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Abstract We seek signatures of the current experimental 12 C α , γ 16 O reaction rate probability distribution function in the pulsation periods of carbon–oxygen white dwarf (WD) models. We find that adiabatic gmodes trapped by the interior carbonrich layer offer potentially useful signatures of this reaction rate probability distribution function. Probing the carbonrich region is relevant because it forms during the evolution of lowmass stars under radiative heliumburning conditions, mitigating the impact of convective mixing processes. We make direct quantitative connections between the pulsation periods of the identified trapped gmodes in variable WD models and the current experimental 12 C α , γ 16 O reaction rate probability distribution function. We find an average spread in relative period shifts of Δ P / P ≃ ±2% for the identified trapped gmodes over the ±3 σ uncertainty in the 12 C α , γ 16 O reaction rate probability distribution function—across the effective temperature range of observed DAV and DBV WDs and for different WD masses, helium shell masses, and hydrogen shell masses. The gmode pulsation periods of observed WDs are typically given to six to seven significant figures of precision. This suggests that an astrophysical constraint on themore »Free, publiclyaccessible full text available August 1, 2023

Abstract Gravitationalwave (GW) detections of binary black hole (BH) mergers have begun to sample the cosmic BH mass distribution. The evolution of single stellar cores predicts a gap in the BH mass distribution due to pairinstability supernovae (PISNe). Determining the upper and lower edges of the BH mass gap can be useful for interpreting GW detections of merging BHs. We use
MESA to evolve single, nonrotating, massive helium cores with a metallicity ofZ = 10^{−5}, until they either collapse to form a BH or explode as a PISN, without leaving a compact remnant. We calculate the boundaries of the lower BH mass gap for Sfactors in the range S(300 keV) = (77,203) keV b, corresponding to the ±3σ uncertainty in our highresolution tabulated^{12}C(α ,γ )^{16}O reaction rate probability distribution function. We extensively test temporal and spatial resolutions for resolving the theoretical peak of the BH mass spectrum across the BH mass gap. We explore the convergence with respect to convective mixing and nuclear burning, finding that significant time resolution is needed to achieve convergence. We also test adopting a minimum diffusion coefficient to help lowerresolution models reach convergence. We establish a new lower edge of the upper mass gap asM _{lower}≃ ${60}_{14}^{+32}$M _{⊙}from the ±3σ uncertainty inmore » 
Context. At present, there are strong indications that white dwarf (WD) stars with masses well below the Chandrasekhar limit ( M Ch ≈ 1.4 M ⊙ ) contribute a significant fraction of SN Ia progenitors. The relative fraction of stable irongroup elements synthesized in the explosion has been suggested as a possible discriminant between M Ch and sub M Ch events. In particular, it is thought that the higherdensity ejecta of M Ch WDs, which favours the synthesis of stable isotopes of nickel, results in prominent [Ni II ] lines in latetime spectra (≳150 d past explosion). Aims. We study the explosive nucleosynthesis of stable nickel in SNe Ia resulting from M Ch and sub M Ch progenitors. We explore the potential for lines of [Ni II ] in the optical an nearinfrared (at 7378 Å and 1.94 μm) in latetime spectra to serve as a diagnostic of the exploding WD mass. Methods. We reviewed stable Ni yields across a large variety of published SN Ia models. Using 1D M Ch delayeddetonation and sub M Ch detonation models, we studied the synthesis of stable Ni isotopes (in particular, 58 Ni) and investigated the formation of [Ni II ] lines usingmore »

Abstract We update the capabilities of the openknowledge software instrument Modules for Experiments in Stellar Astrophysics (
MESA ). The newauto _diff module implements automatic differentiation inMESA , an enabling capability that alleviates the need for hardcoded analytic expressions or finitedifference approximations. We significantly enhance the treatment of the growth and decay of convection inMESA with a new model for timedependent convection, which is particularly important during latestage nuclear burning in massive stars and electrondegenerate ignition events. We strengthenMESA ’s implementation of the equation of state, and we quantify continued improvements to energy accounting and solver accuracy through a discussion of different energy equation features and enhancements. To improve the modeling of stars inMESA , we describe key updates to the treatment of stellar atmospheres, molecular opacities, Compton opacities, conductive opacities, element diffusion coefficients, and nuclear reaction rates. We introduce treatments of starspots, an important consideration for lowmass stars, and modifications for superadiabatic convection in radiationdominated regions. We describe new approaches for increasing the efficiency of calculating monochromatic opacities and radiative levitation, and for increasing the efficiency of evolving the late stages of massive stars with a new operatorsplit nuclear burning mode. We close by discussing major updates toMESA ’s software infrastructure that enhance source code development and communitymore » 
Abstract Using groundbased gravitationalwave detectors, we probe the mass function of intermediatemass black holes (IMBHs) wherein we also include BHs in the upper mass gap at ∼60–130 M ⊙ . Employing the projected sensitivity of the upcoming LIGO and Virgo fourth observing run (O4), we perform Bayesian analysis on quasicircular nonprecessing, spinning IMBH binaries (IMBHBs) with total masses 50–500 M ⊙ , mass ratios 1.25, 4, and 10, and dimensionless spins up to 0.95, and estimate the precision with which the sourceframe parameters can be measured. We find that, at 2 σ , the mass of the heavier component of IMBHBs can be constrained with an uncertainty of ∼10%–40% at a signaltonoise ratio of 20. Focusing on the stellarmass gap with new tabulations of the 12 C( α , γ ) 16 O reaction rate and its uncertainties, we evolve massive helium core stars using MESA to establish the lower and upper edges of the mass gap as ≃ 59 − 13 + 34 M ⊙ and ≃ 139 − 14 + 30 M ⊙ respectively, where the error bars give the mass range that follows from the ±3 σ uncertainty in the 12 C( α , γ ) 16more »

Abstract Stellar evolution and numerical hydrodynamics simulations depend critically on access to fast, accurate, thermodynamically consistent equations of state. We present Skye, a new equation of state for fully ionized matter. Skye includes the effects of positrons, relativity, electron degeneracy, Coulomb interactions, nonlinear mixing effects, and quantum corrections. Skye determines the point of Coulomb crystallization in a selfconsistent manner, accounting for mixing and composition effects automatically. A defining feature of this equation of state is that it uses analytic free energy terms and provides thermodynamic quantities using automatic differentiation machinery. Because of this, Skye is easily extended to include new effects by simply writing new terms in the free energy. We also introduce a novel
thermodynamic extrapolation scheme for extending analytic fits to the free energy beyond the range of the fitting data while preserving desirable properties like positive entropy and sound speed. We demonstrate Skye in action in theMESA stellar evolution software instrument by computing white dwarf cooling curves. 
Abstract The collapse of degenerate oxygen–neon cores (i.e., electroncapture supernovae or accretioninduced collapse) proceeds through a phase in which a deflagration wave (“flame”) forms at or near the center and propagates through the star. In models, the assumed speed of this flame influences whether this process leads to an explosion or to the formation of a neutron star. We calculate the laminar flame speeds in degenerate oxygen–neon mixtures with compositions motivated by detailed stellar evolution models. These mixtures include trace amounts of carbon and have a lower electron fraction than those considered in previous work. We find that trace carbon has little effect on the flame speeds, but that material with electron fraction has laminar flame speeds that are times faster than those at . We provide tabulated flame speeds and a corresponding fitting function so that the impact of this difference can be assessed via full star hydrodynamical simulations of the collapse process.