Table of contents

Collisionless plasma shocks are efficient sources of nonthermal particle acceleration in space and astrophysical systems. We use hybrid (kinetic ion—fluid electron) simulations to examine the nonlinear feedback of the self-generated energetic particles (cosmic rays, CRs) on the shock hydrodynamics. When CR acceleration is efficient, we find evidence of both an upstream precursor, where the inflowing plasma is compressed and heated, and a downstream postcursor, where the energy flux in CRs and amplified magnetic fields play a dynamical role. For the first time, we assess how nonlinear magnetic fluctuations in the postcursor preferentially travel away from the shock at roughly the local Alfvén speed with respect to the downstream plasma. The drift of both magnetic and CR energy with respect to the thermal plasma substantially increases the shock compression ratio with respect to the standard prediction, in particular exceeding 4 for strong shocks. Such modifications also have implications for the spectrum of the particles accelerated via diffusive shock acceleration, a significant result detailed in a companion paper.

Diffusive shock acceleration is a prominent mechanism for producing energetic particles in space and in astrophysical systems. Such energetic particles have long been predicted to affect the hydrodynamic structure of the shock, in turn leading to CR spectra flatter than the test-particle prediction. However, in this work along with a companion paper, we use self-consistent hybrid (kinetic ion–fluid electron) simulations to show for the first time how CR-modified shocks actually produce steeper spectra. The steepening is driven by the enhanced advection of CRs embedded in magnetic turbulence downstream of the shock, in what we call the "postcursor." These results are consistent with multiwavelength observations of supernovae and supernova remnants and have significant phenomenological implications for space/astrophysical shocks in general.

In this paper, we present the SDSS g'-, the Cousins R_c-, and I_c-band magnitudes and associated colors of Starlink's STARLINK-1113 (one of the standard Starlink satellites) and 1130 (Darksat) with a darkening treatment to its surface. Using the 105 cm Murikabushi telescope/MITSuME, simultaneous multicolor observations for the above satellites were conducted four times: on 2020 April 10 and May 18 (for Darksat), and 2020 June 11 (for Darksat and STARLINK-1113). We found that (1) the SDSS g'-band apparent magnitudes of Darksat (6.95 ± 0.11–7.65 ± 0.11 mag) are comparable to or brighter than that of STARLINK-1113 (7.69 ± 0.16 mag), (2) the shorter the observed wavelength is, the fainter the satellite magnitudes tend to become, (3) the reflected flux by STARLINK-1113 is extremely (>1.0 mag) redder than that of Darksat, (4) there is no clear correlation between the solar phase angle and orbital altitude-scaled magnitude, and (5) by flux model fitting of the satellite trails with the blackbody radiation, it is found that the albedo of Darksat is about half that of STARLINK-1113. In particular, result (1) is inconsistent with previous studies. However, considering both solar and observer phase angles and atmospheric extinction, the brightness of STARLINK-1113 can be drastically reduced in the SDSS g' and the Cousins R_c band. Simultaneous multicolor–multispot observations of more than three colors would give us more detailed information regarding the impact of low-Earth-orbit satellite constellations.

Semianalytic models (SAMs) are a promising means of tracking the physical processes associated with galaxy formation, but many of their approximations have not been rigorously tested. As part of the Simulating Multiscale Astrophysics to Understand Galaxies project, we compare predictions from the FIRE-2 hydrodynamical "zoom-in" simulations to those from the Santa Cruz SAM run on the same halo merger trees, with an emphasis on the global mass flow cycle. Our study includes 13 halos spanning low-mass dwarfs (M_vir ∼ 10¹⁰M_⊙ at z = 0), intermediate-mass dwarfs (M_vir ∼ 10¹¹M_⊙), and Milky Way–mass galaxies (M_vir ∼ 10¹²M_⊙). The SAM and FIRE-2 predictions agree relatively well with each other in terms of stellar and interstellar medium mass but differ dramatically on circumgalactic medium mass (the SAM is lower than FIRE-2 by ∼3 orders of magnitude for dwarfs). Strikingly, the SAM predicts higher gas accretion rates for dwarfs compared to FIRE-2 by factors of ∼10–100, and this is compensated for with higher mass outflow rates in the SAM. We argue that the most severe model discrepancies are caused by the lack of preventative stellar feedback and the assumptions for halo gas cooling and recycling in the SAM. As a first step toward resolving these model tensions, we present a simple yet promising new preventative stellar feedback model in which the energy carried by supernova-driven winds is allowed to heat some fraction of gas outside of halos to at least the virial temperature such that accretion is suppressed.

Observations of debris disks, the products of the collisional evolution of rocky planetesimals, can be used to trace planetary activity across a wide range of stellar types. The most common end points of stellar evolution are no exception, as debris disks have been observed around several dozen white dwarf stars. But instead of planetary formation, post-main-sequence debris disks are a signpost of planetary destruction, resulting in compact debris disks from the tidal disruption of remnant planetesimals. In this work, we present the discovery of five new debris disks around white dwarf stars with gaseous debris in emission. All five systems exhibit excess infrared radiation from dusty debris, emission lines from gaseous debris, and atmospheric absorption features indicating ongoing accretion of metal-rich debris. In four of the systems, we detect multiple metal species in emission, some of which occur at strengths and transitions previously unseen in debris disks around white dwarf stars. Our first year of spectroscopic follow-up hints at strong variability in the emission lines that can be studied in the future, expanding the range of phenomena these post-main-sequence debris disks exhibit.

The recent discovery of a spiral feature in the Z − V_Z phase plane in the solar neighborhood implies that the galactic disk has been remarkably affected by a dwarf galaxy passing through it some hundreds of millions of years ago. Using 429,500 Large Sky Area Multi-Object Fibre Spectroscopic Telescope K giants stars, we show that the spiral feature exists not only in the solar vicinity but it also extends to about 15 kpc from the Galactic center and then disappears beyond this radius. Moreover, we find that when the spiral features in a plot of V_ϕ as a function of position in the Z − V_Z plane at various galactocentric radii are remapped to the R − Z plane, the spiral can explain well the observed asymmetric velocity substructures. This is evidence that the phase spiral features are the same as the bulk motions found in previous work as well as this work. Test particle simulations and N-body simulations show that an encounter with a dwarf galaxy a few hundred million years ago will induce a perturbation in the galactic disk. In addition, we find that the last impact of Sgr dSph can also contribute to the flare. As a consequence of the encounter, the distribution function of disk stars at a large range of radii is imprinted by the gravitational perturbation.

We present a microlensing analysis of updated light curves in three filters, the g-band, r-band, and H-band, for the gravitationally lensed quasars Q0957+561 and SBS0909+532. Both systems display prominent microlensing features which we analyze using our Bayesian Monte Carlo technique to constrain the quasar continuum emission region sizes in each band. We report sizes as half-light radii scaled to a 60° inclination angle. For Q0957+561 we measure $\mathrm{log}({r}_{1/2}/\mathrm{cm})={16.54}_{-0.33}^{+0.33}$, ${16.66}_{-0.62}^{+0.37}$, and ${17.37}_{-0.40}^{+0.49}$ in g-, r-, and H-band, respectively. For SBS0909+532 we measure $\mathrm{log}({r}_{1/2}/\mathrm{cm})={15.83}_{-0.33}^{+0.33}$, ${16.21}_{-0.62}^{+0.37}$, and ${17.90}_{-0.63}^{+0.61}$ in the g-, r-, and H-band respectively. With size measurements in three bands spanning the quasar rest frame ultraviolet to optical, we can place constraints on the scaling of accretion disk size with wavelength, $r\propto {\lambda }^{1/\beta }$. In a joint analysis of both systems we find a slope shallower than that predicted by thin disk theory, $\beta ={0.35}_{-0.08}^{+0.16}$, consistent with other constraints from multi-epoch microlensing studies.

We present an analytical treatment for time-dependent diffusive shock acceleration at shocks inside magnetic clouds (MCs) observed near 1 au. The model includes the effects of (i) spatial diffusion of test particles upstream and downstream of the shock, (ii) proton advection with the plasma inside MCs, (iii) a reflecting boundary at distance L upstream of the shock to mimic the boundary of the MCs, and (iv) particle leakage out of the system at a constant rate, possibly through open field lines introduced by magnetic reconnection between the closed field lines of the MC and open field lines in the corona or heliosphere. The analysis reveals that the mean time for accelerating particles from p₀ to p is naturally reduced if the MC characteristic length is much smaller than the spatial diffusion length of energetic protons upstream of the shock. However, because most shocks inside MCs observed at 1 au are located in the back half of the MC, the time that the shock has propagated into the MCs is not sufficient to cause significant SEP enhancement—even with a reflecting boundary—if particles are only injected from the low-beta plasma inside MCs. To cause large SEP enhancements inside the shock–MC structure, magnetic reconnection at the back MC is essential to allow particles energized by the shock prior to its interaction with the MC to enter the MC. These particles consequently become the seed energetic protons that are reaccelerated at the shock inside MC.

In this work, we compare two powerful parameter estimation methods, namely Bayesian inference and neural network based learning, to study the quark matter equation of state with constant speed of sound parameterization and the structure of the quark stars within the two-family scenario. We use the mass and radius estimations from several X-ray sources and also the mass and tidal deformability measurements from gravitational wave events to constrain the parameters of our model. The results found from the two methods are consistent. The predicted speed of sound is compatible with the conformal limit.

We discuss properties of a Type IV burst, which was observed on 2017 September 6, as a result of the powerful flare X 9.3. At decameter wavelengths this burst was observed by the radio telescopes STEREO A, URAN-2, and the Nancay Decameter Array at frequencies 5–35 MHz. This moving Type IV burst was associated with a coronal mass ejection (CME) propagating in the southwest direction with a speed of 1570 km s⁻¹. The maximum radio flux of this burst was about 300 s.f.u. and the polarization was more than 40%. In the frequency range of 8–33 MHz it continued for more than 2 hr. For STEREO A the associated CME was behind the limb, and its longitudinal angle was about 160°. This moving Type IV burst was observed by STEREO A at frequencies of 5–15 MHz in spite of the low sensitivity of STEREO A. This means that the radio emission directivity of a Type IV burst is rather wide. Assuming the plasma mechanism of Type IV radio emission we derived the plasma density distribution in the CME core at distances of 5.6 R_s and 9.8 R_s (R_s is the solar radius), and its mass to be about 10¹⁶ g. It is planned that the minimum perihelion of the Parker Solar Probe (PSP) spacecraft will be at about 9 R_s. So we discuss in what conditions PSP will be in if it crosses a similar CME core.

Rotating star clusters near supermassive black holes are studied using Touma–Tremaine thermodynamics of gravitationally interacting orbital ellipses. A simple numerical procedure for calculating thermodynamic equilibrium states for an arbitrary distribution of stars over masses and semimajor axes is described. Spontaneous symmetry breaking and breakdown of thermodynamics at low positive temperatures are rigorously proven for nonrotating clusters. Rotation is introduced through a second temperature-like parameter. Both axially symmetric and lopsided rotational equilibria are found; the lopsided equilibria precess with the angular velocity that is given by the ratio of the two temperatures. The eccentric stellar disk in the nucleus of the Andromeda galaxy may be an example of a lopsided thermodynamic equilibrium of a rotating black hole star cluster. Stellar-mass black holes occupy highly eccentric orbits in broken-symmetry star clusters, and form flattened disklike configurations in rotating star clusters. They are attracted to orbits that are stationary in the frame of reference rotating with the angular velocity of the cluster. In spherical clusters, stellar-mass black holes' orbits are significantly more eccentric than those of the lighter stars if the temperature is negative and more circular if the temperature is positive. Finally, we note that planets, comets, dark matter particles, and other light bodies tend to form a spherically symmetric nonrotating subcluster with maximum-entropy eccentricity distribution ${s}_{\mathrm{cr}}P(e)=2e$, even if their host cluster is rotating and lopsided.

