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<jats:title>Abstract</jats:title> <jats:p>We present a direct comparison of the Pan-Andromeda Archaeological Survey (PAndAS) observations of the stellar halo of M31 with the stellar halos of six galaxies from the Auriga simulations. We process the simulated halos through the A<jats:sc>uriga</jats:sc>2PA<jats:sc>nd</jats:sc>AS pipeline and create PAndAS-like mocks that fold in all observational limitations of the survey data (foreground contamination from the Milky Way stars, incompleteness of the stellar catalogs, photometric uncertainties, etc.). This allows us to study the survey data and the mocks in the same way and generate directly comparable density maps and radial density profiles. We show that the simulations are overall compatible with the observations. Nevertheless, some systematic differences exist, such as a preponderance for metal-rich stars in the mocks. While these differences could suggest that M31 had a different accretion history or has a different mass compared with the simulated systems, it is more likely a consequence of an underquenching of the star formation history of galaxies, related to the resolution of the A<jats:sc>uriga</jats:sc> simulations. The direct comparison enabled by our approach offers avenues to improve our understanding of galaxy formation as they can help pinpoint the observable differences between observations and simulations. Ideally, this approach will be further developed through an application to other stellar halo simulations. To facilitate this step, we release the pipeline to generate the mocks, along with the six mocks presented and used in this contribution.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>This project aims to investigate the most important weak interaction nuclei in the presupernova evolution of massive stars. To achieve this goal, an ensemble containing 728 nuclei in the mass range of <jats:italic>A</jats:italic> = 1–100 was considered. We computed the mass fractions of these nuclei using Saha’s equation for predetermined values of <jats:italic>T</jats:italic>, <jats:italic>ρ</jats:italic>, and <jats:italic>Y</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> and assuming nuclear statistical equilibrium. The nuclear partition functions were obtained using a newly introduced recipe where excited states, up to 10 MeV, were treated as discrete. The weak interaction rates (electron capture (ec) and <jats:italic>β</jats:italic>-decay (bd)) were calculated in a totally microscopic fashion using the proton–neutron quasiparticle random phase approximation model and without assuming the Brink–Axel hypothesis. The calculated rates were coupled with the computed mass fractions to investigate the time rate of change of lepton to baryon fraction of the stellar matter. We compare our results with the previous calculations reported in the literature. Noticeable differences up to orders of magnitude are reported with previous calculations. These differences may influence the evolution of the star in the later stages of presupernova. We present a list of the top 50 ec and bd nuclei, which have the largest effect on <jats:italic>Y</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> for conditions after silicon core burning. The competition between the ec and bd rates in the stellar core was investigated and it was found that <jats:italic>Y</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> = 0.424–0.455 is the interval where the bd results are bigger than the ec rates.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Detections of the tidal disruption flares (TDFs) of stars by supermassive black holes (SMBHs) are rapidly accumulating as optical surveys improve. These detections may provide constraints on SMBH demographics, stellar dynamics, and stellar evolution in galaxies. To maximize this scientific impact, we require a better understanding of how astrophysical parameters interact with survey selection effects in setting the properties of detected flares. We develop a framework for modeling the distributions of optical TDF detections in surveys across attributes of the host galaxies and the flares themselves. This model folds in effects of the stellar disruption rate in each galaxy, the flare luminosity and temperature distributions, the effects of obscuration and reddening by dust in the host galaxy, and survey selection criteria. We directly apply this model to the sample of TDFs detected by the Zwicky Transient Facility, and find that the overall flare detection rate is in line with simple theoretical expectation. The model can also reproduce the distribution of total stellar mass and redshift of the host galaxies, but fails to match all details of the detected flares, such as their luminosity and temperature distributions. We also find that dust obscuration likely plays an important role in suppressing the TDF detection rate in star-forming galaxies. While we do not find that the unusual preference of TDFs to have hosts in post-starburst galaxies in the “green valley” can be entirely explained by selection effects, our model can help to quantify the true rate enhancement in those galaxies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present rest-frame <jats:italic>B</jats:italic> and <jats:italic>I</jats:italic> imaging of 35 low-redshift (<jats:inline-formula> <jats:tex-math> <?CDATA $z\lt 0.5$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe8d4ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>) Palomar–Green quasars using the Hubble Space Telescope Wide Field Camera 3. We perform multicomponent two-dimensional image decomposition to separate the host galaxy from its bright active nucleus, characterize its morphology, and measure its photometric properties. Special care is devoted to quantifying the structural parameters of the galaxy bulge, determining its <jats:italic>B</jats:italic> − <jats:italic>I</jats:italic> color, and