Pulsation properties of ultra-massive DA white dwarf stars with ONe cores

DE GERÓNIMO, FRANCISCO C.; CÓRSICO, ALEJANDRO H.; ALTHAUS, LEANDRO G.; WACHLIN, FELIPE C.; CAMISASSA, MARÍA E.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Año: 2019 vol. 621

0004-6361

Context. Ultra-massive hydrogen-rich white dwarf stars are expected toharbor oxygen/neon cores resulting from the progenitor evolution throughthe super-asymptotic giant branch phase. As evolution proceeds duringthe white dwarf cooling phase, a crystallization process resulting fromCoulomb interactions in very dense plasmas is expected to occur, leadingto the formation of a highly crystallized core. In particular, pulsatingultra-massive white dwarfs offer a unique opportunity to infer and testthe occurrence of crystallization in white dwarf interiors as well asphysical processes related with dense plasmas. Aims: We aim toassess the adiabatic pulsation properties of ultra-massive hydrogen-richwhite dwarfs with oxygen/neon cores. Methods: We studied thepulsation properties of ultra-massive hydrogen-rich white dwarf starswith oxygen/neon cores. We employed a new set of ultra-massive whitedwarf evolutionary sequences of models with stellar masses in the range1.10 ≤ M⋆/M⊙ ≤ 1.29 computed bytaking into account the complete evolution of the progenitor stars andthe white dwarf stage. During the white dwarf cooling phase, weconsidered element diffusion. When crystallization set on in our models,we took into account latent heat release and also the expected changesin the core chemical composition that are due to phase separationaccording to a phase diagram suitable for oxygen and neon plasmas. Wecomputed nonradial pulsation g-modes of our sequences of models at theZZ Ceti phase by taking into account a solid core. We explored theimpact of crystallization on their pulsation properties, in particular,the structure of the period spectrum and the distribution of the periodspacings. Results: We find that it would be possible, inprinciple, to discern whether a white dwarf has a nucleus made of carbonand oxygen or a nucleus of oxygen and neon by studying the spacingbetween periods. Conclusions: The features found in theperiod-spacing diagrams could be used as a seismological tool to discernthe core composition of ultra-massive ZZ Ceti stars, this should becomplemented with detailed asteroseismic analysis using the individualobserved periods.