The composition of massive white dwarfs and their dependence on the C-burning modeling

Abstract

Recent computations of the interior composition of ultra-massive white dwarfs (WD) have suggested that some white dwarfs could be composed of neon (Ne)-dominated cores. This result is at variance with our previous understanding of the chemical structure of massive white dwarfs, where oxygen is the predominant element. In addition, it is not clear whether some hybrid carbon (C) oxygen (O)-Ne white dwarfs might form when convective boundary mixing is accounted for during the propagation of the C-flame, in the C-burning stage. Both Ne-dominated and hybrid CO-Ne core would have measurable consequences for asteroseismological studies based on evolutionary models. In this work we explore in detail to which extent differences in the adopted micro- and macro-physics can explain the different final white dwarf compositions that have been found by different authors. Additionally, we explored the impact of such differences in the cooling times, crystallization and pulsational properties of pulsating WDs. We explore the impact of the intensity of convective boundary mixing during the C-flash, extreme mass-loss rates, and the size of the adopted nuclear networks on the final composition, age, crystallization and pulsational properties of white dwarfs. Based on the insight coming from 3D hydro-dynamical simulations, we expect that the very slow propagation of the carbon flame will be altered by turbulent entrainment affecting the inward propagation of the flame. Also, we find that Ne-dominated chemical profiles of massive WDs recently reported appear in their modeling due to the overlooking of a key nuclear reaction. We find that the inaccuracies in the chemical composition of ultra-massive white dwarfs recently reported lead to differences of 10% in the cooling times and degree of crystallization and about 8% in the period spacing of the models once they reach the ZZ Ceti instability strip.

arXiv comment10 pages, 9 figures, 1 table, accepted for publication in A&A

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