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1. General relativistic pulsations of ultra-massive ZZ Ceti stars

   https://doi.org/10.1093/mnras/stad2248  

   Córsico, Alejandro_H; Boston, S_Reece; Althaus, Leandro_G; Kilic, Mukremin; Kepler, S_O; Camisassa, María_E; Torres, Santiago (July 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Ultra-massive white dwarf stars are currently being discovered at a considerable rate, thanks to surveys such as the Gaia space mission. These dense and compact stellar remnants likely play a major role in Type Ia supernova explosions. It is possible to probe the interiors of ultra-massive white dwarfs through asteroseismology. In the case of the most massive white dwarfs, general relativity could affect their structure and pulsations substantially. In this work, we present results of relativistic pulsation calculations employing relativistic ultra-massive ONe-core white dwarf models with hydrogen-rich atmospheres and masses ranging from 1.29 to $$1.369 \ \mathrm{M}_{\odot }$$ with the aim of assessing the impact of general relativity on the adiabatic gravity (g)-mode period spectrum of very high mass ZZ Ceti stars. Employing the relativistic Cowling approximation for the pulsation analysis, we find that the critical buoyancy (Brunt–Väisälä) and acoustic (Lamb) frequencies are larger for the relativistic case, compared to the Newtonian case, due to the relativistic white dwarf models having smaller radii and higher gravities for a fixed stellar mass. In addition, the g-mode periods are shorter in the relativistic case than those in the Newtonian computations, with relative differences of up to ∼$50$ per cent for the highest mass models ($$1.369 \ \mathrm{M}_{\odot }$$) and for effective temperatures typical of the ZZ Ceti instability strip. Hence, the effects of general relativity on the structure, evolution, and pulsations of white dwarfs with masses larger than ∼$$1.29 \ \mathrm{M}_{\odot }$$ cannot be ignored in the asteroseismological analysis of ultra-massive ZZ Ceti stars. 

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2. Search for Ultraheavy Dark Matter from Observations of Dwarf Spheroidal Galaxies with VERITAS

   https://doi.org/10.3847/1538-4357/acbc7b  

   Acharyya, A.; Archer, A.; Bangale, P.; Bartkoske, J. T.; Batista, P.; Baumgart, M.; Benbow, W.; Buckley, J. H.; Falcone, A.; Feng, Q.; et al (March 2023, The Astrophysical Journal)

   Abstract Dark matter is a key piece of the current cosmological scenario, with weakly interacting massive particles (WIMPs) a leading dark matter candidate. WIMPs have not been detected in their conventional parameter space (100 GeV ≲ M χ ≲ 100 TeV), a mass range accessible with current Imaging Atmospheric Cherenkov Telescopes. As ultraheavy dark matter (UHDM; M χ ≳ 100 TeV) has been suggested as an underexplored alternative to the WIMP paradigm, we search for an indirect dark matter annihilation signal in a higher mass range (up to 30 PeV) with the VERITAS γ -ray observatory. With 216 hr of observations of four dwarf spheroidal galaxies, we perform an unbinned likelihood analysis. We find no evidence of a γ -ray signal from UHDM annihilation above the background fluctuation for any individual dwarf galaxy nor for a joint-fit analysis, and consequently constrain the velocity-weighted annihilation cross section of UHDM for dark matter particle masses between 1 TeV and 30 PeV. We additionally set constraints on the allowed radius of a composite UHDM particle. 

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3. An INTEGRAL/SPI view of reticulum II: particle dark matter and primordial black holes limits in the MeV range

   https://doi.org/10.1093/mnras/stac008  

   Siegert, Thomas; Boehm, Celine; Calore, Francesca; Diehl, Roland; Krause, Martin G; Serpico, Pasquale D; Vincent, Aaron C (February 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Reticulum II (Ret II) is a satellite galaxy of the Milky Way (MW) and presents a prime target to investigate the nature of dark matter (DM) because of its high mass-to-light ratio. We evaluate a dedicated INTEGRAL observation campaign data set to obtain γ-ray fluxes from Ret II and compare those with expectations from DM. Ret II is not detected in the γ-ray band 25–8000 keV, and we derive a flux limit of $${\lesssim}10^{-8}\, \mathrm{erg\, cm^{-2}\, s^{-1}}$$. The previously reported 511 keV line is not seen, and we find a flux limit of $${\lesssim}1.7 \times 10^{-4}\, \mathrm{ph\, cm^{-2}\, s^{-1}}$$. We construct spectral models for primordial black hole (PBH) evaporation and annihilation/decay of particle DM, and subsequent annihilation of e+s produced in these processes. We exclude that the totality of DM in Ret II is made of a monochromatic distribution of PBHs of masses $${\lesssim}8 \times 10^{15}\, \mathrm{g}$$. Our limits on the velocity-averaged DM annihilation cross section into e+e− are $$\langle \sigma v \rangle \lesssim 5 \times 10^{-28} \left(m_{\rm DM} / \mathrm{MeV} \right)^{2.5}\, \mathrm{cm^3\, s^{-1}}$$. We conclude that analysing isolated targets in the MeV γ-ray band can set strong bounds on DM properties without multi-year data sets of the entire MW, and encourage follow-up observations of Ret II and other dwarf galaxies. 