Mass segregation, a tendency for more massive galaxies to be distributed closer to the cluster center, is naturally expected from dynamical friction, but its presence is still controversial. Using deep optical observations of 14 Abell clusters (KYDISC) and a set of hydrodynamic simulations (YZiCS), we find in some cases a hint of mass segregation inside the virial radius. Segregation is visible more clearly when the massive galaxy fraction is used instead of mean stellar mass. The trend is more significant in the simulations than in the observations. To find out the mechanisms affecting mass segregation, we look into the evolution of individual simulated clusters. We find that the degree of mass segregation is different for different clusters: the trend is visible only for low-mass clusters. We compare the masses of galaxies and their dark halos at the time of infall and at the present epoch to quantify the amount of tidal stripping. We then conclude that satellites that get accreted at earlier epochs, or galaxies in more massive clusters, go through more tidal stripping. These combined effects result in a correlation between the host halo mass and the degree of stellar mass segregation.

When multiple species interact with an electrostatic ion acoustic wave, they can exchange momentum, despite the lack of momentum in the field itself. The resulting force on the electrons can have a curl, and thus give rise to compensating electric fields with curl on magnetohydrodynamic timescales. As a result, a magnetic field can be generated. Surprisingly, in some astrophysical settings, this mechanism can seed magnetic fields with growth rates even larger than through the traditional Biermann battery.

Turbulence is a key process in many fields of astrophysics. Advances in numerical simulations of fluids over the last several decades have revolutionized our understanding of turbulence and related processes such as star formation and cosmic ray propagation. However, data from numerical simulations of astrophysical turbulence are often not made public. We introduce a new simulation-oriented database for the astronomical community: the Catalogue for Astrophysical Turbulence Simulations (CATS), located at www.mhdturbulence.com. CATS includes magnetohydrodynamic (MHD) turbulent box simulation data products generated by the public codes athena++, arepo, enzo, and flash. CATS also includes several synthetic observational data sets, such as turbulent HI data cubes. We also include measured power spectra and three-point correlation functions from some of these data. We discuss the importance of open-source statistical and visualization tools for the analysis of turbulence simulations such as those found in CATS.

We investigated the thermal properties of prominence formation using time series analysis of Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA) data. Here, we report the first time-lag measurements derived from SDO/AIA observations of a prominence and its cavity on the solar limb, made possible by AIA's different wave bands and high time resolution. With our time-lag analysis, which tracks the thermal evolution using emission formed at different temperatures, we find that the prominence cavity exhibited a mixture of heating and cooling signatures. This is in contrast to prior time-lag studies of multiple active regions that chiefly identified cooling signatures and very few heating signatures, which is consistent with nanoflare heating. We also computed time lags for the same pairs of SDO/AIA channels using output from a one-dimensional hydrodynamic model of prominence material forming through thermal nonequilibrium (TNE). We demonstrate that the SDO/AIA time lags for flux tubes undergoing TNE are predicted to be highly complex, changing with time and location along the flux tube, and are consistent with the observed time-lag signatures in the cavity surrounding the prominence. Therefore, the time-lag analysis is a sensitive indicator of the heating and cooling processes in different coronal regions. The time lags calculated for the simulated prominence flux tube are consistent with the behavior deduced from the AIA data, thus supporting the TNE model of prominence formation. Future investigations of time lags predicted by other models for the prominence mass could be a valuable method for discriminating among competing physical mechanisms.

We present rest-frame optical spectroscopic observations of 24 Hot Dust-Obscured Galaxies (Hot DOGs) at redshifts 1.7–4.6 with KECK/NIRES. Our targets are selected, based on their extreme red colors, to be the highest-luminosity sources from the WISE infrared survey. In 20 sources with well-detected emission, we fit the key [O iii], Hβ, Hα, [N ii], and [S ii] diagnostic lines to constrain physical conditions. Of the 17 targets with a clear detection of the [O iii]λ5007 Å emission line, 15 display broad blueshifted and asymmetric line profiles, with widths ranging from 1000 to 8000 km s⁻¹ and blueshifts up to 3000 km s⁻¹. These kinematics provide strong evidence for the presence of massive ionized outflows of up to $8000\ {M}_{\odot }\,{\mathrm{yr}}^{-1}$, with a median of $150\ {M}_{\odot }\,{\mathrm{yr}}^{-1}$. As many as eight sources show optical emission line ratios consistent with vigorous star formation. Balmer-line star formation rates, uncorrected for reddening, range from 30 to 1300 ${M}_{\odot }\,{\mathrm{yr}}^{-1}$, with a median of $50\ {M}_{\odot }\,{\mathrm{yr}}^{-1}$. Estimates of the SFR from Spectral Energy Distribution fitting of mid- and far-infrared photometry suggest significantly higher values. We estimate the central black hole masses to be of order ${10}^{8-10}\,{M}_{\odot }$, assuming the present-day ${M}_{\mathrm{BH}}\mbox{--}{\sigma }_{* }$ relation. The bolometric luminosities and the estimated masses of the central black holes of these galaxies suggest that many of the active galactic nucleus-dominated Hot DOGs are accreting at or above their Eddington limit. The combination of ongoing star formation, massive outflows, and high Eddington ratios suggest Hot DOGs are a transitional phase in galaxy evolution.

We describe a new algorithm for reconstruction of differential emission measures (DEMs) in the solar corona. Although a number of such algorithms currently exist, they can have difficulty converging for some cases and can be complex, slow, or idiosyncratic in their output (i.e., their inversions can have features that are a result of the inversion code and instrument response, not of the solar source); we will document some of these issues in this paper. The new algorithm described here significantly reduces these drawbacks and is particularly notable for its simplicity; it is reproduced here, in full, on a single page. After we describe the algorithm, we compare its performance and fidelity with some prevalent methods. Although presented here for extreme-ultraviolet data, the algorithm is robust and extensible to any other wavelengths (e.g., X-rays) where the DEM treatment is valid.

Low-mass X-ray binaries (LMXBs) containing neutron stars are both extremely luminous and compact, emitting up to ∼10⁶${L}_{\odot }$ within a kilometer-scale boundary layer. This combination allows for easy modulation, motivating an X-ray Search for Extraterrestrial Intelligence. When X-ray lenses with radii $100\mbox{--}1000\ \mathrm{km}$ magnify the LMXB boundary layer, it brightens by a factor of several thousand for a fraction of a second. In addition, there should be occultation events where the neutron star is blocked out. Passive X-ray lenses could require little internal power, and the LMXB light source itself shines for millions of years, with potential for an effective beacon for interstellar communication. A very large number of lenses would be needed to ensure frequent signals in all directions, however, and gathering material to construct them could be very difficult. Avoiding collisions between lenses, aiming them, and building and maintaining their precise shapes pose additional challenges. "Lens flares" of bright LMXBs are easily detectable in the Galaxy, although they would be rare events, occurring perhaps once per decade. Our more sensitive X-ray instruments could detect the eclipses of Galactic LMXBs and possibly intergalactic flares, but it is unlikely they would be observing the LMXB at the right time.

We carry out a suite of simulations of the evolution of cosmic-ray (CR) driven, radiatively cooled cold clouds embedded in hot material, as found in galactic outflows. In such interactions, CRs stream toward the cloud at the Alfvén speed, which decreases dramatically at the cloud boundary, leading to a bottleneck in which pressure builds up in front of the cloud. At the same time, CRs stream along the sides of the cloud, forming a boundary layer where large filaments develop. Shear in this boundary layer is the primary mode of cloud destruction, which is relatively slow in all cases, but slowest in the cases with the lowest Alfvén speeds. Thus, the CR pressure in the bottleneck region has sufficient time to accelerate the cold clouds efficiently. Furthermore, radiative cooling has relatively little impact on these interactions. Our simulations are two-dimensional and limited by a simplified treatment of CR dynamics, the neglect of CR heating, and an idealized magnetic field geometry. Nevertheless, our results suggest that CRs, when acting as the primary source of momentum input, are capable of accelerating clouds to velocities comparable to those observed in galaxy outflows.

We present the first release of a large-scale study of relatively bright (V < 13.5) metal-poor stars observed with the Southern African Large Telescope (SALT), based on high-resolution spectra of 50 stars with a resolving power of R ∼ 40,000 and S/N ∼ 20 per pixel at 4300 Å. The elemental abundances of C, Sr, Ba, and Eu are reported, as well as several α-elements (Mg, Ca, Sc, Ti, and V) and iron-peak elements (Mn, Co, Ni, and Zn). We find a diverse array of abundance patterns, including several consistent with the signatures of carbon-enhanced metal-poor CEMP-i and CEMP-r stars. We find that 15 of 50 (30%) are carbon enhanced (with [C/Fe] > +0.70), and that a large fraction (26 of 50, 52%) are enhanced in r-process elements. Among the r-process-enhanced stars, five are strongly enhanced r-II ([Eu/Fe] > +1.0) stars (two of which are newly discovered) and 21 are newly discovered moderately enhanced r-I (+0.3 ≤  [Eu/Fe]  ≤ +1.0) stars. There are eight stars in our sample that, on the basis of their abundances and kinematics, are possible members of the metal-weak thick-disk population. We also compare our measured abundances to progenitor-enrichment models, and find that the abundance patterns for the majority of our stars can be attributed to a single (rather than multiple) enrichment event.

Gravitational waves from the merger of binary neutron stars (BNSs) are accompanied by electromagnetic counterparts, making it possible to identify the associated host galaxy. In this work, we explore how properties of the hosts relate to the astrophysical processes leading to the mergers. It is thought that the BNS merger rate within a galaxy at a given epoch depends primarily on the galaxy's star formation history, as well as the underlying merger time-delay distribution of the binary systems. The stellar history of a galaxy, meanwhile, depends on the cosmological evolution of the galaxy through time, and is tied to the growth of structure in the universe. We study the hosts of BNS mergers in the context of structure formation by populating the UniverseMachine simulations with gravitational wave (GW) events, based on a simple time-delay model. We find that different time-delay distributions predict different properties of the associated host galaxies, including the distributions of stellar mass, star formation rate, halo mass, and local and large-scale clustering of hosts. Moreover, BNSs merging today with short delay times occur preferentially in hosts with high star formation rates, while those with long delay times live in dense regions within massive halos that have low star formation. We show that with ${ \mathcal O }(10)$ events from current GW detector networks, it is possible to make preliminary distinctions between formation channels which trace stellar mass, halo mass, or star formation rate. We also find that strategies to follow-up GW events with electromagnetic telescopes can be significantly optimized using the clustering properties of their hosts.

Next-generation experiments allow for the possibility of testing the neutrino flavor oscillation model to very high levels of accuracy. Here, we explore the possibility that the dark matter in the current universe is made of two particles, a sterile neutrino and a very light dark matter particle. By using a 3+1 neutrino flavor oscillation model, we study how such a type of dark matter imprints the solar neutrino fluxes, spectra, and survival probabilities of electron neutrinos. The current solar neutrino measurements allow us to define an upper limit for the ratio of the mass of a light dark matter particle m_ϕ and the Fermi constant G_ϕ, such that G_ϕ/m_ϕ must be smaller than 10³⁰G_F eV⁻¹ to be in agreement with current solar neutrino data from the Borexino, Sudbury Neutrino Observatory, and Super-Kamiokande detectors. Moreover, for models with a very small Fermi constant, the amplitude of the time variability must be lower than 3% to be consistent with current solar neutrino data. We also found that solar neutrino detectors like Darwin, able to measure neutrino fluxes in the low-energy range with high accuracy, will provide additional constraints to this class of models that complement the ones obtained from the current solar neutrino detectors.

Studies of solar radio bursts play an important role in understanding the dynamics and acceleration processes behind solar space weather events, and the influence of solar magnetic activity on solar system planets. Similar low-frequency bursts detected from active M-dwarfs are expected to probe their space weather environments and therefore the habitability of their planetary companions. Active M-dwarfs produce frequent, powerful flares which, along with radio emission, reveal conditions within their atmospheres. However, to date, only one candidate solar-like coherent radio burst has been identified from these stars, preventing robust observational constraints on their space weather environment. During simultaneous optical and radio monitoring of the nearby dM5.5e star Proxima Centauri, we detected a bright, long-duration optical flare, accompanied by a series of intense, coherent radio bursts. These detections include the first example of an interferometrically detected coherent stellar radio burst temporally coincident with a flare, strongly indicating a causal relationship between these transient events. The polarization and temporal structure of the trailing long-duration burst enable us to identify it as a type IV burst. This represents the most compelling detection of a solar-like radio burst from another star to date. Solar type IV bursts are strongly associated with space weather events such as coronal mass ejections and solar energetic particle events, suggesting that stellar type IV bursts may be used as a tracer of stellar coronal mass ejections. We discuss the implications of this event for the occurrence of coronal mass ejections from Proxima Cen and other active M-dwarfs.

