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1. Black hole, neutron star, and white dwarf merger rates in AGN discs

   https://doi.org/10.1093/mnras/staa2681  

   McKernan, B; Ford, K E; O’Shaughnessy, R (September 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Advanced LIGO and Advanced Virgo are detecting a large number of binary stellar origin black hole (BH) mergers. A promising channel for accelerated BH merger lies in active galactic nucleus (AGN) discs of gas around supermasssive BHs. Here, we investigate the relative number of compact object (CO) mergers in AGN disc models, including BH, neutron stars (NS), and white dwarfs, via Monte Carlo simulations. We find the number of all merger types in the bulk disc grows ∝ t1/3 which is driven by the Hill sphere of the more massive merger component. Median mass ratios of NS–BH mergers in AGN discs are $$\tilde{q}=0.07\pm 0.06(0.14\pm 0.07)$$ for mass functions (MF) M−1(− 2). If a fraction fAGN of the observed rate of BH–BH mergers (RBH–BH) come from AGN, the rate of NS–BH (NS–NS) mergers in the AGN channel is $${R}_{\mathrm{ BH}\!-\!\mathrm{ NS}} \sim f_{\mathrm{ AGN}}[10,300]\, \rm {Gpc}^{-3}\, \rm {yr}^{-1},({\mathit{ R}}_{NS\!-\!NS} \le \mathit{ f}_{AGN}400\, \rm {Gpc}^{-3}\, \rm {yr}^{-1}$$). Given the ratio of NS–NS/BH–BH LIGO search volumes, from preliminary O3 results the AGN channel is not the dominant contribution to observed NS–NS mergers. The number of lower mass gap events expected is a strong function of the nuclear MF and mass segregation efficiency. CO merger ratios derived from LIGO can restrict models of MF, mass segregation, and populations embedded in AGN discs. The expected number of electromagnetic (EM) counterparts to NS–BH mergers in AGN discs at z < 1 is $$\sim [30,900]\, {\rm {yr}}^{-1}(f_{\mathrm{ AGN}}/0.1)$$. EM searches for flaring events in large AGN surveys will complement LIGO constraints on AGN models and the embedded populations that must live in them. 

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2. The effects of local stellar radiation and dust depletion on non-equilibrium interstellar chemistry

   https://doi.org/10.1093/mnras/stac2338  

   Richings, Alexander J.; Faucher-Giguère, Claude-André; Gurvich, Alexander B.; Schaye, Joop; Hayward, Christopher C. (August 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Interstellar chemistry is important for galaxy formation, as it determines the rate at which gas can cool, and enables us to make predictions for observable spectroscopic lines from ions and molecules. We explore two central aspects of modelling the chemistry of the interstellar medium (ISM): (1) the effects of local stellar radiation, which ionizes and heats the gas, and (2) the depletion of metals on to dust grains, which reduces the abundance of metals in the gas phase. We run high-resolution (400 M⊙ per baryonic particle) simulations of isolated disc galaxies, from dwarfs to Milky Way-mass, using the fire galaxy formation models together with the chimes non-equilibrium chemistry and cooling module. In our fiducial model, we couple the chemistry to the stellar fluxes calculated from star particles using an approximate radiative transfer scheme; and we implement an empirical density-dependent prescription for metal depletion. For comparison, we also run simulations with a spatially uniform radiation field, and without metal depletion. Our fiducial model broadly reproduces observed trends in H i and H2 mass with stellar mass, and in line luminosity versus star formation rate for [C ii]$$_{158 \rm {\mu m}}$$, [O i]$$_{63 \rm {\mu m}}$$, [O iii]$$_{88 \rm {\mu m}}$$, [N ii]$$_{122 \rm {\mu m}}$$, and H α6563Å. Our simulations with a uniform radiation field predict fainter luminosities, by up to an order of magnitude for [O iii]$$_{88 \rm {\mu m}}$$ and H α6563Å, while ignoring metal depletion increases the luminosity of carbon and oxygen lines by a factor ≈ 2. However, the overall evolution of the galaxy is not strongly affected by local stellar fluxes or metal depletion, except in dwarf galaxies where the inclusion of local fluxes leads to weaker outflows and hence higher gas fractions. 

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3. A Reanalysis of the Composition of K2-106b: An Ultra-short-period Super-Mercury Candidate

   https://doi.org/10.3847/1538-3881/acb04b  

   Rodríguez Martínez, Romy; Gaudi, B. Scott; Schulze, Joseph G.; Acuña, Lorena; Kolecki, Jared; Johnson, Jennifer A.; Asnodkar, Anusha Pai; Boley, Kiersten M.; Deleuil, Magali; Mousis, Olivier; et al (February 2023, The Astronomical Journal)

   We present a reanalysis of the K2-106 transiting planetary system, with a focus on the composition of K2-106b, an ultra-short-period, super-Mercury candidate. We globally model existing photometric and radial velocity data and derive a planetary mass and radius for K2-106b of Mp = 8.53 ± 1.02 M⊕ and = - + Rp 1.71 0.057 RÅ 0.069 , which leads to a density of r = - + 9.4 p 1.5 1.6 g cm−3 , a significantly lower value than previously reported in the literature. We use planet interior models that assume a two-layer planet comprised of a liquid, pure Fe core and an iron-free, MgSiO3 mantle, and we determine that the range of the core mass fractions are consistent with the observed mass and radius. We use existing high-resolution spectra of the host star to derive the Fe/Mg/Si abundances ([Fe/ H] = −0.03 ± 0.01, [Mg/H] = 0.04 ± 0.02, [Si/H] = 0.03 ± 0.06) to infer the composition of K2-106b. We find that K2-106b has a density and core mass fraction ( - + 44 %15 12 ) consistent with that of Earth (CMF⊕ = 32%). Furthermore, its composition is consistent with what is expected, assuming that it reflects the relative refractory abundances of its host star. K2-106b is therefore unlikely to be a super-Mercury, as has been suggested in previous literature. 

