 2MASS, and Gaia catalogs. Starting from an MSX position it is, in general, simple to match 2MASS and WISE counterparts. However, in order to obtain a Gaia match to the MSX source it is required to use a two-step approach due to the large number of nearby candidates and low initial positional accuracy of the MSX data. We show that the closest comparable position to the SiO maser in our limited sample never is the MSX position. When a plausible source with a characteristic signature of an evolved star with a circumstellar shell can be found in the area, the best indicator of the maser position is provided by the Gaia position, with the 2MASS position being second best. Typical positional offsets from all catalogs to the SiO masers are reported.

We study the photo-desorption occurring in H₂O:CO:NH₃ ice mixtures irradiated with monochromatic (550 and 900 eV) and broadband (250–1250 eV) soft X-rays generated at the National Synchrotron Radiation Research Center (Hsinchu, Taiwan). We detect many masses photo-desorbing, from atomic hydrogen (m/z = 1) to complex species with m/z = 69 (e.g., C₃H₃NO, C₄H₅O, C₄H₇N), supporting the enrichment of the gas phase. At low numbers of absorbed photons, substrate-mediated, exciton-promoted desorption dominates the photo-desorption yield, inducing the release of weakly bound (to the surface of the ice) species; as the number of weakly bound species declines, the photo-desorption yield decreases about one order of magnitude, until porosity effects, reducing the surface/volume ratio, produce a further drop of the yield. We derive an upper limit to the CO photo-desorption yield, which in our experiments varies from 1.4 to 0.007 molecules photon⁻¹ in the range ∼10¹⁵–10²⁰ absorbed photons cm⁻². We apply these findings to a protoplanetary disk model irradiated by a central T Tauri star.

To test a technique to be used on the white-light imager onboard the recently launched Parker Solar Probe mission, we performed a numerical differentiation of the brightness profiles along the photometric axis of the F-corona models that are derived from STEREO Ahead Sun Earth Connection Heliospheric Investigation observations recorded with the HI-1 instrument between 2007 December and 2014 March. We found a consistent pattern in the derivatives that can be observed from any S/C longitude between about 18° and 23° elongation with a maximum at about 21°. These findings indicate the presence of a circumsolar dust density enhancement that peaks at about 23° elongation. A straightforward integration of the excess signal in the derivative space indicates that the brightness increase over the background F-corona is on the order of 1.5%–2.5%, which implies an excess dust density of about 3%–5% at the center of the ring. This study has also revealed (1) a large-scale azimuthal modulation of the inner boundary of the pattern, which is in clear association with Mercury's orbit; and (2) a localized modulation of the inner boundary that is attributable to the dust trail of Comet 2P/Encke, which occurs near ecliptic longitudes corresponding to the crossing of Encke's and Mercury's orbital paths. Moreover, evidence of dust near the S/C in two restricted ranges of ecliptic longitudes has also been revealed by this technique, which is attributable to the dust trails of (1) comet 73P/Schwassmann–Wachmann 3, and (2) 169P/NEAT.

We report on the discovery of a merger-driven starburst at z = 1.52, PACS-787, based on high signal-to-noise ALMA observations. CO(5–4) and continuum emission (850 μm) at a spatial resolution of 03 reveal two compact (r_1/2 ∼ 1 kpc) and interacting molecular gas disks at a separation of 8.6 kpc, indicative of an early stage in a merger. With an SFR of 991 M_⊙ yr⁻¹, this starburst event should occur closer to final coalescence, as is usually seen in hydrodynamical simulations. From the CO size, inclination, and velocity profile for both disks, the dynamical mass is calculated through a novel method that incorporates a calibration using simulations of galaxy mergers. Based on the dynamical mass, we measure (1) the molecular gas mass, independent from the CO luminosity, (2) the ratio of the total gas mass and the CO(1–0) luminosity (${\alpha }_{\mathrm{CO}}\equiv {M}_{\mathrm{gas}}/{L}_{\mathrm{CO}\,1-0}^{{\prime} }$), and (3) the gas-to-dust ratio, with the latter two being lower than typically assumed. We find that the high star formation triggered in both galaxies is caused by a set of optimal conditions: a high gas mass/fraction, a short depletion time (τ_depl = 85 and 67 Myr) to convert gas into stars, and the interaction of likely counter-rotating molecular disks that may accelerate the loss of angular momentum. The state of interaction is further established by the detection of diffuse CO and continuum emission, tidal debris that bridges the two nuclei and is associated with stellar emission seen by HST/WFC3. This observation demonstrates the power of ALMA to study the dynamics of galaxy mergers at high redshift.