We present simulations of the capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA) and of a next-generation Very Large Array (ngVLA) to detect and resolve substructures due to terrestrial planets and super-Earths in nearby planet-forming disks. We adopt the results of global 2D hydrodynamical planet–disk simulations that account for the dynamics of gas and dust in a disk with an embedded planet. Our simulations follow the combined evolution of gas and dust for several thousand planetary orbits. We show that long integrations (several tens of hours) with the ngVLA can detect and spatially resolve dust structures due to low-mass rocky planets in the terrestrial planet formation regions of nearby disks (stellocentric radii r = 1–3 au), under the assumption that the disk viscosity in those regions is low (α ≤ 10⁻⁵). ALMA is instead unable to resolve these structures in these disk regions. We also show that high-resolution ngVLA observations separated by several days to a few weeks would allow us to detect the proper motion of the azimuthally asymmetric structures expected in the disk regions of terrestrial planet formation.

We observed the W51 high-mass star-forming complex with the Atacama Large Millimeter/submillimeter Array's longest-baseline configurations, achieving an angular resolution of ∼20 mas, corresponding to a linear resolution of ∼100 au at D_W51 = 5.4 kpc. The observed region contains three high-mass protostars in which the dust continuum emission at 1.3 mm is optically thick up to a radius ≲1000 au and has brightness temperatures ≳200 K. The high luminosity (≳10⁴L_⊙) in the absence of free–free emission suggests the presence of massive stars (M ≳ 20 M_⊙) at the earliest stages of their formation. Our continuum images reveal remarkably complex and filamentary structures arising from compact cores. Molecular emission shows no clear signs of rotation or infall on scales from 150 to 2000 au; we do not detect disks. The central sources drive young (t_dyn ∼ 100 yr), fast (v ∼ 100 km s⁻¹), powerful ($\dot{M}\gt {10}^{-4}$M_⊙ yr⁻¹), collimated outflows. These outflows provide indirect evidence of accretion disks on scales r ≲ 100–500 au (depending on the object). The active outflows are connected to fossil flows that have different orientations on larger spatial scales, implying that the orientations of these small disks change over time. These results together support a variant of an accretion model for high-mass star formation in which massive protostars do not form a large, stable Keplerian disk during their early stages but instead accrete material from multiple massive flows with different angular momentum vectors. This scenario therefore contrasts with the simplified classic paradigm of a stable disk+jet system, which is the standard model for low-mass star formation, and provides experimental confirmation of a multidirectional and unsteady accretion model for massive star formation.

Using extreme-ultraviolet images, we recently proposed a new and alternative formation mechanism for coronal rain along magnetically open field lines due to interchange magnetic reconnection. In this paper we report coronal rain at chromospheric and transition region temperatures originating from the coronal condensations facilitated by reconnection between open and closed coronal loops. For this, we employ the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory. Around 2013 October 19, a coronal rain along curved paths was recorded by IRIS over the southeastern solar limb. Related to this, we found reconnection between a system of higher-lying open features and lower-lying closed loops that occurs repeatedly in AIA images. In this process, the higher-lying features form magnetic dips. In response, two sets of newly reconnected loops appear and retract away from the reconnection region. In the dips, seven events of cooling and condensation of coronal plasma repeatedly occur due to thermal instability over several days, from October 18 to 20. The condensations flow downward to the surface as coronal rain, with a mean interval between condensations of ∼6.6 hr. In the cases where IRIS data were available we found the condensations to cool all the way down to chromospheric temperatures. Based on our observations we suggest that some of the coronal rain events observed at chromospheric temperatures could be explained by the new and alternative scenario for the formation of coronal rain, where the condensation is facilitated by interchange reconnection.

We present arepo-mcrt, a novel Monte Carlo radiative transfer radiation-hydrodynamics (RHD) solver for the unstructured moving-mesh code arepo. Our method is designed for general multiple scattering problems in both optically thin and thick conditions. We incorporate numerous efficiency improvements and noise reduction schemes to help overcome efficiency barriers that typically inhibit convergence. These include continuous absorption and energy deposition, photon weighting and luminosity boosting, local packet merging and splitting, path-based statistical estimators, conservative (face-centered) momentum coupling, adaptive convergence between time steps, implicit Monte Carlo algorithms for thermal emission, and discrete-diffusion Monte Carlo techniques for unresolved scattering, including a novel advection scheme. We primarily focus on the unique aspects of our implementation and discussions of the advantages and drawbacks of our methods in various astrophysical contexts. Finally, we consider several test applications including the levitation of an optically thick layer of gas by trapped infrared radiation. We find that the initial acceleration phase and revitalized second wind are connected via self-regulation of the RHD coupling, such that the RHD method accuracy and simulation resolution each leave important imprints on the long-term behavior of the gas.

Large galaxies may contain an "atmosphere" of hot interstellar X-ray gas, and the temperature and radial density profile of this gas can be used to measure the total mass of the galaxy contained within a given radius r. We use this technique for 102 early-type galaxies with stellar masses M_⋆ > 10¹⁰M_⊙, to evaluate the mass fraction of dark matter (DM) within the fiducial radius r = 5r_e, denoted f₅ = f_DM(5r_e). On average, these systems have a median $\overline{{f}_{5}}\simeq 0.8\mbox{--}0.9$ with a typical galaxy-to-galaxy scatter ±0.15. Comparisons with mass estimates made through the alternative techniques of satellite dynamics (e.g., velocity distributions of globular clusters, planetary nebulae, satellite dwarfs) as well as strong lensing show encouraging consistency over the same range of stellar mass. We find that many of the disk galaxies (S0/SA0/SB0) have a significantly higher mean f₅ than do the pure ellipticals, by Δf₅ ≃ 0.1. We suggest that this higher level may be a consequence of sparse stellar haloes and quieter histories with fewer major episodes of feedback or mergers. Comparisons are made with the Magneticum Pathfinder suite of simulations for both normal and centrally dominant "Brightest Cluster" galaxies. Though the observed data exhibit somewhat larger scatter at a given galaxy mass than do the simulations, the mean level of DM mass fraction for all classes of galaxies is in good first-order agreement with the simulations. Finally, we find that the group galaxies with stellar masses near M_⋆ ∼ 10¹¹M_⊙ have relatively more outliers at low f₅ than in other mass ranges, possibly the result of especially effective AGN feedback in that mass range leading to expansion of their DM halos.

Physical processes such as reignition, enhancement, and fading of active galactic nuclei (AGN) are not entirely understood because the timeline of these events is expected to last many years. However, it is well known that the differences in the energy budget between AGN components, like the optical ionizing region and the mid-infrared (MIR) dust echoes, can be interpreted as a hint of AGN evolution. Here we present a catalog of 88 AGN candidates showing hints of the fading and rising of their activity in the nearby universe. We use AGN scaling relations to select them from an initial sample of 877 candidates using publicly available optical, X-ray, and MIR luminosities. We then use the multiwavelength information to discard sources contaminated with extranuclear emission and those with an X-ray luminosity not well corrected for absorption. We find that 96% of our candidates are fading sources. This result suggests a scenario where the universe had its peak of AGN activity somewhere in the past and is dominated by a fading phase at the present time. Alternatively, the fading phase is longer than the rising phase, which is consistent with galaxy merger simulations. Around 50% of these fading candidates are associated with merging or interacting systems. Finally, we also find the existence of jets in ∼30% of these candidates and that the preferred AGN dust geometry is torus-like instead of wind-like. Our results are compatible with the fading of nuclear activity, expected if they are in an inefficient state.

We have revisited the problem of off-pulse emission in pulsars, where a detailed search for the presence of low-level radio emission outside the pulse window is carried out. The presence of off-pulse emission was earlier reported in two long-period pulsars, PSR B0525+21 and B2046–16, at frequencies below 1 GHz using the Giant Metrewave Radio Telescope (GMRT). However, subsequent studies did not detect off-pulse emission from these pulsars at higher radio frequencies (>1 GHz). We have carefully inspected the analysis scheme used in the earlier detections and found an anomaly with data editing routines used, which resulted in leakage of signal from the on-pulse to the off-pulse region. We show that the earlier detections from PSR B0525+21 and B2046–16 were a result of this leakage. The above analysis scheme has been modified and offline gating has been used to search for off-pulse emission in 21 long-period pulsars (P > 1.2 s) at different observing frequencies of GMRT. The presence of low-level off-pulse emission of the peak flux 0.5 mJy was detected in the brightest pulsar in this list, PSR 0B0628–28, with an off-pulse to average pulsar flux ratio of 0.25%. We suggest that coherent radio emission resulting due to cyclotron resonance near the light cylinder can be a possible source for the off-pulse emission in this pulsar.

We investigate the impact of ram pressure stripping due to the intracluster medium (ICM) on star-forming disk galaxies with a multiphase interstellar medium maintained by strong stellar feedback. We carry out radiation-hydrodynamic simulations of an isolated disk galaxy embedded in a 10¹¹M_⊙ dark matter halo with various ICM winds mimicking the cluster outskirts (moderate) and the central environment (strong). We find that both star formation quenching and triggering occur in ram pressure–stripped galaxies, depending on the strength of the winds. H i and H₂ in the outer galactic disk are significantly stripped in the presence of moderate winds, whereas turbulent pressure provides support against ram pressure in the central region, where star formation is active. Moderate ICM winds facilitate gas collapse, increasing the total star formation rates by ∼40% when the wind is oriented face-on or by ∼80% when it is edge-on. In contrast, strong winds rapidly blow away neutral and molecular hydrogen gas from the galaxy, suppressing star formation by a factor of 2 within ∼200 Myr. Dense gas clumps with n_H ≳ 10 M_⊙ pc⁻² are easily identified in extraplanar regions, but no significant young stellar populations are found in such clumps. In our attempts to enhance radiative cooling by adopting a colder ICM of T = 10⁶ K, only a few additional stars are formed in the tail region, even if the amount of newly cooled gas increases by an order of magnitude.

Using photometry collected with the Zwicky Transient Facility, we are conducting an ongoing survey for binary systems with short orbital periods (${P}_{{\rm{b}}}\lt 1\,\mathrm{hr})$ with the goal of identifying new gravitational-wave sources detectable by the upcoming Laser Interferometer Space Antenna (LISA). We present a sample of 15 binary systems discovered thus far, with orbital periods ranging from 6.91 to 56.35 minutes. Of the 15 systems, seven are eclipsing systems that do not show signs of significant mass transfer. Additionally, we have discovered two AM Canum Venaticorum systems and six systems exhibiting primarily ellipsoidal variations in their lightcurves. We present follow-up spectroscopy and high-speed photometry confirming the nature of these systems, estimates of their LISA signal-to-noise ratios, and a discussion of their physical characteristics.

Radial-velocity monitoring has revealed the presence of moving broad emission lines in some quasars, potentially indicating the presence of a subparsec binary system. Phase-referenced, near-infrared interferometric observations could map out the binary orbit by measuring the photocenter difference between a broad emission line and the hot dust continuum. We show that astrometric data over several years may be able to detect proper motions and accelerations, confirming the presence of a binary and constraining system parameters. The brightness, redshifts, and astrometric sizes of current candidates are well matched to the capabilities of the upgraded Very Large Telescope Interferometer/GRAVITY+ instrument, and we identify a first sample of 10 possible candidates. The astrometric signature depends on the morphology and evolution of hot dust emission in supermassive black hole binary systems. Measurements of the photocenter offset may reveal binary motion whether the hot dust emission region is fixed to the inner edge of the circumbinary disk, or moves in response to the changing irradiation pattern from an accreting secondary black hole.

