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

TL;DR

This paper introduces new fully evolutionary models for ultra-massive white dwarf stars, focusing on their pulsational properties and core composition, to improve understanding of crystallization effects and stellar evolution.

Contribution

It presents the first detailed asteroseismic analysis of the ultra-massive ZZ Ceti star BPM~37093 using evolutionary models with crystallized cores, and outlines plans for further core composition studies.

Findings

Initial asteroseismic analysis of BPM~37093 with crystallized models.

Plans to compare C/O core models to determine core composition.

Future extension to other stars observed by Kepler and TESS.

Abstract

Ultra-massive hydrogen-rich (DA spectral type) white dwarf (WD) stars ($M_{\star} > 1M_{\odot}$) coming from single-star evolution are expected to harbor cores made of $^{16}$O and $^{20}$Ne, resulting from semi-degenerate carbon burning when the progenitor star evolves through the super asymptotic giant branch (S-AGB) phase. These stars are expected to be crystallized by the time they reach the ZZ Ceti instability strip ($T_{\rm eff} \sim 12\,500$ K). Theoretical models predict that crystallization leads to a separation of $^{16}$O and $^{20}$Ne in the core of ultra-massive WDs, which impacts their pulsational properties. This property offers a unique opportunity to study the processes of crystallization. Here, we present 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 this 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 possible evolutionary origin. We also plan to extend this kind of analyses to other stars observed from the ground and also from space missions like Kepler and TESS.