New fully evolutionary models for asteroseismology of ultra-massive white-dwarf stars

CAMISASSA, MARIA E.; DE GERONIMO, FRANCISCO; CORSICO, ALEJANDRO H.; ALTHAUS, LEANDRO G.

Viena

Workshop; Stars and their Variability Observed from Space,; 2019

Universidad de Viena

Ultra-massive hydrogen-rich white dwarf (WD) stars of spectral type DA (M⋆>1M⊙) coming from single-star evolution are expected to harbour cores made of 16O and 20Ne, resulting from semi-degenerate carbon burning when the progenitor star evolved through the super-asymptotic giant branch phase. These stars are expected to be crystallized by the time they reach the ZZ Ceti instability strip (Teff∼12500 K). Theoretical models predict that crystallization leads to a separation of 16O and 20Ne in the core of ultra-massive WDs, which has an impact on their pulsational properties. That property offers a unique opportunity to study the processes of crystallization. This paper presented the first results of a detailed asteroseismic analysis of the best-studied ultra-massive ZZ Ceti star, BPM 37093. As a second step, we plan to repeat the analysis using ultra-massive DA WD models with C/O cores in order to study the possibility of elucidating the core chemical composition of BPM 37093 and shed some light on its evolutionary origin. We also plan to extend this kind of analysis to other stars observed from the ground and with space instruments like Kepler and TESS.