The processes that shape the extended atmospheres of red supergiants, heat their chromospheres, create molecular reservoirs, drive mass loss, and create dust remain poorly understood. Betelgeuse's V-band "Great Dimming" event of 2019 September/2020 February and its subsequent rapid brightening provides a rare opportunity to study these phenomena. Two different explanations have emerged to explain the dimming; new dust appeared in our line of sight attenuating the photospheric light, or a large portion of the photosphere had cooled. Here we present five years of Wing three-filter (A, B, and C band) TiO and near-IR photometry obtained at the Wasatonic Observatory. These reveal that parts of the photosphere had a mean effective temperature (T_eff) significantly lower than that found by Levesque & Massey. Synthetic photometry from MARCS-model photospheres and spectra reveal that the V band, TiO index, and C-band photometry, and previously reported 4000–6800 Å spectra can be quantitatively reproduced if there are multiple photospheric components, as hinted at by Very Large Telescope (VLT)-SPHERE images in Montargès et al. If the cooler component has ΔT_eff ≥ 250 K cooler than 3650 K, then no new dust is required to explain the available empirical constraints. A coincidence of the dominant short- (∼430 days) and long-period (∼5.8 yr) V-band variations occurred near the time of deep minimum (Guinan et al. 2019a). This is in tandem with the strong correlation of V mag and photospheric radial velocities, recently reported by Dupree et al. (2020b). These suggest that the cooling of a large fraction of the visible star has a dynamic origin related to the photospheric motions, perhaps arising from pulsation or large-scale convective motions.

Supernova (SN) explosions are a major feedback mechanism, regulating star formation in galaxies through their momentum input. We review the observations of SNRs in radiative stages in the Milky Way, to validate theoretical results regarding the momentum/energy injection from a single SN explosion. For seven supernova remnants (SNRs) where we can observe fast-expanding, atomic radiative shells, we show that the shell momentum inferred from H i 21 cm line observations is in the range of (0.5–4.5) × 10⁵M_⊙ km s⁻¹. In two SNRs (W44 and IC 443), shocked molecular gas with momentum comparable to that of the atomic SNR shells has also been observed. We compare the momentum and kinetic/thermal energy of these seven SNRs with the results from 1D and 3D numerical simulations. The observation-based momentum and kinetic energy agree well with the expected momentum/energy input from an SN explosion of ∼10⁵¹ erg. It is much more difficult to use data/model comparisons of thermal energy to constrain the initial explosion energy, however, due to rapid cooling and complex physics at the hot/cool interface in radiative SNRs. We discuss the observational and theoretical uncertainties of these global parameters and explosion energy estimates for SNRs in complex environments.

Dielectronic recombination (DR) rate coefficients for carbon-like ⁴⁰Ca¹⁴⁺ forming nitrogen-like ⁴⁰Ca¹³⁺ have been measured using the electron–ion merged-beam technique at the heavy-ion storage ring CSRm at the Institute of Modern Physics in Lanzhou, China. The measured DR rate coefficients in the energy range from 0 to 92 eV cover most of the DR resonances associated with 2s²2p² → 2s²2p² and 2s²2p² → 2s2p³ core transitions (ΔN = 0). Theoretical calculations of the DR cross sections were carried out by using two different state-of-the-art atomic theoretical techniques, multiconfiguration Breit–Pauli (MCBP) code AUTOSTRUCTURE and relativistic configuration interaction code FAC, to compare with the experimental rate coefficients. The theoretical calculations agree with the experimental results at collision energy higher than 10 eV. However, significant discrepancies of resonance energies and strengths can be found at collision energy below 8 eV. Temperature-dependent plasma recombination rate coefficients were derived from the measured DR rate coefficients in the energy range from 0.1 to 1000 eV and compared with the recommended atomic data from the literature. The theoretical data of Gu et al. and Zatsarinny et al. are 30% lower than the experimental results at the temperatures of photoionized plasmas, but have a very good agreement at the temperatures of collisionally ionized plasmas. Other previously published theoretical data of Jacobs et al. and Mazzotta et al. by using Burgess formula and LS-coupling calculations significantly underestimate the plasma rate coefficients in the low temperature range. The present results comprise a set of benchmark data suitable for astrophysical modeling.

To understand how planetary spin evolves and traces planet formation processes, we measure rotational line broadening in eight planetary-mass objects (PMOs) of various ages (1–800 Myr) using near-infrared high-resolution spectra from NIRSPEC/Keck. Combining these with published rotation rates, we compile 27 PMO spin velocities, 16 of which derive from our NIRSPEC/Keck program. Our data are consistent with spin velocities v scaling with planetary radius R as v ∝ 1/R. We conclude that spin angular momentum is conserved as objects cool and contract over the sampled age range. The PMOs in our sample spin at rates that are approximately an order of magnitude below their break-up values, consistent with the hypothesis that they were spun down by magnetized circum-PMO disks (CPDs) during the formation era at ages ≲a few Myr. There is a factor of 4–5 variation in spin velocity that has yet to be understood theoretically. It also remains to be seen whether spin evolves on timescales ≳1 Gyr for PMOs, as it does for stars and high-mass brown dwarfs emitting magnetized winds.

Close white dwarf binaries consisting of a white dwarf and an A-, F-, G-, or K-type main-sequence star, henceforth close WD+AFGK binaries, are ideal systems to understand the nature of type Ia supernovae progenitors and to test binary evolution models. In this work we identify 775 WD+AFGK candidates from TGAS (The Tycho-Gaia Astrometric Solution) and Gaia Data Release 2 (DR2), a well-defined sample of stars with available parallaxes, and we measure radial velocities (RVs) for 275 of them with the aim of identifying close binaries. The RVs have been measured from high-resolution spectra obtained at the Xinglong 2.16 m Telescope and the San Pedro Mártir 2.12 m Telescope and/or from available LAMOST DR6 (low-resolution) and RAVE DR5 (medium-resolution) spectra. We identify 23 WD+AFGK systems displaying more than 3σ RV variation among 151 systems for which the measured values are obtained from different nights. Our WD+AFGK binary sample contains both AFGK dwarfs and giants, with a giant fraction ∼43%. The close binary fractions we determine for the WD+AFGK dwarf and giant samples are ≃24% and ≃15%, respectively. We also determine the stellar parameters (i.e., effective temperature, surface gravity, metallicity, mass, and radius) of the AFGK companions with available high-resolution spectra. The stellar parameter distributions of the AFGK companions that are members of close and wide binary candidates do not show statistically significant differences.

The aim of this paper is to investigate the impact of energy transfer on the evolution of contact binaries. Horizontal turbulence is triggered by the differential rotation induced by meridional circulation, which is a direct consequence of nonuniform heating at the inner critical Roche lobes because of strong rotational and tidal distortions. Thermal energy is transferred by the horizontal turbulence from the more massive star to the less massive one, and horizontal turbulence can be responsible for the redistribution of what is a significant fraction of the total core luminosity. The secondary becomes overluminous and oversized owing to energy transfer from the companion star, whereas the primary shifts toward smaller luminosity and is undersized. The convective regions for primaries are enlarged by the improved radiative temperature gradient. The main region for energy transport is located at the bottom of the common envelope because of a higher local density and enthalpy difference. One can find that thermal structure can be disturbed and display periodic thermal relaxation oscillations between the semidetached stage and the contact stage. W-type W UMa contact binaries acquire efficient energy transfer, which can cause the temperature of secondaries to exceed that of the primaries. However, angular momentum loss owing to nonconservative mass transfer can make the system maintain shallow contact and not evolve from overcontact to semidetached configurations, and the system may appear as an A-type W UMa contact binary.

How massive early-type galaxies (ETGs) assembled their mass, on which timescales the star formation quenched, and when their supersolar metallicity has been established are still open and debated issues. Thanks to very deep spectroscopic observations carried out at the Large Binocular Telescope, we simultaneously measured stellar age, metallicity, and velocity dispersion for C1-23152, an ETG at redshift z = 3.352, corresponding to an epoch when the universe was ∼1.8 Gyr old. The analysis of its spectrum shows that this galaxy, hosting an active galactic nucleus (AGN), formed and assembled ∼2 × 10¹¹M_⊙, shaping its morphology within the ∼600 Myr preceding the observations, since z ∼ 4.6. The stellar population has a mean mass-weighted age of ${400}_{-70}^{+30}$ Myr, and it is formed between ∼600 and ∼150 Myr before the observed epoch, the latter being the time since quenching. Its high stellar velocity dispersion, σ_e = 409 ± 60 km s⁻¹, confirms the high mass (M_dyn = 2.2 (±0.4) × 10¹¹M_⊙) and the high mass density (${{\rm{\Sigma }}}_{e}^{{M}^{* }}$ = Σ_1kpc = 3.2 (±0.7) × 10¹⁰M_⊙ kpc⁻²), suggesting a fast dissipative process at its origin. The analysis points toward a supersolar metallicity, [Z/H] = 0.25${}_{-0.10}^{+0.006}$, in agreement with the above picture, suggesting a star formation efficiency much higher than the replenishment time. However, subsolar-metallicity values cannot be firmly ruled out by our analysis. Quenching must have been extremely efficient to reduce the star formation to SFR < 6.5 M_⊙ yr⁻¹ in less than 150 Myr. This could be explained by the presence of the AGN, even if a causal relation cannot be established from the data. C1-23152 has the same stellar and physical properties of the densest ETGs in the local universe of comparable mass, suggesting that they are C1-23152-like galaxies that evolved to z = 0 unperturbed.

Broadband X-ray spectroscopy of the X-ray emission produced in the coronae of active galactic nuclei (AGNs) can provide important insights into the physical conditions very close to their central supermassive black holes. The temperature of the Comptonizing plasma that forms the corona is manifested through a high-energy cutoff that has been difficult to directly constrain even in the brightest AGN because it requires high-quality data at energies above 10 keV. In this paper we present a large collection of coronal cutoff constraints for obscured AGNs based on a sample of 130 AGNs selected in the hard X-ray band with Swift/BAT and observed nearly simultaneously with NuSTAR and Swift/XRT. We find that under a reasonable set of assumptions regarding partial constraints the median cutoff is well constrained to 290 ± 20 keV, where the uncertainty is statistical and given at the 68% confidence level. We investigate the sensitivity of this result to our assumptions and find that consideration of various known systematic uncertainties robustly places the median cutoff between 240 and 340 keV. The central 68% of the intrinsic cutoff distribution is found to be between about 140 and 500 keV, with estimated uncertainties of 20 and 100 keV, respectively. In comparison with the literature, we find no clear evidence that the cutoffs in obscured and unobscured AGNs are substantially different. Our analysis highlights the importance of carefully considering partial and potentially degenerate constraints on the coronal high-energy cutoff in AGNs.

How do active galactic nuclei with low optical luminosities produce powerful radio emission? Recent studies of active galactic nuclei with moderate radio and low optical luminosities (Fanaroff & Riley class I, FR I) searching for broad nuclear emission lines in polarized light, as predicted by some active galactic nucleus unification models, have found heterogeneous results. These models typically consist of a central engine surrounded by a torus of discrete dusty clouds. These clouds would absorb and scatter optical emission, blocking broad nuclear emission lines, and reradiate in mid-infrared. Some scattered broad-line emission may be observable, depending on geometry, which would be polarized. We present a wide-band infrared spectroscopic analysis of 10 nearby FR I radio galaxies to determine whether there is significant emission from a dusty obscuring structure. We used Markov Chain Monte Carlo algorithms to decompose Spitzer/IRS spectra of our sample. We constrained the wide-band behavior of our models with photometry from the Two Micron All Sky Survey, Spitzer/IRAC, Spitzer/MIPS, and Herschel/SPIRE. We find that one galaxy is best fit by a clumpy torus and three others show some thermal mid-infrared component. This suggests that in those three there is likely some obscuring dust structure that is inconsistent with our torus models and there must be some source of photons heating the dust. We conclude that 40% of our FR I radio galaxies show evidence of obscuring dusty material, possibly some other form of hidden broad-line nucleus, but only 10% favor the clumpy torus model specifically.