estimating its stellar mass. Roughly half of the sample, comprising the less luminous (<jats:inline-formula> <jats:tex-math> <?CDATA ${L}_{5100}\lesssim {10}^{45}\,\mathrm{erg}\,{{\rm{s}}}^{-1}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe8d4ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>) but most high Eddington ratio quasars, reside in disk galaxies that are often barred and possess pseudo bulges. The large stellar masses, large effective radii, and faint surface brightnesses suggest that the host galaxies of the most luminous quasars are mostly ellipticals. Major mergers constitute only a small percentage (≲20%) of our sample. Our quasar sample roughly obeys the scaling relations between black hole mass and host galaxy (bulge, core, total) stellar mass. Hosts with black holes more massive than <jats:inline-formula> <jats:tex-math> <?CDATA $\sim {10}^{8}\,{M}_{\odot }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe8d4ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> behave similarly to classical bulges and early-type galaxies, while those with less massive black holes, particularly the narrow-line Seyfert 1s, are consistent with pseudo bulges in late-type galaxies. The host galaxy bulges, irrespective of whether they are classical or pseudo, follow a relatively tight inverse relation between the effective radius and the mean effective surface brightness of inactive classical bulges and ellipticals. We argue that pseudo bulges experience recent or ongoing nuclear star formation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Active galactic nuclei (AGNs) are powered by geometrically thin accretion disks surrounding a central supermassive black hole. Here we explore the evolution of stars embedded in these extreme astrophysical environments (AGN stars). Because AGN disks are much hotter and denser than most components of the interstellar medium, AGN stars are subject to very different boundary conditions than normal stars. They are also strongly affected by both mass accretion, which can run away given the vast mass of the disk, and mass loss due to super-Eddington winds. Moreover, chemical mixing plays a critical role in the evolution of these stars by allowing fresh hydrogen accreted from the disk to mix into their cores. We find that, depending on the local AGN density and sound speed and the duration of the AGN phase, AGN stars can rapidly become very massive (<jats:italic>M</jats:italic> &gt; 100 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>). These stars undergo core collapse, leave behind compact remnants, and contribute to polluting the disk with heavy elements. We show that the evolution of AGN stars can have a profound impact on the evolution of AGN metallicities, as well as the production of gravitational wave sources observed by LIGO-Virgo. We point to our Galactic Center as a region well suited to testing some of our predictions for this exotic stellar evolutionary channel.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We introduce a new method to estimate the probability that an extragalactic transient source is associated with a candidate host galaxy. This approach relies solely on simple observables: sky coordinates and their uncertainties, galaxy fluxes, and angular sizes. The formalism invokes Bayes’ rule to calculate the posterior probability <jats:inline-formula> <jats:tex-math> <?CDATA $P\left({O}_{i}| x\right)$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe8d2ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> from the galaxy prior <jats:italic>P</jats:italic>(<jats:italic>O</jats:italic>), observables <jats:italic>x</jats:italic>, and an assumed model for the true distribution of transients in/around their host galaxies. Using simulated transients placed in the well-studied Cosmic Evolution Survey field, we consider several agnostic and physically motivated priors and offset distributions to explore the method sensitivity. We then apply the methodology to the set of 13 fast radio bursts (FRBs) localized with an uncertainty of several arcseconds. Our methodology finds nine of these are securely associated to a single host galaxy, <jats:inline-formula> <jats:tex-math> <?CDATA $P\left({O}_{i}| x\right)\gt 0.95$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe8d2ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>. We examine the observed and intrinsic properties of these secure FRB hosts, recovering distributions similar to those found in previous works. Furthermore, we find a strong correlation between the apparent magnitude of the securely identified host galaxies and the estimated cosmic dispersion measures of the corresponding FRBs, which results from the Macquart relation. Future work with FRBs will leverage this relation and other measures from the secure hosts as priors for future associations. The methodology is generic to transient type, localization error, and image quality. We encourage its application to other transients where host galaxy associations are critical to the science, e.g., gravitational wave events, gamma-ray bursts, and supernovae. We have encoded the technique in Python on GitHub: <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://github.com/FRBs/astropath" xlink:type="simple">https://github.com/FRBs/astropath</jats:ext-link>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A key component of the baryon cycle in galaxies is the depletion of metals from the gas to the dust phase in the neutral interstellar medium (ISM). The METAL (Metal Evolution, Transport, and Abundance in the Large Magellanic Cloud) program on the Hubble Space Telescope acquired UV spectra toward 32 sight lines in the half-solar metallicity LMC, from which we derive interstellar depletions (gas-phase fractions) of Mg, Si, Fe, Ni, S, Zn, Cr, and Cu. The depletions of different elements are tightly correlated, indicating a common origin. Hydrogen column