Asteroseismological analysis of the ultra-massive ZZ Ceti stars BPM~37093, GD~518, and SDSS~J0840+5222

Abstract

Ultra-massive hydrogen-rich (DA) white dwarfs (WD) are expected to have a substantial portion of their cores in a crystalline state at the effective temperatures characterizing the ZZ Ceti instability strip, as a result of Coulomb interactions. Asteroseismological analyses of these WDs can provide valuable information related to the crystallization process, the core chemical composition and the evolutionary origin of these stars. We present a thorough asteroseismological analysis of the ultra-massive ZZ Ceti star BPM~37093, which exhibits a rich period spectrum, on the basis of a complete set of fully evolutionary models that represent ultra-massive oxygen/neon(ONe)-core DA WD stars harbouring a range of hydrogen (H) envelope thicknesses. We also carry out preliminary asteroseismological inferences on two other ultra-massive ZZ Ceti stars that exhibit fewer periods, GD~518, and SDSS~J0840+5222. We found a mean period spacing of $\Delta \Pi \sim 17$ s in the data of BPM~37093, associated to $\ell =2$ modes. We found asteroseismological models for the three objects under analysis, two of them (BPM~37093 and GD~518) characterized by canonical (thick) H envelopes, and the third one (SDSS~J0840+5222) with a thinner H envelope. We also derive asteroseismological distances which are somewhat different to the astrometric measurements of {\it Gaia} for these stars. Asteroseismological analyses like the one presented in this paper could lead to a more complete knowledge of the processes occurring during crystallization inside WDs. Also, they could make it possible to deduce the core chemical composition of ultra-massive WDs, and in this way, to infer their evolutionary origin, i.e., if the star has a ONe core and then is the result of single-star evolution, or if it harbour a carbon/oxygen (CO) core and is the product of a merger of the two components of a binary system.

13 pages, 10 figures, 7 tables. To be published in Astronomy and Astrophysics

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• The evolution of ultra-massive carbon oxygen white dwarfs
• Structure and evolution of ultra-massive white dwarfs in general relativity
• A Deficit of Massive White Dwarfs in Gaia Astrometric Binaries
• Cooling Delays from Iron Sedimentation and Iron Inner Cores in White Dwarfs
• WD J004917.14$-$252556.81, the Most Massive Pulsating White Dwarf
• Can we reveal the core-chemical composition of ultra-massive white dwarfs through their magnetic fields?
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• The composition of massive white dwarfs and their dependence on the C-burning modeling
• The impact of Coulomb diffusion of ions on the pulsational properties of DA white dwarfs
• The pulsational properties of ultra-massive DB white dwarfs with carbon-oxygen cores coming from single-star evolution
• Three-component Phase Separation for Ultramassive White Dwarf Models
• Seismology and diffusion of ultramassive white dwarf magnetic fields