Recent developments in astronomical radio telescopes opened new opportunities in imaging and spectroscopy of solar radio bursts at subsecond timescales. Imaging in narrow frequency bands has revealed temporal variations in the positions and source sizes that do not fit into the standard picture of type III solar radio bursts, and require a better understanding of radio-wave transport. In this paper, we utilize 3D Monte Carlo ray-tracing simulations that account for the anisotropic density turbulence in the inhomogeneous solar corona to quantitatively explain the image dynamics at the fundamental (near plasma frequency) and harmonic (double) plasma emissions observed at ∼32 MHz. Comparing the simulations with observations, we find that anisotropic scattering from an instantaneous emission point source can account for the observed time profiles, centroid locations, and source sizes of the fundamental component of type III radio bursts (generated where f_pe ≈ 32 MHz). The best agreement with observations is achieved when the ratio of the perpendicular to the parallel component of the wavevector of anisotropic density turbulence is around 0.25. Harmonic emission sources observed at the same frequency (∼32 MHz, but generated where f_pe ≈ 16 MHz) have apparent sizes comparable to those produced by the fundamental emission, but demonstrate a much slower temporal evolution. The simulations of radio-wave propagation make it possible to quantitatively explain the variations of apparent source sizes and positions at subsecond timescales both for the fundamental and harmonic emissions, and can be used as a diagnostic tool for the plasma turbulence in the upper corona.

We present a study of far-UV (FUV) bright horizontal branch (HB) stars to understand the peculiarities seen in the HB sequence of the globular cluster NGC 1851, using ground- and space-based multiwavelength data. Optical and UV color–magnitude diagrams are used to classify HB stars and their membership from Hubble Space Telescope and Gaia DR2 data. The spectral energy distributions (SEDs) of the hot HB stars located from the core to tidal radii are constructed. The SEDs reveal that the HB stars near the "Grundahl jump" show a decrease in the FUV flux when atmospheric models of cluster metallicity are used for fitting, but a better fit is found with higher-metallicity models, as expected due to atmospheric diffusion. We report on four particularly interesting extreme HB (EHB) stars, two each in the inner and outer regions. We detect a subluminous EHB and "blue-hook" candidates with temperatures T_eff  ∼  25,000 K and 31,000 K, respectively. We found an EHB star (T_eff  ∼  17,000 K) with a radius that lies between the BHB and normal EHB stars. The most peculiar of our EHB stars (T_eff  ∼  28,000 K) is found to be a photometric binary to a blue straggler star (BSS; T_eff  ∼  7000 K), which is an important target for spectroscopic study. This discovery of the candidate EHB+BSS binary system could help to explain the mass loss in the red giant branch phase, leading to the formation of EHB stars.

Here, we present the results of the first solid-phase ex situ analysis of cosmic grain analogs produced at low temperature (<200 K) in the NASA Ames COsmic SImulation Chamber (COSmIC) from small hydrocarbon precursors, methane (CH₄) and acetylene (C₂H₂), seeded in an argon supersonic jet expansion and submitted to a plasma discharge. The plasma-induced chemical reactions, initiated between the precursor molecules and their atomic and molecular fragments, radicals and ions, produce larger molecules and eventually solid particles that are collected in situ under controlled conditions. Scanning electron microscopy (SEM) imaging was used to provide insight on the morphology and growth structure of the grains produced in COSmIC, and to investigate how the precursors used to produce the grains affect these parameters. This SEM study has shown that under identical experimental conditions with fixed physical and chemical parameters (precursor density, temperature, energy, and reaction time), heavier precursors in the initial mixture produce larger grains and in larger quantity, most likely as a result of a more complex chemistry: most of the grains produced in the Ar/CH₄ (95:5) gas mixture ranged from 15 to 385 nm in diameter with an average density of 2.1 grains μm⁻², while the grains produced in the Ar/C₂H₂ (95:5) gas mixture ranged from 40 to 650 nm with a density of 3.5 grains μm⁻². Changes in the morphology were also observed, with grains produced from acetylene (C₂H₂) precursors tending to be more spherical than grains produced from methane (CH₄) precursors. This change in morphology could be associated with different stages of growth formation at low temperature from a more "planar" growth at first, followed by coagulation into more spherical particles. This study demonstrates that the COSmIC experimental setup can be used to investigate carbon grain formation from small gas-phase molecular precursors at low temperature (<200 K), i.e., under a temperature regime that is representative of the dust condensation zone and outer region of circumstellar envelopes.

We present the distance-calibrated spectral energy distribution (SED) of the d/sdL7 SDSS J14162408+1348263A (J1416A) and an updated SED for SDSS J14162408+1348263B (J1416B). We also present the first retrieval analysis of J1416A using the Brewster retrieval code base and the second retrieval of J1416B. We find that the primary is best fit by a nongray cloud opacity with a power-law wavelength dependence but is indistinguishable between the type of cloud parameterization. J1416B is best fit by a cloud-free model, consistent with the results from Line et al. Most fundamental parameters derived via SEDs and retrievals are consistent within 1σ for both J1416A and J1416B. The exceptions include the radius of J1416A, where the retrieved radius is smaller than the evolutionary model-based radius from the SED for the deck cloud model, and the bolometric luminosity, which is consistent within 2.5σ for both cloud models. The pair's metallicity and carbon-to-oxygen ratio point toward formation and evolution as a system. By comparing the retrieved alkali abundances while using two opacity models, we are able to evaluate how the opacities behave for the L and T dwarf. Lastly, we find that relatively small changes in composition can drive major observable differences for lower-temperature objects.

The connection between galaxy star formation rate (SFR) and dark matter (DM) is of paramount importance for the extraction of cosmological information from next-generation spectroscopic surveys that will target emission line star-forming galaxies. Using publicly available mock galaxy catalogs obtained from various semianalytic models (SAMs), we explore the SFR–DM connection in relation to the speed-from-light method for inferring the growth rate, f, from luminosity/SFR shifts. Emphasis is given to the dependence of the SFR distribution on the environmental density on scales of 10–100 s Mpc. We show that the application of the speed-from-light method to a Euclid-like survey is not biased by environmental effects. In all models, the precision on the measured β = f/b parameter is σ_β ≲ 0.17 at z = 1. This translates into errors of σ_f ∼ 0.22 and ${\sigma }_{(f{\sigma }_{8})}\sim 0.1$ without invoking assumptions on the mass power spectrum. These errors are in the same ballpark as recent analyses of the redshift space distortions in galaxy clustering. In agreement with previous studies, the bias factor, b, is roughly a scale-independent, constant function of the SFR for star-forming galaxies. Its value at z = 1 ranges from 1.2 to 1.5 depending on the SAM recipe. Although in all SAMs, denser environments host galaxies with higher stellar masses, the dependence of the SFR on the environment is more involved. In most models, the SFR probability distribution is skewed to larger values in denser regions. One model exhibits an inverted trend, where high SFR is suppressed in dense environments.

The recent discovery by LIGO/Virgo of a merging binary having a $\sim 23\,{M}_{\odot }$ black hole and a $\sim 2.6\,{M}_{\odot }$ compact companion has triggered a debate regarding the nature of the secondary, which falls into the so-called mass gap. Here we explore some consequences of the assumption that the secondary was a neutron star (NS). We show with concrete examples of heretofore viable equations of state (EOSs) that rapid uniform rotation may neither be necessary for some EOSs nor sufficient for others to explain the presence of an NS. Absolute upper limits for the maximum mass of a spherical NS derived from GW170817 already suggest that this unknown compact companion might be a slowly or even a nonrotating NS. However, several soft NS EOSs favored by GW170817 with maximum spherical masses $\lesssim 2.1\,{M}_{\odot }$ cannot be invoked to explain this object, even allowing for maximum uniform rotation. By contrast, sufficiently stiff EOSs that yield $2.6\,{M}_{\odot }$ NSs that are slowly rotating or, in some cases, nonrotating, and are compatible with GW170817 and the results of the Neutron Star Interior Composition Explorer (NICER), can account for the black hole companion.

Previous analyses of large databases of Milky Way stars have revealed the stellar disk of our Galaxy to be warped and that this imparts a strong signature on the kinematics of stars beyond the solar neighborhood. However, due to the limitation of accurate distance estimates, many attempts to explore the extent of these Galactic features have generally been restricted to a volume near the Sun. By combining the Gaia DR2 astrometric solution, StarHorse distances, and stellar abundances from the APOGEE survey, we present the most detailed and radially expansive study yet of the vertical and radial motions of stars in the Galactic disk. We map velocities of stars with respect to their Galactocentric radius, angular momentum, and azimuthal angle and assess their relation to the warp. A decrease in vertical velocity is discovered at Galactocentric radius R = 13 kpc and angular momentum L_z = 2800 kpc km s⁻¹. Smaller ripples in vertical and radial velocity are also discovered superposed on the main trend. We also discovered that trends in the vertical velocity with azimuthal angle are not symmetric about the peak, suggesting the warp is lopsided. To explain the global trend in vertical velocity, we built a simple analytical model of the Galactic warp. Our best fit yields a starting radius of ${8.87}_{-0.09}^{+0.08}\ \mathrm{kpc}$ and precession rate of ${13.57}_{-0.18}^{+0.20}\ \mathrm{km}\ {{\rm{s}}}^{-1}\ {\mathrm{kpc}}^{-1}$. These parameters remain consistent across stellar age groups, a result that supports the notion that the warp is the result of an external, gravitationally induced phenomenon.

In the universe's most massive galaxies, active galactic nucleus (AGN) feedback appears to limit star formation. The accumulation of cold gas near the central black hole fuels powerful AGN outbursts, keeping the ambient medium in a state marginally unstable to condensation and formation of cold gas clouds. However, the ability of that mechanism to self-regulate may depend on numerous environmental factors, including the depth of the potential well and the pressure of the surrounding circumgalactic medium (CGM). Here we present a suite of numerical simulations, with halo mass ranging from 2 × 10¹²M_⊙ to 8 × 10¹⁴M_⊙, exploring the dependence of AGN feedback on those environmental factors. We include the spatially extended mass and energy input from the massive galaxy's old stellar population capable of sweeping gas out of the galaxy if the confining CGM pressure is sufficiently low. Our simulations show that this feedback mechanism is tightly self-regulating in a massive galaxy with a deep central potential and low CGM pressure, permitting only small amounts of multiphase gas to accumulate and allowing no star formation. In a similar-mass galaxy with shallower central potential and greater CGM pressure the feedback mechanism is more episodic, producing extended multiphase gas and allowing small rates of star formation (∼0.1 M_⊙ yr⁻¹). At the low-mass end, the mechanism becomes implausibly explosive, perhaps because the CGM initially has no angular momentum, which would have reduced the amount of condensed gas capable of fueling feedback.

We present X-SHOOTER near-IR spectroscopy of a large sample of 38 luminous (M₁₄₅₀ = −29.0 to −24.4) quasars at 5.78 < z < 7.54, which have complementary [C ii]_158μm observations from ALMA. This X-SHOOTER/ALMA sample provides us with the most comprehensive view of reionization-era quasars to date, allowing us to connect the quasar properties with those of its host galaxy. In this work we introduce the sample, discuss data reduction and spectral fitting, and present an analysis of the broad emission line properties. The measured Fe ii/Mg ii flux ratio suggests that the broad-line regions of all quasars in the sample are already enriched in iron. We also find the Mg ii line to be on average blueshifted with respect to the [C ii] redshift with a median of −391 km s⁻¹. A significant correlation between the Mg ii−[C ii]_158μm and C iv−[C ii]_158μm velocity shifts indicates a common physical origin. Furthermore, we fRequently detect large C iv–Mg ii emission line velocity blueshifts in our sample with a median value of −1848 km s⁻¹. While we find all other broad emission line properties not to be evolving with redshift, the median C iv–Mg ii blueshift is much larger than found in low-redshift, luminosity-matched quasars (−800 km s⁻¹). Dividing our sample into two redshift bins, we confirm an increase of the average C iv–Mg ii blueshift with increasing redshift. Future observations of the rest-frame optical spectrum with the James Webb Space Telescope will be instrumental in further constraining the possible evolution of quasar properties in the epoch of reionization.

