About the existence of warm H-rich pulsating white dwarfs

Abstract

Context. The possible existence of warm (T eff ∼ 19 000 K) pulsating DA white dwarf (WD) stars, hotter than ZZ Ceti stars, was predicted in theoretical studies more than 30 yr ago. These studies reported the occurrence of g-mode pulsational instabilities due to the κ mechanism acting in the partial ionization zone of He below the H envelope in models of DA WDs with very thin H envelopes (M H /M ⋆ 10 −10 ). However, to date, no pulsating warm DA WD has been discovered, despite the varied theoretical and observational evidence suggesting that a fraction of WDs should be formed with a range of very low H content. Aims. We re-examine the pulsational predictions for such WDs on the basis of new full evolutionary sequences. We analyze all the warm DAs observed by TESS satellite up to Sector 9 in order to search for the possible pulsational signal. Methods. We compute WD evolutionary sequences of masses 0.58 and 0.80 M ⊙ with H content in the range −14.5 log(M H /M ⋆ ) −10, appropriate for the study of pulsational instability of warm DA WDs. Initial models were extracted from progenitors that were evolved through very late thermal pulses on the early cooling branch. We use LPCODE stellar code to which we have incorporated a new full-implicit treatment of time-dependent element diffusion for precisely modeling the H/He transition zone in evolving WD models with very low H content. The non-adiabatic pulsations of our warm DA WD models were computed in the effective temperature range of 30 000 − 10 000 K, focusing on ℓ = 1 g modes with periods in the range 50 − 1500 s. Results. We find that traces of H surviving the very late thermal pulse float to the surface, eventually forming growing, thin pure H envelopes and rather extended H/He transition zones. We find that such extended transition zones inhibit the excitation of g modes due to partial ionization of He below the H envelope. Only in the case that the H/He transition is assumed much more abrupt than predicted by diffusion, models do exhibit pulsational instability. In this case, instabilities are found only in WD models with H envelopes in the range of −14.5 log(M H /M ⋆ ) −10 and at effective temperatures higher than those typical of ZZ Ceti stars, in agreement with previous studies. None of the 36 warm DAs observed so far by TESS satellite are found to pulsate. Conclusions. Our study suggests that the non-detection of pulsating warm DAs, if WDs with very thin H envelopes do exist, could be attributed to the presence of a smooth and extended H/He transition zone. This could be considered as an indirect proof that element diffusion indeed operates in the interior of WDs.

White-Dwarf Asteroseismology With the Kepler Space Telescope

“…Altogether, the Kepler space telescope has had a strong impact (and will continue to have) on the area of white-dwarf asteroseismology. This first step will be multiplied by current and future space missions, such as the Transiting Exoplanet Survey Satellite (TESS; Ricker et al, 2015), which is already in full operation and is providing the first results on pulsating white dwarfs (Bell et al, 2019;Althaus et al, 2020;Bognár et al, 2020), and other space missions that will become operational in the coming years, such as Cheops (Moya et al, 2018) and Plato (Piotto, 2018).…”

Exploring the pulsational properties of two ZZ Ceti stars

“…Hermes et al 2017a,b;Bell et al 2017b;Córsico 2020). In addition, published results have already demonstrated the value of the TESS data, focussing on a DBV star (Bell et al 2019), several ZZ Ceti stars (Bognár et al 2020;Althaus et al 2020), and GW Vir variables (Córsico et al 2021).…”

The formation of ultra-massive carbon-oxygen core white dwarfs and their evolutionary and pulsational properties

“…LPCODE has been tested and calibrated with other stellar evolutionary codes in different evolutionary stages, such as the red giant phase (Silva Aguirre et al 2020;Christensen-Dalsgaard et al 2020) and the WD stage (Salaris et al 2013). Relevant for the present work, LPCODE considers a new full-implicit treatment of time-dependent element diffusion that includes thermal and chemical diffusion and gravitational settling (Althaus et al 2020), outer boundary conditions provided by nongray model atmospheres (Rohrmann et al 2012;Camisassa et al 2017;Rohrmann 2018, for references), and a full treatment of energy sources, in particular the energy contribution ensuing from phase separation of core chemical species upon crystallization. The treatment of crystallization is based on the most up-to-date phase diagrams of Horowitz et al (2010) for dense CO mixtures, and that of Medin & Cumming (2010) for ONe mixtures.…”