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4. Can magnetized turbulence set the mass scale of stars?

   https://doi.org/10.1093/mnras/staa1883  

   Guszejnov, Dávid; Grudić, Michael Y; Hopkins, Philip F; Offner, Stella S; Faucher-Giguère, Claude-André (August 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Understanding the evolution of self-gravitating, isothermal, magnetized gas is crucial for star formation, as these physical processes have been postulated to set the initial mass function (IMF). We present a suite of isothermal magnetohydrodynamic (MHD) simulations using the gizmo code that follow the formation of individual stars in giant molecular clouds (GMCs), spanning a range of Mach numbers found in observed GMCs ($$\mathcal {M} \sim 10\!-\!50$$). As in past works, the mean and median stellar masses are sensitive to numerical resolution, because they are sensitive to low-mass stars that contribute a vanishing fraction of the overall stellar mass. The mass-weighted median stellar mass M50 becomes insensitive to resolution once turbulent fragmentation is well resolved. Without imposing Larson-like scaling laws, our simulations find $$M_\mathrm{50} \,\, \buildrel\propto \over \sim \,\,M_\mathrm{0} \mathcal {M}^{-3} \alpha _\mathrm{turb}\, \mathrm{SFE}^{1/3}$$ for GMC mass M0, sonic Mach number $$\mathcal {M}$$, virial parameter αturb, and star formation efficiency SFE = M⋆/M0. This fit agrees well with previous IMF results from the ramses, orion2, and sphng codes. Although M50 has no significant dependence on the magnetic field strength at the cloud scale, MHD is necessary to prevent a fragmentation cascade that results in non-convergent stellar masses. For initial conditions and SFE similar to star-forming GMCs in our Galaxy, we predict M50 to be $$\gt 20 \, \mathrm{M}_{\odot }$$, an order of magnitude larger than observed ($$\sim 2 \, \mathrm{M}_\odot$$), together with an excess of brown dwarfs. Moreover, M50 is sensitive to initial cloud properties and evolves strongly in time within a given cloud, predicting much larger IMF variations than are observationally allowed. We conclude that physics beyond MHD turbulence and gravity are necessary ingredients for the IMF. 

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5. But what about...: cosmic rays, magnetic fields, conduction, and viscosity in galaxy formation

   https://doi.org/10.1093/mnras/stz3321  

   Hopkins, Philip F; Chan, T K; Garrison-Kimmel, Shea; Ji, Suoqing; Su, Kung-Yi; Hummels, Cameron B; Kereš, Dušan; Quataert, Eliot; Faucher-Giguère, Claude-André (March 2020, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present and study a large suite of high-resolution cosmological zoom-in simulations, using the FIRE-2 treatment of mechanical and radiative feedback from massive stars, together with explicit treatment of magnetic fields, anisotropic conduction and viscosity (accounting for saturation and limitation by plasma instabilities at high β), and cosmic rays (CRs) injected in supernovae shocks (including anisotropic diffusion, streaming, adiabatic, hadronic and Coulomb losses). We survey systems from ultrafaint dwarf ($$M_{\ast }\sim 10^{4}\, \mathrm{M}_{\odot }$$, $$M_{\rm halo}\sim 10^{9}\, \mathrm{M}_{\odot }$$) through Milky Way/Local Group (MW/LG) masses, systematically vary uncertain CR parameters (e.g. the diffusion coefficient κ and streaming velocity), and study a broad ensemble of galaxy properties [masses, star formation (SF) histories, mass profiles, phase structure, morphologies, etc.]. We confirm previous conclusions that magnetic fields, conduction, and viscosity on resolved ($$\gtrsim 1\,$$ pc) scales have only small effects on bulk galaxy properties. CRs have relatively weak effects on all galaxy properties studied in dwarfs ($$M_{\ast } \ll 10^{10}\, \mathrm{M}_{\odot }$$, $$M_{\rm halo} \lesssim 10^{11}\, \mathrm{M}_{\odot }$$), or at high redshifts (z ≳ 1–2), for any physically reasonable parameters. However, at higher masses ($$M_{\rm halo} \gtrsim 10^{11}\, \mathrm{M}_{\odot }$$) and z ≲ 1–2, CRs can suppress SF and stellar masses by factors ∼2–4, given reasonable injection efficiencies and relatively high effective diffusion coefficients $$\kappa \gtrsim 3\times 10^{29}\, {\rm cm^{2}\, s^{-1}}$$. At lower κ, CRs take too long to escape dense star-forming gas and lose their energy to collisional hadronic losses, producing negligible effects on galaxies and violating empirical constraints from spallation and γ-ray emission. At much higher κ CRs escape too efficiently to have appreciable effects even in the CGM. But around $$\kappa \sim 3\times 10^{29}\, {\rm cm^{2}\, s^{-1}}$$, CRs escape the galaxy and build up a CR-pressure-dominated halo which maintains approximate virial equilibrium and supports relatively dense, cool (T ≪ 106 K) gas that would otherwise rain on to the galaxy. CR ‘heating’ (from collisional and streaming losses) is never dominant. 

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