We perform a systematic search for high-redshift ($z\,\gt$ 1.5) extreme variability quasars (EVQs) using repeat spectra from the Sixteenth Data Release of the Sloan Digital Sky Survey, which provides a baseline spanning up to ∼18 yr in the observed frame. We compile a sample of 348 EVQs with a maximum continuum variability at rest frame 1450 Å of more than 100% (i.e., δV ≡ (Max − Min)/Mean > 1). The EVQs show a range of emission-line variability, including 23 where at least one line in our redshift range disappears below detectability, which can then be seen as analogous to low-redshift changing-look quasars (CLQs). Importantly, spurious CLQs caused by problematic SDSS spectral flux calibration, e.g., fiber-drop issue, have been rejected. The similar properties (e.g., continuum/line, difference-composite spectra and Eddington ratio) of normal EVQs and CLQs imply that they are basically the same physical population with analogous intrinsic variability mechanisms, as a tail of a continuous distribution of normal quasar properties. In addition, we find no reliable evidence (≲1σ) to support that CLQs are a subset of EVQs with less efficient accretion. Finally, we also confirm the antibreathing of C iv (i.e., the line width increases as luminosity increases) in EVQs and find that in addition to the ∼0.4 dex systematic uncertainty in single-epoch C iv virial black hole mass estimates, an extra scatter of ∼0.3 dex will be introduced by extreme variability.

Recent discussions about supernova magnitude evolution have raised doubts about the robustness of the late-universe acceleration. In a previous letter, Huang did a null test of the cosmic acceleration by using a Parameterization based on the cosmic Age (PAge), which covers a broad class of cosmological models including the standard Λ cold dark matter model and its many extensions. In this work, we continue to explore the cosmic expansion history with the PAge approximation. Using baryon acoustic oscillations (without a CMB prior on the acoustic scale), gravitational strong lens time delay, and passively evolving early galaxies as cosmic chronometers, we obtain  ≳ 4σ detections of cosmic acceleration for both flat and nonflat PAge universes. In the nonflat case, we find a novel ≳3σ tension between the spatial curvatures derived from baryon acoustic oscillations and strong lens time delay. Implications and possible systematics are discussed.

Since gravitational waves (GWs) propagate freely through a perfect fluid, coalescing compact binary systems as standard sirens allow us to measure the luminosity distance directly and provide distance measurements unaffected by the cosmic opacity. DECi-hertz Interferometer Gravitational-wave Observatory (DECIGO) is a future Japanese space gravitational-wave antenna sensitive to frequency range between target frequencies of the Laser Interferometric Space Antenna and ground-based detectors. Combining the predicted future GW observations from DECIGO and three current popular astrophysical probes (H ii regions, SNe Ia Pantheon sample, quasar sample) in electromagnetic domains, one would be able to probe the opacity of the universe at different redshifts. In this paper, we show that the cosmic-opacity parameter can be constrained to a high precision (Δ ∼ 10⁻²) out to high redshifts (z ∼ 5). In order to reconstruct the evolution of cosmic opacity without assuming any particular functional form of it, the cosmic-opacity tests should be applied to individual redshift bins independently. Therefore, we also calculate the optical depth at individual redshifts and averaged τ(z) within redshift bins. Our findings indicate that, compared with the results obtained from the H ii galaxies and Pantheon SNe Ia, there is an improvement in precision when the quasar sample is considered. While nonzero optical depth is statistically significant only for redshift ranges 0 < z < 0.5, 1 < z < 2, and 2.5 < z < 3.5, such a tendency is different from that obtained in the framework of its parameterized form. Therefore, the importance of a cosmic-opacity test without a prescribed phenomenological function should be emphasized.

We present a large sample of 2.5–38 μm galaxy spectra drawn from a cross-archival comparison in the AKARI–Spitzer Extragalactic Spectral Survey, and investigate a subset of 113 star-forming galaxies with prominent polycyclic aromatic hydrocarbon (PAH) emission spanning a wide range of star formation properties. With AKARI's extended 2.5–5 μm wavelength coverage, we self-consistently model for the first time all PAH emission bands using a modified version of Pahfit. We find L_{PAH 3.3}/L_IR ∼ 0.1%, and the 3.3 μm PAH feature contributes ∼1.5%–3% to the total PAH power—somewhat less than earlier dust models have assumed. We establish a calibration between 3.3 μm PAH emission and star formation rate, but also find regimes where it loses reliability, including at high luminosity and low metallicity. The 3.4 μm aliphatic emission and a broad plateau feature centered at 3.47 μm are also modeled. As the PAH feature with the shortest wavelength, the one at 3.3 μm is susceptible to attenuation, leading to differences of a factor of ∼3 in the inferred star formation rate at high obscuration with different assumed attenuation geometries. Surprisingly, L_{PAH 3.3}/L_{Σ PAH} shows no sign of decline at high luminosities, and the low-metallicity dwarf galaxy II Zw 40 exhibits an unusually strong 3.3 μm band; both results suggest either that the smallest PAHs are better able to survive under intense radiation fields than presumed, or that PAH emission is shifted to shorter wavelengths in intense and high-energy radiation environments. A photometric surrogate for 3.3 μm PAH luminosity using JWST/NIRCam is provided and found to be highly reliable at low redshift.

Optical spectroscopic observations of white dwarf stars selected from catalogs based on the Gaia DR2 database reveal nine new gaseous debris disks that orbit single white dwarf stars, about a factor of 2 increase over the previously known sample. For each source we present gas emission lines identified and basic stellar parameters, including abundances for lines seen with low-resolution spectroscopy. Principle discoveries include (1) the coolest white dwarf (T_eff ≈ 12,720 K) with a gas disk; this star, WD0145+234, has been reported to have undergone a recent infrared outburst; (2) co-location in velocity space of gaseous emission from multiple elements, suggesting that different elements are well mixed; (3) highly asymmetric emission structures toward SDSS J0006+2858, and possibly asymmetric structures for two other systems; (4) an overall sample composed of approximately 25% DB and 75% DA white dwarfs, consistent with the overall distribution of primary atmospheric types found in the field population; and (5) never-before-seen emission lines from Na in the spectra of Gaia J0611−6931, semi-forbidden Mg, Ca, and Fe lines toward WD 0842+572, and Si in both stars. The currently known sample of gaseous debris disk systems is significantly skewed toward northern hemisphere stars, suggesting a dozen or so emission line stars are waiting to be found in the southern hemisphere.

We investigated the 65, 71, 79, 84, 119, and 163 μm OH doublets of 178 local (0 < z < 0.35) galaxies. They were observed using the Herschel/Photoconductor Array Camera and Spectrometer, including Seyfert galaxies, low-ionization nuclear emission-line regions, and star-forming galaxies. We observe these doublets exclusively in absorption (OH71), primarily in absorption (OH65, OH84), mostly in emission (OH79), only in emission (OH163), and an approximately even mix of the both (OH119). In 19 galaxies we find P Cygni or reverse P Cygni line profiles in the OH doublets. We use several galaxy observables to probe spectral classification, brightness of a central active galactic nucleus (AGN)/starburst component, and radiation field strength. We find that OH79, OH119, and OH163 are more likely to display strong emission for bright, unobscured AGNs. For less luminous, obscured AGNs and nonactive galaxies, we find populations of strong absorption (OH119), weaker emission (OH163), and a mix of weak emission and weak absorption (OH79). For OH65, OH71, and OH84, we do not find significant correlations with the observables listed above. For OH79 and OH119, we find relationships with both the 9.7 μm silicate feature and Balmer decrement dust extinction tracers in which more dust leads to weaker emission/stronger absorption. The origin of emission for the observed OH doublets, whether from collisional excitation or from radiative pumping by infrared photons, is discussed.

Using the Zwicky Transient Facility alert stream, we are conducting a large spectroscopic campaign to construct a complete, volume-limited sample of transients brighter than 20 mag, and coincident within 100'' of galaxies in the Census of the Local Universe catalog. We describe the experiment design and spectroscopic completeness from the first 16 months of operations, which have classified 754 supernovae. We present results from a systematic search for calcium-rich gap transients in the sample of 22 low-luminosity (peak absolute magnitude M > −17), hydrogen-poor events found in the experiment. We report the detection of eight new events, and constrain their volumetric rate to ≳15% ± 5% of the SN Ia rate. Combining this sample with 10 previously known events, we find a likely continuum of spectroscopic properties ranging from events with SN Ia–like features (Ca-Ia objects) to those with SN Ib/c–like features (Ca-Ib/c objects) at peak light. Within the Ca-Ib/c events, we find two populations distinguished by their red (g − r ≈ 1.5 mag) or green ($g-r\approx 0.5$ mag) colors at the r-band peak, wherein redder events show strong line blanketing features and slower light curves (similar to Ca-Ia objects), weaker He lines, and lower [Ca ii]/[O i] in the nebular phase. We find that all together the spectroscopic continuum, volumetric rates, and striking old environments are consistent with the explosive burning of He shells on low-mass white dwarfs. We suggest that Ca-Ia and red Ca-Ib/c objects arise from the double detonation of He shells, while green Ca-Ib/c objects are consistent with low-efficiency burning scenarios like detonations in low-density shells or deflagrations.

Helioseismic inferences of large-scale flows in the solar interior necessitate accounting for the curvature of the Sun, both in interpreting systematic trends introduced in measurements as well as the sensitivity kernel that relates photospheric measurements to subsurface flow velocities. Additionally, the inverse problem that relates measurements to model parameters needs to be well posed to obtain accurate inferences, which necessitates a sparse set of parameters. Further, the sensitivity functions need to be computationally easy to evaluate. In this work, we address these issues by demonstrating that the sensitivity kernels for flow velocities may be computed efficiently on the basis of vector spherical harmonics. We are also able to account for line-of-sight projections in Doppler measurements, as well as center-to-limb differences in line-formation heights. We show that given the assumed spherical symmetry of the background model, it is often cheap to simultaneously compute the kernels for pairs of observation points that are related by a rotation. Such an approach is therefore particularly well suited to inverse problems for large-scale flows in the Sun, such as meridional circulation.

Recent work has shown that Milky Way–mass galaxies display an incredible range of stellar halo properties, yet the origin of this diversity is unclear. The nearby galaxy M81—currently interacting with M82 and NGC 3077—sheds unique light on this problem. We present a Subaru Hyper Suprime-Cam survey of the resolved stellar populations around M81, revealing M81's stellar halo in never-before-seen detail. We resolve the halo to unprecedented V-band equivalent surface brightnesses of 33 mag arcsec ⁻² and produce the first-ever global stellar mass density map for a Milky Way–mass stellar halo outside of the Local Group. Using the minor axis, we confirm M81's halo as one of the lowest mass and metal poorest known (M_⋆ ≃ 1.16 × 10⁹M_⊙, [Fe/H] ≃ −1.2)—indicating a relatively quiet prior accretion history. Yet, our global halo census finds that tidally unbound material from M82 and NGC 3077 provides a substantial infusion of metal-rich material (M_⋆ ≃ 5.4 × 10⁸M_⊙, [Fe/H] ≃−0.9). We further show that, following the accretion of its massive satellite M82 (and the LMC-like NGC 3077), M81 will host one of the most massive and metal-rich stellar halos in the nearby universe. Thus, the saga of M81: following a passive history, M81's merger with M82 will completely transform its halo from a low-mass, anemic halo rivaling the Milky Way, to a metal-rich behemoth rivaled only by systems such as M31. This dramatic transformation indicates that the observed diversity in stellar halo properties is primarily driven by diversity in the largest mergers these galaxies have experienced.

We report the observational findings of the Sh2-112 H ii region by using the multiwavelength data analysis ranging from optical to radio wavelengths. This region is powered by the massive O8V-type star BD +45 3216. The surface density distribution and minimum spanning tree analyses of the young stellar object (YSO) candidates in the region reveal their groupings toward the western periphery of the H ii region. A GMRT radio continuum emission peak is found toward the northwest boundary of the H ii region and is investigated as a compact/ultracompact H ii region candidate powered by a B0–B0.5-type star. Toward the southwest direction, a prominent curved rim-like structure is found in the Hα image and GMRT radio continuum maps, where the H₂ and ¹³CO emission is also observed. These results suggest the existence of the ionized boundary layer (IBL) on the surface of the molecular cloud. This IBL is found to be overpressured with respect to the internal pressure of the surrounding molecular cloud. This implies that the shocks are propagating/propagated into the molecular cloud, and the young stars identified within it are likely triggered due to the massive star. It is also found that this region is ionization-bounded toward the west and density-bounded toward the east. Based on the distribution of the ionized gas, molecular material, and YSO candidates, we propose that the Sh2-112 H ii region is a good candidate for the blister-type H ii region that has been evolved on the surface of a cylindrical molecular cloud.

