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<jats:title>Abstract</jats:title> <jats:p>We investigate the influence of orbital eccentricity and interplanetary magnetic field (IMF) direction on the dimensions of Mercury’s magnetosphere using four Earth years of data collected by the <jats:italic>MErcury Surface, Space ENvironment, GEochemistry, and Ranging</jats:italic> spacecraft. The variations in magnetopause subsolar distance, flank distance, and tail radius are compared to determine the global dimensions and compressibility of the magnetosphere. As Mercury moves from perihelion (aphelion) to aphelion (perihelion) its magnetosphere expands (contracts) globally by ∼15% on average. After mapping all of the magnetopause crossings to a fixed radial distance from the Sun, we find that IMF cone angle changes lead to reconfigurations of the magnetosphere of comparable magnitude to radial solar distance effects. Furthermore, the magnetosphere is found to expand globally under the quasi-radial IMF, while contracting under the quasi-perpendicular conditions. A new Mercury magnetopause model parameterized by the heliocentric distance and IMF cone angle was constructed. Unlike Earth, the model gives a closed magnetopause for the nightside in most cases, and its flaring decreases with the contraction of the magnetosphere. The results demonstrate the highly variable reconfigurations of Mercury’s magnetosphere under its extreme environmental conditions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We model explosions driven by the coalescence of a black hole or neutron star with the core of its massive-star companion. Upon entering a common-envelope phase, a compact object may spiral all the way to the core. The concurrent release of energy is likely to be deposited into the surrounding common envelope, powering a merger-driven explosion. We use hydrodynamic models of binary coalescence to model the common-envelope density distribution at the time of coalescence. We find toroidal profiles of material, concentrated in the binary’s equatorial plane and extending to many times the massive star’s original radius. We use the spherically averaged properties of this circumstellar material (CSM) to estimate the emergent light curves that result from the interaction between the blast wave and the CSM. We find that typical merger-driven explosions are brightened by up to three magnitudes by CSM interaction. From population synthesis models, we discover that the brightest merger-driven explosions, <jats:italic>M</jats:italic> <jats:sub> <jats:italic>V</jats:italic> </jats:sub> ∼ −18 to −20, are those involving black holes because they have the most massive and extended CSM. Black hole coalescence events are also common; they represent about 50% of all merger-driven explosions and approximately 0.25% of the core-collapse rate. Merger-driven explosions offer a window into the highly uncertain physics of common-envelope interactions in binary systems by probing the properties of systems that merge rather than eject their envelopes.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Using archival <jats:italic>Chandra</jats:italic> observations with a total effective exposure of 323 ks, we derive an updated catalog of point sources in the bulge globular cluster M28. The catalog contains 502 X-ray sources within an area of ∼475 arcmin<jats:sup>2</jats:sup>, and more than 90% of these sources are first detected in this cluster. We find significant dips in the radial distribution profiles of X-ray sources in M28, and the projected distance and width of the distribution dip for bright (<jats:italic>L</jats:italic> <jats:sub>X</jats:sub> ≳ 4.5 × 10<jats:sup>30</jats:sup> erg s<jats:sup>−1</jats:sup>) X-ray sources are larger than for faint (<jats:italic>L</jats:italic> <jats:sub>X</jats:sub> ≲ 4.5 × 10<jats:sup>30</jats:sup> erg s<jats:sup>−1</jats:sup>) sources. Fitting with the “generalized King model” gives a slightly larger average mass for the bright sources (1.30 ± 0.15 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) than for the faint ones (1.09 ± 0.14 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>), which supports a universal delay in mass segregation between heavy objects in globular clusters. We show that the dynamical age of M28 is comparable to that of Terzan 5 and much smaller than that of 47 Tuc, but M28 is evolving faster (i.e., with a shorter two-body relaxation timescale) than 47 Tuc. These features may suggest an acceleration effect of cluster dynamical evolution by tidal shock in M28. Besides, we find an abnormal deficiency of X-ray sources in the central region (<jats:italic>R</jats:italic> ≲ 1.′5) of M28 compared with its outskirts, which indicates that M28 may have suffered an early phase of primordial binary disruption within its central region, and the mass segregation effect will erase such a phenomenon as clusters evolve to an older dynamical age.