New DA white dwarf models for asteroseismology of ZZ Ceti stars

Abstract

Context. Asteroseismology is a powerful tool used to infer the evolutionary status and chemical stratification of white dwarf stars and to gain insights into the physical processes that lead to their formation. This is particularly true for the variable hydrogen-rich atmosphere (DA) white dwarfs, known as DAV or ZZ Ceti stars. They constitute the most numerous class of pulsating white dwarfs. Aims. We present a new grid of white dwarf models that take into account advances made over the last decade in modeling and input physics of both the progenitor and the white dwarf stars. As a result, it is possible to avoid several shortcomings present in the set of white dwarf models employed in the asteroseismological analyses of ZZ Ceti stars that we carried out in our previous works. Methods. We generate white dwarf stellar models appropriate for ZZ Ceti stars with masses from ∼0.52 to ∼0.83 M⊙, resulting from the whole evolution of initially 1.5–4.0 M⊙ mass star models. These new models are derived from a self-consistent way with the changes in the internal chemical distribution that result from the mixing of all the core chemical components induced by mean molecular-weight inversions, from 22Ne diffusion, Coulomb sedimentation, and from residual nuclear burning. In addition, the expected nuclear-burning history and mixing events along the progenitor evolution are accounted for, in particular the occurrence of third dredge-up, which determines the properties of the core and envelope of post-AGB and white dwarf stars, as well as the white dwarf initial-final mass relation. The range of hydrogen envelopes of our new ZZ Ceti models extends from the maximum residual hydrogen content predicted by the progenitor history, log(MH/M⊙)∼ − 4 to −5, to log(MH/M⊙) = − 13.5, thus allowing for the first stellar models that would enable the search for seismological solutions for ZZ Ceti stars with extremely thin hydrogen envelopes – if, indeed, they do exist in nature. We computed the adiabatic gravity(g)-mode pulsation periods of these models. Calculations of our new evolutionary and pulsational ZZ Ceti models were performed with the LPCODE stellar evolution code and the LP-PUL stellar pulsation code. Results. Our new hydrogen-burning post-AGB models predict chemical structures for ZZ Ceti stars that are substantially different from those we used in our previous works, particularly in connection with the chemical profiles of oxygen and carbon near the stellar centre. We also discuss the implications of these new models for the pulsational spectrum of ZZ Ceti stars. Specifically, we find that the pulsation periods of g modes and the mode-trapping properties of the new models differ significantly from those characterizing the ZZ Ceti models of our previous works, particularly for long periods. Conclusions. The improvements in the modeling of ZZ Ceti stars we present here lead to substantial differences in the predicted pulsational properties of ZZ Ceti stars, which are expected to impact the asteroseismological inferences of these stars. This is extremely relevant in view of the abundant amount of photometric data from current and future space missions, resulting in discoveries of numerous ZZ Ceti stars.

Seismological Studies of Pulsating DA White Dwarfs Observed with the Kepler Space Telescope and K2 Campaigns 1–8

“…The parameters chosen are listed in Table 6. For the central oxygen abundance, we did not go below 0.50, consistent with the predictions of stellar evolution (e.g., Metcalfe et al 2002;Althaus & Córsico 2022). We then compared fits for KIC 7594781 and EPIC 201719578 with each of these core combinations.…”

Stellar Neutrino Emission across the Mass–Metallicity Plane

“…Plasmon neutrino rates scale as Y e 3 , leading to a smaller E ν; hence more metal-rich models have a larger E γ /E ν than metal-poor models in this M ZAMS range. The dependence of CO WD on the 22 Ne mass fraction, the degree of neutronization, may have implications for the progenitor Type Ia supernova (Timmes et al 2003;Townsley et al 2009;Bravo et al 2010;Piersanti et al 2022) and the pulsation periods of variable WD (Campante et al 2016;Chidester et al 2021;Althaus & Córsico 2022). Farag et al (2020) showed L γ /L ν ; 40 for a standard solar model.…”

The Impact of Breathing Pulses during Core Helium Burning on the Core Chemical Structure and Pulsations of Hydrogen-rich Atmosphere White Dwarfs

“…All of the sequences were computed for metallicity Z = 0.01 starting from the ZAMS and evolved through the CHeB phase to the thermal pulses on the AGB and finally to the domain of the ZZ Ceti stars at advanced stages of WD cooling. For this purpose, we have used the stellar evolution code LPCODE, developed by the La Plata group (Althaus et al 2005;Salaris et al 2013;Althaus et al 2015;Miller Bertolami 2016;Althaus & Córsico 2022). Our treatment for the progenitor evolution considers new observational constraints and recent advances in the micro-and macrophysics involved in the modeling of CHeB, AGB, and thermally pulsing phases (see Miller Bertolami 2016 for details).…”

“…Recent asteroseismological studies of pulsating DA (H-rich atmospheres) WDs-also called DAV or ZZ Ceti starsand pulsating DB (He-rich atmospheres) WDs-also known as DBV or V777 Her stars-based on static parametric models of WDs have been carried out by Giammichele et al (2018Giammichele et al ( , 2021 and Charpinet et al (2021). In the case of four DAV stars (SDSS J1136+0409, EPIC 220347759, KIC 11911480, and L 19−2), and a DBV star (KIC 08626021), these studies have resulted in asteroseismic models characterized by cores and central O abundances that are substantially larger than those predicted by canonical evolutionary calculations of WD progenitors (Althaus et al 2010a;Renedo et al 2010;Salaris et al 2010;Althaus & Córsico 2022). In this sense, Timmes et al (2018) have shown that the inclusion of neutrino cooling in the models of DB WDs could lead to a different asteroseismological model for KIC 08626021 than the one found by Giammichele et al (2018).…”