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4. Effects of cloud geometry and metallicity on shattering and coagulation of cold gas, and implications for cold streams penetrating virial shocks

   https://doi.org/10.1093/mnras/stae2771  

   Yao, Zhiyuan; Mandelker, Nir; Oh, S_Peng; Aung, Han; Dekel, Avishai (December 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Theory and observations reveal that the circumgalactic medium (CGM) and the cosmic web at high redshifts are multiphase, with small clouds of cold gas embedded in a hot, diffuse medium. We study the ‘shattering’ of large, thermally unstable clouds into tiny cloudlets of size $$\ell _{\rm shatter}\sim {\rm min}(c_{\rm s}t_{\rm cool})$$ using idealized numerical simulations. We expand upon previous works by exploring the effects of cloud geometry (spheres, streams, and sheets), metallicity, and an ionizing ultraviolet background. We find that ‘shattering’ is mainly triggered by clouds losing sonic contact and rapidly imploding, leading to a reflected shock that causes the cloud to re-expand and induces Richtmyer–Meshkov instabilities at its interface. The fragmented cloudlets experience a drag force from the surrounding hot gas, leading to recoagulation into larger clouds. We distinguish between ‘fast’ and ‘slow’ coagulation regimes. Sheets are always in the ‘fast’ coagulation regime, while streams and spheres transition to ‘slow’ coagulation above a critical overdensity, which is smallest for spheres. Surprisingly, $$\ell _\mathrm{shatter}$$ does not appear to be a characteristic clump size even if it is well resolved. Rather, fragmentation continues until the grid scale with a mass distribution of $$N(\gt m)\propto m^{-1}$$. We apply our results to cold streams feeding massive ($$M_{\rm v}\lower.5ex\rm{\,\, \buildrel\gt \over \sim \,\,}10^{12}\, {\rm M}_\odot$$) galaxies at $$z\lower.5ex\rm{\,\, \buildrel\gt \over \sim \,\,}2$$ from the cosmic web, finding that streams likely shatter upon entering the hot CGM through the virial shock. This could explain the large clumping factors and covering fractions of cold gas around such galaxies, and may be related to galaxy quenching by preventing cold streams from reaching the central galaxy. 

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5. A unicorn in monoceros: the 3 M⊙ dark companion to the bright, nearby red giant V723 Mon is a non-interacting, mass-gap black hole candidate

   https://doi.org/10.1093/mnras/stab907  

   Jayasinghe, T; Stanek, K Z; Thompson, Todd A; Kochanek, C S; Rowan, D M; Vallely, P J; Strassmeier, K G; Weber, M; Hinkle, J T; Hambsch, F-J; et al (May 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We report the discovery of the closest known black hole candidate as a binary companion to V723 Mon. V723 Mon is a nearby ($$d\sim 460\, \rm pc$$), bright (V ≃ 8.3 mag), evolved (Teff, giant ≃ 4440 K, and Lgiant ≃ 173 L⊙) red giant in a high mass function, f(M) = 1.72 ± 0.01 M⊙, nearly circular binary (P = 59.9 d, e ≃ 0). V723 Mon is a known variable star, previously classified as an eclipsing binary, but its All-Sky Automated Survey, Kilodegree Extremely Little Telescope, and Transiting Exoplanet Survey Satellite light curves are those of a nearly edge-on ellipsoidal variable. Detailed models of the light curves constrained by the period, radial velocities, and stellar temperature give an inclination of $$87.0^{\circ ^{+1.7^\circ }}_{-1.4^\circ }$$, a mass ratio of q ≃ 0.33 ± 0.02, a companion mass of Mcomp = 3.04 ± 0.06 M⊙, a stellar radius of Rgiant = 24.9 ± 0.7 R⊙, and a giant mass of Mgiant = 1.00 ± 0.07 M⊙. We identify a likely non-stellar, diffuse veiling component with contributions in the B and V band of $${\sim }63{{\ \rm per\ cent}}$$ and $${\sim }24{{\ \rm per\ cent}}$$, respectively. The SED and the absence of continuum eclipses imply that the companion mass must be dominated by a compact object. We do observe eclipses of the Balmer lines when the dark companion passes behind the giant, but their velocity spreads are low compared to observed accretion discs. The X-ray luminosity of the system is $$L_{\rm X}\simeq 7.6\times 10^{29}~\rm ergs~s^{-1}$$, corresponding to L/Ledd ∼ 10−9. The simplest explanation for the massive companion is a single compact object, most likely a black hole in the ‘mass gap’. 

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