The physical mechanism driving mass ejection during a nova eruption is still poorly understood. Possibilities include ejection in a single ballistic event, a common-envelope interaction, a continuous wind, or some combination of these processes. Here, we present a study of 12 Galactic novae, for which we have premaximum high-resolution spectroscopy. All 12 novae show the same spectral evolution. Before optical peak, they show a slow P Cygni component. After peak, a fast component quickly arises, while the slow absorption remains superimposed on top of it, implying the presence of at least two physically distinct flows. For novae with high-cadence monitoring, a third, intermediate-velocity component is also observed. These observations are consistent with a scenario where the slow component is associated with the initial ejection of the accreted material and the fast component with a radiation-driven wind from the white dwarf. When these flows interact, the slow flow is swept up by the fast flow, producing the intermediate component. These colliding flows may produce the γ-ray emission observed in some novae. Our spectra also show that the transient heavy-element absorption lines seen in some novae have the same velocity structure and evolution as the other lines in the spectrum, implying an association with the nova ejecta rather than a preexisting circumbinary reservoir of gas or material ablated from the secondary. While this basic scenario appears to qualitatively reproduce multiwavelength observations of classical novae, substantial theoretical and observational work is still needed to untangle the rich diversity of nova properties.

We report on the discovery of the companion star to the millisecond pulsar J1631+3627F in the globular cluster M13. By means of a combination of optical and near-UV high-resolution observations obtained with the Hubble Space Telescope, we identified the counterpart at the radio source position. Its location in the color–magnitude diagrams reveals that the companion star is a faint ($V\approx 24.3$) He-core white dwarf. We compared the observed companion magnitudes with those predicted by state-of-the-art binary evolution models and found out that it has a mass of $0.23\pm 0.03\,{M}_{\odot }$, a radius of ${0.033}_{-0.005}^{+0.004}\,{R}_{\odot }$, and a surface temperature of $11,{500}_{-1300}^{+1900}$ K. Combining the companion mass with the pulsar mass function is not enough to determine the orbital inclination and the neutron star mass; however, the last two quantities become correlated: we found that either the system is observed at a low-inclination angle, or the neutron star is massive. In fact, assuming that binaries are randomly aligned with respect to the observer line of sight, there is a $\sim 70 \%$ of probability that this system hosts a neutron star more massive than $1.6\,{M}_{\odot }$. In fact, the maximum and median mass of the neutron star, corresponding to orbital inclination angles of 90° and 60°, are ${M}_{\mathrm{NS},\max }=3.1\pm 0.6\,{M}_{\odot }$ and ${M}_{\mathrm{NS},\mathrm{med}}=2.4\pm 0.5\,{M}_{\odot }$, respectively. On the other hand, also assuming an empirical neutron star mass probability distribution, we found that this system could host a neutron star with a mass of $1.5\pm 0.1\,{M}_{\odot }$ if orbiting with a low-inclination angle around 40°.

Thanks to the observational and simulation works, the importance of the nonideal magnetohydrodynamic (MHD) effects, i.e., Hall effect, ohmic resistivity, and ambipolar diffusion, have been well established at various stages of cloud evolution. To get a comparison between the Hall effect with other effects, we aim to model the time evolution of a rotating filamentary molecular cloud during the isothermal/polytropic collapse phase in the presence of the Hall drift. Three components of the velocity vector are investigated when the angular momentum is fully coupled with the magnetic field at large radii of a filament. For this purpose, the nonideal MHD equations in the self-similar formalism are considered at large radii of a molecular cloud where the magnetic field evolution is affected by the Hall drift. Then, the connection between the self-similar approach with the observational data from the filamentary clouds is examined to get a realistic model. Due to the existence of Hall drift, the significant changes on the rotation of the cloud can be seen when the cloud switches from the isothermal collapse phase to the polytropic collapse phase. Also, the results of this model are useful in the study of the multiple star formation process as well as the initial conditions for driving the outflows during the collapse of the filamentary clouds. Finally, we found that there are some conditions for the comparability of the Hall effect with the ambipolar diffusion in the outer regions of the clouds.

We present new molecular modeling for ¹⁴NO and ¹⁵NO and a deep, blind molecular line survey at low radio frequencies (99–129 MHz). This survey is the third in a series completed with the Murchison Widefield Array (MWA), but in comparison with the previous surveys, uses 4 times more data (17 hr versus 4 hr) and is 3 times better in angular resolution (1' versus 3'). The new molecular modeling for nitric oxide and its main isotopologue has seven transitions within the MWA frequency band (although we also present the higher-frequency transitions). Although we did not detect any new molecular lines at a limit of 0.21 Jy beam⁻¹, this work is an important step in understanding the data processing challenges for the future Square Kilometre Array and places solid limits on what is expected in the future of low-frequency surveys. The modeling can be utilized for future searches of nitric oxide.

The Marshall Grazing Incidence Spectrometer (MaGIXS) is a sounding rocket experiment that will observe the soft X-ray spectrum of the Sun from 24 to 6.0 Å (0.5–2.0 keV) and is scheduled for launch in 2021. Component- and instrument-level calibrations for the MaGIXS instrument are carried out using the X-ray and Cryogenic Facility (XRCF) at NASA Marshall Space Flight Center. In this paper, we present the calibration of the incident X-ray flux from the electron impact source with different targets at the XRCF using a CCD camera; the photon flux at the CCD was low enough to enable its use as a "photon counter," i.e., the ability to identify individual photon hits and calculate their energy. The goal of this paper is two-fold: (1) to confirm that the flux measured by the XRCF beam normalization detectors is consistent with the values reported in the literature and therefore reliable for MaGIXS calibration and (2) to develop a method of counting photons in CCD images that best captures their number and energy.

We investigate a short-period (P ≈ 4.4 days) eclipsing binary KIC 8301013 using high-quality Kepler photometry and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopic data. Through the light-curve and radial-velocity synthesis using the Wilson–Devinney method, it reveals that the binary is an almost circular (e ≈ 0.001), detached system composed of two late F-type main-sequence stars, with masses and radii of ${M}_{1}=1.29\pm 0.02{M}_{\odot }$, R₁ = 1.45 ± 0.01R_⊙ and M₂ = 1.11 ± 0.05M_⊙, R₂ = 1.20 ± 0.01R_⊙ for the primary and secondary, respectively. Besides the light variations due to the eclipses, the light curve shows quasi-sinusoidal variations that could be ascribed to starspot modulation. After removing the synthetic binary light curve from the detrended Kepler data, we measure the periods of the active region rotation by using the autocorrelation function (ACF) and Lomb–Scargle periodograms, the decay timescale of the active region by fitting the ACF of out-of-eclipse residuals, and the size of the active region represented by the rms scatter of the out-of-eclipse residuals. The activity level on the binary is significantly stronger than the Sun and has a better agreement with individual F-type stars. No periodic changes are detected in the active region evolution. Thus, KIC 8301013 is an interesting sample for the study of starspot modulation.

Recent surveys of protoplanetary disks show that substructure in dust thermal continuum emission maps is common in protoplanetary disks. These substructures, most prominently rings and gaps, shape and change the chemical and physical conditions of the disk, along with the dust size distributions. In this work, we use a thermochemical code to focus on the chemical evolution that is occurring within the gas-depleted gap and the dust-rich ring often observed behind it. The compositions of these spatial locations are of great import, as the gas and ice-coated grains will end up being part of the atmospheres of gas giants and/or the seeds of rocky planets. Our models show that the dust temperature at the midplane of the gap increases, enough to produce local sublimation of key volatiles and pushing the molecular layer closer to the midplane, while it decreases in the dust-rich ring, causing a higher volatile deposition onto the dust grain surfaces. Further, the ring itself presents a freeze-out trap for volatiles in local flows powered by forming planets, becoming a site of localized volatile enhancement. Within the gas-depleted gap, the line emission depends on several different parameters, such as the depth of the gap in surface density, the location of the dust substructure, and the abundance of common gas tracers, such as CO. In order to break this uncertainty between abundance and surface density, other methods, such as disk kinematics, become necessary to constrain the disk structure and its chemical evolution.

We augment the heliospheric network of galactic cosmic ray (GCR) monitors using 2012–2017 penetrating radiation measurements from the New Horizons (NH) Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI), obtaining intensities of ≳75 MeV particles. The new, predominantly GCR observations provide critical links between the Sun and Voyager 2 and Voyager 1 (V2 and V1), in the heliosheath and local interstellar medium (LISM), respectively. We provide NH, Advanced Composition Explorer (ACE), V2, and V1 GCR observations, using them to track solar cycle variations and short-term Forbush decreases from the Sun to the LISM, and to examine the interaction that results in the surprising, previously reported V1 LISM anisotropy episodes. To investigate these episodes and the hitherto unexplained lagging of associated in situ shock features at V1, propagating disturbances seen at ACE, NH, and V2 were compared to V1. We conclude that the region where LISM magnetic field lines drape around the heliopause is likely critical for communicating solar disturbance signals upstream of the heliosheath to V1. We propose that the anisotropy-causing physical process that suppresses intensities at ∼90° pitch angles relies on GCRs escaping from a single compression in the draping region, not on GCRs trapped between two compressions. We also show that NH suprathermal and energetic particle data from PEPSSI are consistent with the interpretation that traveling shocks and corotating interaction region (CIR) remnants can be distinguished by the existence or lack of Forbush decreases, respectively, because turbulent magnetic fields at local shocks inhibit GCR transport while older CIR structures reaching the outer heliosphere do not.

Quasi-periodic pulsations (QPPs) are routinely observed in a range of wavelengths during flares, but in most cases the mechanism responsible is unknown. We present a method to detect and characterize QPPs in time series such as light curves for solar or stellar flares based on forward modeling and Bayesian analysis. We include models for QPPs as oscillations with finite lifetimes and nonmonotonic amplitude modulation, such as wave trains formed by dispersive evolution in structured plasmas. By quantitatively comparing different models using Bayes factors, we characterize the QPPs according to five properties: sinusoidal or nonsinusoidal, finite or indefinite duration, symmetric or asymmetric perturbations, monotonic or nonmonotonic amplitude modulation, and constant or varying period of oscillation. We demonstrate our method and show examples of these five characteristics by analyzing QPPs in white-light stellar flares observed by the Kepler space telescope. Different combinations of properties may be able to identify particular physical mechanisms and so improve our understanding of QPPs and allow their use as seismological diagnostics. We propose that three observational classes of QPPs can be distinguished: decaying harmonic oscillations, finite wave trains, and nonsinusoidal pulsations.

Small, rocky planets have been found orbiting in extreme proximity to their host stars, sometimes down to only ∼2 stellar radii. These ultra-short-period planets (USPs) likely did not form in their present-day orbits, but rather migrated from larger initial separations. While tides are the probable cause of this migration, the tidal source has remained uncertain. Here, we introduce planetary obliquity tides as a natural pathway for the production of USPs within close-in multiplanet systems. The crucial idea is that tidal dissipation generally forces planetary spin vectors to equilibrium configurations called "Cassini states," in which the planetary obliquities (axial tilts) are nonzero. In these cases, sustained tidal dissipation and inward orbital migration are inevitable. Migration then increases the obliquity and strengthens the tides, creating a positive feedback loop. Thus, if a planet's initial semimajor axis is small enough (a ≲ 0.05 au), it can experience runaway orbital decay, which is stalled at ultra-short orbital periods when the forced obliquity reaches very high values (∼85°) and becomes unstable. We use secular dynamics to outline the parameter space in which the innermost member of a prototypical Kepler multiple-planet system can become a USP. We find that these conditions are consistent with many observed features of USPs, such as period ratios, mutual inclinations, and occurrence rate trends with stellar type. Future detections of stellar obliquities and close-in companions, together with theoretical explorations of the potential for chaotic obliquity dynamics, can help constrain the prevalence of this mechanism.