density is the main driver for depletion variations. Correlations are weaker with volume density, probed by C <jats:sc>i</jats:sc> fine-structure lines, and distance to the LMC center. The latter correlation results from an east–west variation of the gas-phase metallicity. Gas in the east, compressed side of the LMC encompassing 30 Doradus and the southeast H <jats:sc>i</jats:sc> over-density is enriched by up to +0.3 dex, while gas in the west side is metal deficient by up to −0.5 dex. Within the parameter space probed by METAL, no correlation with molecular fraction or radiation-field intensity are found. We confirm the factor of three to four increase in dust-to-metal and dust-to-gas ratios between the diffuse (log N(H) ∼ 20 cm<jats:sup>−2</jats:sup>) and molecular (log N(H) ∼ 22 cm<jats:sup>−2</jats:sup>) ISM observed from far-infrared, 21 cm, and CO observations. The variations of dust-to-metal and dust-to-gas ratios with column density have important implications for the sub-grid physics of chemical evolution, gas and dust mass estimates throughout cosmic times, and for the chemical enrichment of the Universe measured via spectroscopy of damped Ly<jats:italic>α</jats:italic> systems.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In the next decade, transient searches from the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope will increase the sample of known Type Ia supernovae (SNe Ia) from ∼10<jats:sup>3</jats:sup> to 10<jats:sup>5</jats:sup>. With this reduction of statistical uncertainties on cosmological measurements, new methods are needed to reduce systematic uncertainties. Characterizing the underlying spectroscopic evolution of SN Ia remains a major systematic uncertainty in current cosmological analyses, motivating a new simulation tool for the next era of SN Ia cosmology: Build Your Own Spectral Energy Distribution (<jats:sc>byosed</jats:sc>). <jats:sc>byosed</jats:sc> is used within the SNANA framework to simulate light curves by applying spectral variations to model SEDs, enabling flexible testing of possible systematic shifts in SN Ia distance measurements. We test the framework by comparing a nominal Roman SN Ia survey simulation using a baseline SED model to simulations using SEDs perturbed with <jats:sc>byosed</jats:sc>, and investigating the impact of ignoring specific SED features in the analysis. These features include semiempirical models of two possible, predicted relationships: between SN ejecta velocity and light-curve observables, and a redshift-dependent relationship between SN Hubble residuals and host-galaxy mass. We analyze each <jats:sc>byosed</jats:sc> simulation using the SALT2 and BEAMS with Bias Corrections framework, and estimate changes in the measured value of the dark-energy equation-of-state parameter, <jats:italic>w</jats:italic>. We find a difference of Δ<jats:italic>w</jats:italic> = −0.023 for SN velocity and Δ<jats:italic>w</jats:italic> = 0.021 for redshift-evolving host mass when compared to simulations without these features. By using <jats:sc>byosed</jats:sc> for SN Ia cosmology simulations, future analyses (e.g., the Rubin and Roman SN Ia samples) will have greater flexibility to constrain or reduce such SN Ia modeling uncertainties.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We develop a new equation-of-state (EOS) table involving thermal (anti)kaons, Bose–Einstein condensate of <jats:italic>K</jats:italic> <jats:sup>−</jats:sup> mesons, and Λ-hyperons for core-collapse supernova and neutron star merger simulations. This EOS table is based on a finite-temperature, density-dependent relativistic hadron field theory where baryon–baryon interaction is mediated by scalar <jats:italic>σ</jats:italic>, vector <jats:italic>ω</jats:italic>, and <jats:italic>ρ</jats:italic> mesons, using the parameter set DD2 for nucleons. The repulsive hyperon–hyperon interaction is mediated by an additional strange <jats:italic>ϕ</jats:italic> meson. The EOS for the <jats:italic>K</jats:italic> <jats:sup>−</jats:sup> condensed matter is also calculated within the framework of the relativistic mean field model, whereas the low-density, inhomogeneous matter is calculated in the extended nuclear statistical equilibrium model. The EOS table is generated for a wide range of values of three parameters—baryon density (10<jats:sup>−12</jats:sup> to ∼1 fm<jats:sup>−3</jats:sup>), positive charge fraction (0.01–0.60), and temperature (0.1–158.48 MeV).</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The merger of binary neutron stars (NS–NS) as the progenitor of short gamma-ray bursts (GRBs) has been confirmed by the discovery of the association of the gravitational-wave (GW) event GW170817 with GRB 170817A. However, the merger product of binary NS remains an open question. An X-ray plateau followed by a steep decay (“internal plateau”) has been found in some short GRBs, implying that a supramassive magnetar operates as the merger remnant and then collapses into a newborn black hole (BH) at the end of the plateau. X-ray bump or second plateau following the “internal plateau” are considered as the expected signature from the fallback accretion onto this newborn BH through the Blandford–Znajek mechanism (BZ). At the same time, a nearly isotropic wind driven by the Blandford–Payne mechanism (BP) from the newborn BH’s disk can produce a bright kilonova. Therefore, the bright kilonova observation for a short GRB with “internal plateau” (and followed by X-ray bump or second plateau) provides further evidence for this scenario. In this paper, we find that GRB 160821B is a candidate of such a case, and the kilonova emission of GRB 160821B is possibly powered by the BP wind from a newborn BH. Future GW detection of GRB 160821B–like events may provide further support to this scenario, enable us to investigate the properties of the magnetar and the newborn BH, and constrain the equation of state of neutron stars.</jats:p>