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Title: The Final Fates of Close Hot Subdwarf–White Dwarf Binaries: Mergers Involving He/C/O White Dwarfs and the Formation of Unusual Giant Stars with C/O-Dominated Envelopes
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The Astrophysical Journal
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Abstract

    The diffusion coefficients of neutron rich nuclei in crystallizing white dwarf (WD) stars are essential microphysics input for modeling the evolution of the composition profile. Recently, molecular dynamics simulations have been used to compute diffusion coefficients for realistic mixtures of C–O and O–Ne WDs with many trace nuclides that could be important sedimentary heat sources such as22Ne,23Na,25Mg, and27Mg. In this brief note, I repeat these simulations but now include56Fe. I find that for the large charge ratios involved in these mixtures the empirical law developed in our earlier work tends to under-predict diffusion coefficients in the moderately coupled regime by 30%–40%. As this formalism is presently implemented in the stellar evolution code MESA, it is important for authors studying mixtures containing heavy nuclides like56Fe to be aware of these systematics. However, the impact on astrophysics is expected to be small.

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  3. Abstract

    The dimmest and most numerous outlier of the Type Ia supernova population, the Type Iax event, is increasingly being found in the results of observational campaigns. There is currently no single accepted model to describe these events. This 2D study explores the viability of modeling Type Iax events as a hybrid C/O/Ne white dwarf progenitor undergoing a deflagration using the multiphysics software FLASH. This hybrid was created using the stellar evolution code MESA, and its C-depleted core and mixed structure have demonstrated lower yields than traditional C/O progenitors in previous deflagration-to-detonation studies. To generate a sample, 30 “realizations” of this simulation were performed, the only difference being the shape of the initial match head used to start the deflagration. Consistent with earlier work, these realizations produce the familiar hot dense bound remnant surrounded by sparse ejecta. Our results indicate that the majority of the star remains unburned (∼70%) and bound (>90%). Our realizations produce total ejecta yields on the order of 10−2–10−1M, ejected56Ni yields on the order of 10−4–10−2M, and ejecta kinetic energies on the order of 1048–1049erg. Compared to yields inferred from recent observations of the dimmest Type Iax events—SN 2007qd, SN 2008ha, SN 2010ae, SN 2019gsc, SN 2019muj, SN 2020kyg, and SN 2021fcg—our simulation produces comparable56Ni yields but too-small total yields and kinetic energies. Reignition of the remnant is also seen in some realizations.

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