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>M dwarfs have enormous potential for our understanding of structure and formation on both Galactic and exoplanetary scales through their properties and compositions. However, current atmosphere models have limited ability to reproduce spectral features in stars at the coolest temperatures (<jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> &lt; 4200 K) and to fully exploit the information content of current and upcoming large-scale spectroscopic surveys. Here we present a catalog of spectroscopic temperatures, metallicities, and spectral types for 5875 M dwarfs in the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and <jats:italic>Gaia-DR2</jats:italic> surveys using <jats:italic>The Cannon</jats:italic>: a flexible, data-driven spectral-modeling and parameter-inference framework demonstrated to estimate stellar-parameter labels (<jats:inline-formula> <jats:tex-math> <?CDATA ${T}_{\mathrm{eff}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab7004ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}g$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab7004ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>, <jats:inline-formula> <jats:tex-math> <?CDATA $[\mathrm{Fe}/{\rm{H}}]$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab7004ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>, and detailed abundances) to high precision. Using a training sample of 87 M dwarfs with optically derived labels spanning <jats:inline-formula> <jats:tex-math> <?CDATA $2860\,{\rm{K}}\lt {T}_{{\rm{e}}{\rm{f}}{\rm{f}}}\lt 4130\,{\rm{K}}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab7004ieqn4.gif" xlink:type="simple" /> </jats:inline-formula> calibrated with bolometric temperatures, and <jats:inline-formula> <jats:tex-math> <?CDATA $-0.5\lt [\mathrm{Fe}/{\rm{H}}]\lt 0.5$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjab7004ieqn5.gif" xlink:type="simple" /> </jats:inline-formula> dex calibrated with FGK binary metallicities, we train a two-parameter model with predictive accuracy (in cross-validation) to 77 K and 0.09 dex respectively. We also train a one-dimensional spectral classification model using 51 M dwarfs with Sloan Digital Sky Survey optical spectral types ranging from M0 to M6, to predictive accuracy of 0.7 types. We find <jats:italic>Cannon</jats:italic> temperatures to be in agreement to within 60 K compared to a subsample of 1702 sources with color-derived temperatures, and <jats:italic>Cannon</jats:italic> metallicities to be in agreement to within 0.08 dex metallicity compared to a subsample of 15 FGK+M or M+M binaries. Finally, our comparison between <jats:italic>Cannon</jats:italic> and APOGEE pipeline (ASPCAP DR14) labels finds that ASPCAP is systematically biased toward reporting higher temperatures and lower metallicities for M dwarfs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Approximately 200 supermassive black holes (SMBHs) have been discovered within the first ∼gigayear after the Big Bang. One pathway for the formation of SMBHs is through the collapse of supermassive stars (SMSs). A possible obstacle to this scenario is that the collapsing gas fragments and forms a cluster of main-sequence stars. Here, we raise the possibility that stellar collisions may be sufficiently frequent and energetic to inhibit the contraction of the massive protostar, avoiding strong UV radiation driven outflows, and allowing it to continue growing into an SMS. We investigate this scenario with semianalytic models incorporating star formation; gas accretion; dynamical friction from stars and gas; stellar collisions; and gas ejection. We find that when the collapsing gas fragments at a density of ≲3 × 10<jats:sup>10</jats:sup> cm<jats:sup>−3</jats:sup>, the central protostar contracts due to infrequent stellar mergers, and in turn photoevaporates the remaining collapsing gas, resulting in the formation of a ≲10<jats:sup>4</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> object. On the other hand, when the collapsing gas fragments at higher densities (expected for a metal-poor cloud with <jats:italic>Z</jats:italic> ≲ 10<jats:sup>−5</jats:sup> <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub> with suppressed H<jats:sub>2</jats:sub> abundance) the central protostar avoids contraction and keeps growing via frequent stellar mergers, reaching masses as high as ∼10<jats:sup>5</jats:sup>–10<jats:sup>6</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. We conclude that frequent stellar mergers represent a possible pathway to form massive BHs in the early universe.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent intense X-ray and UV monitoring campaigns with <jats:italic>Swift</jats:italic> have detected clear UV lags behind X-ray in several local active galactic nuclei (AGNs). The UV to X-ray lags are often larger (by a factor of up to ∼20) than expected if the UV variation is simply due to the X-ray reprocessing. We previously developed a model in which the UV/optical variations are attributed to disk turbulences, and the effect of large-scale turbulence is considered. Our model, which overcomes many severe challenges to the reprocessing scheme, can explain the observed variations in NGC 5548, particularly the correlations and lags among the UV/optical bands. In this work, assuming the corona heating is associated with turbulences in the inner accretion disk, we extend our study to model the correlations and lags between the X-ray and UV/optical bands. We find that our model, without the need of light echoing, can well reproduce the observed UV to X-ray lags and the optical to UV lags simultaneously in four local Seyfert galaxies, including NGC 4151, NGC 4395, NGC 4593, and NGC 5548. In our scenario, relatively larger UV to X-ray lag is expected for AGN with smaller innermost disk radius and thus more compact corona. Interestingly, for these Seyfert galaxies studied in this work, sources with relatively larger UV to X-ray lags do have broader Fe K<jats:italic>α</jats:italic> lines, indicative of relativistic broadening due to more compact corona and smaller innermost disk radius. If confirmed with more X-ray and UV monitoring campaigns, this interesting discovery would provide a new probe to the inner disk/corona.</jats:p>