We report the first observational detection of frequency modulation in the cross-sectional width of spicule structures due to field-aligned plasma flows. Cross-sectional width variations were estimated for the least superimposed off-limb spicules observed in high-resolution Hα imaging spectroscopy data. Analysis of estimated cross-sectional widths suggest periodic oscillations, concurrent with 2D numerical modeling for a jet structure in a stratified solar atmosphere. Spectral analysis for both observed and simulated cross-sectional widths indicate frequency modulation as noticeable shifts in estimated periodicities during rise and fall phases of field-aligned plasma flows in the jet structure. Furthermore, the presence of the first overtone in a dynamic/spicular waveguide is also evident in both the observed and the simulated jet structures. These harmonics can be an important tool for future chromospheric magnetoseismology investigations and applications to dynamic waveguides (like spicules).

We report the ground-level detection of a Galactic cosmic-ray (GCR) flux enhancement lasting ∼17 hr and associated with the passage of a magnetic flux rope (MFR) over the Earth. The MFR was associated with a slow coronal mass ejection (CME) caused by the eruption of a filament on 2016 October 9. Due to the quiet conditions during the eruption and the lack of interactions during the interplanetary CME transport to the Earth, the associated MFR preserved its configuration and reached the Earth with a strong magnetic field, low density, and a very low turbulence level compared to local background, thus generating the ideal conditions to redirect and guide GCRs (in the ∼8–60 GV rigidity range) along the magnetic field of the MFR. An important negative B_Z component inside the MFR caused large disturbances in the geomagnetic field and a relatively strong geomagnetic storm. However, these disturbances are not the main factors behind the GCR enhancement. Instead, we found that the major factor was the alignment between the MFR axis and the asymptotic direction of the observer.

We have performed a search over 3440 deg² of Epoch 1 (2017–2019) of the Very Large Array Sky Survey to identify unobscured quasars in the optical (0.2 < z < 3.2) and obscured active galactic nuclei (AGNs) in the infrared that have brightened dramatically in the radio over the past one to two decades. These sources would have been previously classified as "radio-quiet" quasars based on upper limits from the Faint Images of the Radio Sky at Twenty cm survey (1993–2011), but they are now consistent with "radio-loud" quasars (${L}_{3\mathrm{GHz}}={10}^{40\mbox{--}42}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$). A quasi-simultaneous, multiband (∼1–18 GHz) follow-up study of 14 sources with the VLA has revealed compact sources (<01 or <1 kpc) with peaked radio spectral shapes. The high-amplitude variability over decadal timescales at 1.5 GHz (100% to >2500%) but roughly steady fluxes over a few months at 3 GHz are inconsistent with extrinsic variability due to propagation effects, thus favoring an intrinsic origin. We conclude that our sources are powerful quasars hosting compact/young jets. This challenges the generally accepted idea that "radio-loudness" is a property of the quasar/AGN population that remains fixed on human timescales. Our study suggests that frequent episodes of short-lived AGN jets that do not necessarily grow to large scales may be common at high redshift. We speculate that intermittent but powerful jets on subgalactic scales could interact with the interstellar medium, possibly driving feedback capable of influencing galaxy evolution.

We report on the results of a new spectroscopic monitoring campaign of the quasar PG 0026+129 at the Calar Alto Observatory 2.2 m telescope from 2017 July to 2020 February. Significant variations in the fluxes of the continuum and broad emission lines, including Hβ and He ii, were observed in the first and third years, and clear time lags between them are measured. The broad Hβ line profile consists of two Gaussian components: an intermediate-width H${\beta }_{\mathrm{IC}}$ with an FWHM of 1964 ± 18 $\mathrm{km}\,{{\rm{s}}}^{-1}$ and another very broad H${\beta }_{\mathrm{VBC}}$ with an FWHM of 7570 ± 83 $\mathrm{km}\,{{\rm{s}}}^{-1}$. H${\beta }_{\mathrm{IC}}$ has long time lags of ∼40–60 days in the rest frame, while H${\beta }_{\mathrm{VBC}}$ shows nearly zero time delay with respect to the optical continuum at 5100 Å. The velocity-resolved delays show consistent results: lags of ∼30–50 days at the core of the broad Hβ line and roughly zero lags at the wings. H${\beta }_{\mathrm{IC}}$ has a redshift of ∼400 $\mathrm{km}\,{{\rm{s}}}^{-1}$, which seems to be stable for nearly 30 yr by comparing with archived spectra, and may originate from an infall. The rms spectrum of H${\beta }_{\mathrm{VBC}}$ shows a double-peaked profile with brighter blue peak and extended red wing in the first year, which matches the signature of a thin disk. Both the double-peaked profile and the near-zero lag suggest that H${\beta }_{\mathrm{VBC}}$ comes from a region associated with the part of the accretion disk that emits the optical continuum. Adopting the FWHM (in the rms spectrum) and the time lag measured for the total Hβ line, and a virial factor of 1.5, we obtain a virial mass of ${2.89}_{-0.69}^{+0.60}\times {10}^{7}\,{M}_{\odot }$ for the central black hole in this quasar.

We present a new catalog of TeV gamma-ray sources using 1523 days of data from the High-Altitude Water Cherenkov (HAWC) Observatory. The catalog represents the most sensitive survey of the northern gamma-ray sky at energies above several TeV, with three times the exposure compared to the previous HAWC catalog, 2HWC. We report 65 sources detected at ≥5σ significance, along with the positions and spectral fits for each source. The catalog contains eight sources that have no counterpart in the 2HWC catalog, but are within 1° of previously detected TeV emitters, and 20 sources that are more than 1° away from any previously detected TeV source. Of these 20 new sources, 14 have a potential counterpart in the fourth Fermi Large Area Telescope catalog of gamma-ray sources. We also explore potential associations of 3HWC sources with pulsars in the Australia Telescope National Facility (ATNF) pulsar catalog and supernova remnants in the Galactic supernova remnant catalog.

We report the results of reverberation mapping of three bright Seyfert galaxies, Mrk 79, NGC 3227, and Mrk 841, from a campaign conducted from 2016 December to 2017 May with the Wyoming Infrared Observatory (WIRO) 2.3 m telescope. All three of these targets have shown asymmetric broad Hβ emission lines in the past, although their emission lines were relatively symmetric during our observations. We measured Hβ time lags for all three targets and estimated masses of their black holes—for the first time in the case of Mrk 841. For Mrk 79 and NGC 3227, the data are of sufficient quality to resolve distinct time lags as a function of velocity and to compute two-dimensional velocity-delay maps. Mrk 79 shows smaller time lags for high-velocity gas, but the distribution is not symmetric, and its complex velocity-delay map could result from the combination of both inflowing and outflowing Hβ emitting disks that may be part of a single larger structure. NGC 3227 shows the largest time lags for blueshifted gas, and the two-dimensional velocity-delay map suggests a disk with some inflow. We compare our results with previous work and find evidence for different time lags despite similar luminosities, as well as evolving broad-line region structures.

We present a high spatial resolution Chandra X-ray study of two infrared dark clouds (IRDCs), G034.43+00.24 and G035.39−00.33, which are expected to be in the early phases of star cluster formation. We detect 112 and 209 valid X-ray point sources toward G034.43+00.24 and G035.39−00.33, respectively. We cross-match the X-ray point sources with the Two Micron All Sky Survey (2MASS), Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE), and Wide-field Infrared Survey Explorer (WISE) catalogs and find 53% and 59% of the X-ray sources in G034.43+00.24 and in G035.39−00.33 have corresponding infrared counterparts, respectively. These sources are probable members of young massive clusters in formation, and using stellar isochrones we estimate that a population of 1–2 Myr old, intermediate- to high-mass young stellar objects (YSOs) exist in both IRDCs. Two and 10 Class II counterparts to X-ray sources were identified in G034.43+00.24 and in G035.39−00.33, respectively, which are located in or near dark filaments. The X-ray luminosity function (XLF) of G035.39−00.33 implies that the total mass consists of up to ∼1700M_⊙ of stars, using the XLF of the well-studied Orion Nebula Cluster as a calibrator. This corresponds to a star formation efficiency of at most 5%, indicating the system is still very much gas dominated and in an early stage of the star formation process. The population of G034.43+00.24 is less well determined due to the lower sensitivity of its observations.

We present Gemini Multi-Object Spectrograph (GMOS) integral field unit (IFU) observations of six massive (M_⋆ ≥ 10¹¹M_⊙) A-star dominated post-starburst galaxies at z ∼ 0.6. These galaxies are a subsample of the $\mathrm{SQuIGG}\vec{L}{\rm{E}}$ Survey, which selects intermediate-redshift post-starbursts from the Sloan Digital Sky Survey spectroscopic sample (DR14) with spectral shapes that indicate they have recently shut off their primary epoch of star formation. Using Hδ_A absorption as a proxy for stellar age, we constrain five of the galaxies to have young (∼600 Myr) light-weighted ages at all radii and find that the sample on average has flat age gradients. We examine the spatial distribution of mass-weighted properties by fitting our profiles with a toy model including a young, centrally concentrated burst superimposed on an older, extended population. We find that galaxies with flat Hδ_A profiles are inconsistent with formation via a central secondary starburst. This implies that the mechanism responsible for shutting off this dominant episode of star formation must have done so uniformly throughout the galaxy.

The sputtering rate of presolar silicon carbide grains due to galactic cosmic rays has been computed for their experimentally deduced lifetimes (∼1 Gyr) in the interstellar medium. An ion target simulator, SDTrimSP, was used to model the sputtering of interstellar grains with varying sizes and thicknesses of the ice mantle formed around the grain during their journey through the interstellar medium. Temperature, composition, and density for four different types of molecular cloud environments (quiescent, low-mass young stellar objects (YSOs), intermediate-mass YSOs, and high-mass YSO weak processing) considered indicate the sputtering rate on the mantle ice composition depends on water composition to a certain extent. The model simulations indicate galactic cosmic ray(s) with an energy range from 10 MeV to 1 GeV are just capable of sputtering/destructing ∼13%–15% of the grain itself. This value, stretched over 1 Gyr is not as significant as the other destruction processes and therefore can be classified as a minor destruction process. The effect of galactic cosmic rays on the ice mantle and core is also noted with particular emphasis on amorphization/recoils generated inside the SiC core and their distribution within the grain.

Cepheids in multiple systems provide information on the outcome of the formation of massive stars. They can also lead to exotic end-stage objects. This study concludes our survey of 70 galactic Cepheids using the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) with images at two wavelengths to identify companions closer than 5''. In the entire WFC3 survey we identify 16 probable companions for 13 Cepheids. The 7 Cepheids having resolved candidate companions within 2'' all have the surprising property of themselves being spectroscopic binaries (as compared with a 29% incidence of spectroscopic binaries in the general Cepheid population). This is a strong suggestion that an inner binary is linked to the scenario of a third companion within a few hundred astronomical units. This characteristic is continued for more widely separated companions. Under a model where the outer companion is formed first, it is unlikely that it can anticipate a subsequent inner binary. Rather, it is more likely that a triple system has undergone dynamical interaction, resulting in one star moving outward to its current location. Chandra and Gaia data as well as radial velocities and HST/STIS and IUE spectra are used to derive properties of the components of the Cepheid systems. The colors of the companion candidates show a change in distribution at approximately 2000 au separations, from a range including both hot and cool colors for closer companions, to only low-mass companions for wider separations.

We study the characteristics of near-Earth networks (NENs) of gamma-ray burst (GRB) detectors, with the objective of defining a network with all-sky, full-time localization capability for multimessenger astrophysics. We show that a minimum network consisting of nine identical spacecraft in two orbits with different inclinations provides a good combination of sky coverage with several-degree localization accuracy with detector areas of 100 cm². In order to achieve this, careful attention must be paid to systematics. This includes accurate photon timing (∼0.1 ms), good energy resolution (∼10%), and reduction of Earth albedo, which are all within current capabilities. Such a network can be scaled in both the number and size of detectors to produce increased accuracy. We introduce a new method of localization that does not rely on on-board trigger systems or on the cross-correlation of time histories, but rather it tests positions in ground processing over the entire sky and assigns probabilities to them to detect and localize events. We demonstrate its capabilities with simulations. If the NEN spacecraft can downlink at least several hundred time- and energy-tagged events per second, and the data can be ground-processed as they are received, it can in principle derive GRB positions in near-real time over the entire sky.