The evolution of ultra-massive white dwarfs

CAMISASSA, M. E.; ALTHAUS, L. G.; CÓRSICO, A.H.; DE GERÓNIMO, F. C.; MILLER BERTOLAMI, M. M.; NOVARINO, M. L. ; ROHRMANN, R. D.; WACHLIN, F. C. ; GARCÍA-BERRO, E.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2019 vol. 625 p. 87 - 99

0004-6361

Ultra-massive white dwarfs are powerful tools used to study various physical processes in the asymptotic giant branch (AGB), typeIa supernova explosions, and the theory of crystallization through white dwarf asteroseismology. Despite the interest in these whitedwarfs, there are few evolutionary studies in the literature devoted to them. Here we present new ultra-massive white dwarf evolution-ary sequences that constitute an improvement over previous ones. In these new sequences we take into account for the first time theprocess of phase separation expected during the crystallization stage of these white dwarfs by relying on the most up-to-date phasediagram of dense oxygen/neon mixtures. Realistic chemical profiles resulting from the full computation of progenitor evolution dur-ing the semidegenerate carbon burning along the super-AGB phase are also considered in our sequences. Outer boundary conditionsfor our evolving models are provided by detailed non-gray white dwarf model atmospheres for hydrogen and helium composition.We assessed the impact of all these improvements on the evolutionary properties of ultra-massive white dwarfs, providing updatedevolutionary sequences for these stars. We conclude that crystallization is expected to affect the majority of the massive white dwarfsobserved with effective temperatures below 40 000 K. Moreover, the calculation of the phase separation process induced by crystal-lization is necessary to accurately determine the cooling age and the mass-radius relation of massive white dwarfs. We also providecolors in theGaiaphotometric bands for our H-rich white dwarf evolutionary sequences on the basis of new model atmospheres.Finally, these new white dwarf sequences provide a new theoretical frame to perform asteroseismological studies on the recentlydetected ultra-massive pulsating white dwarfs