Pulsating low-mass white dwarfs in the frame of new evolutionary sequences VI. Thin H-envelope sequences and asteroseismology of ELMV stars revisited

Abstract

Some low-mass white-dwarf (LMWD) stars with H atmospheres show long-period g-mode pulsations, comprising the class of pulsating WDs called extremely low-mass variable (ELMV) stars. It is generally believed that these stars have thick H envelopes. However, from stellar evolution considerations, the existence of LMWDs with thin H envelopes is also possible. We present a thorough asteroseismological analysis of ELMV stars based on a complete set of fully evolutionary models that represents low-mass He-core WD stars harboring a range of H envelope thicknesses. Although there are currently nine ELMVs, here we only focus on those that exhibit more than three periods and whose periods do not show significant uncertainties. We considered g-mode adiabatic pulsation periods for low-mass He-core WD models with $M_{\ast }$ in the range [0.1554-0.4352]$M_{\odot }$, $T_{\mathrm{eff}}$ in the range [6000-10000]K, and H envelope thicknesses in the range -5.8<log($M_{\mathrm{H}}/M_{\ast }$)<-1.7. We explore the effects of employing different H-envelope thicknesses on the adiabatic pulsation properties of low-mass He-core WD models, and perform period-to-period fits to ELMVs to search for a representative model. We found that the mode-trapping effects of g modes depend sensitively on $M_{\mathrm{H}}$, with the trapping cycle and trapping amplitude larger for thinner H envelopes. Also, the asymptotic period spacing is longer for thinner H envelopes. Finally, we found asteroseismological models (when possible) characterized by canonical (thick) and by thin H envelope, with $T_{\mathrm{eff}}$ and $M_{\ast }$ in agreement with the spectroscopic determinations. The fact that we have found asteroseismological solutions with H envelopes thinner than canonical gives a clue of the possible scenario of formation of these stars. Indeed, in the light of our results, some of these stars could have been formed by binary evolution through unstable mass loss.

arXiv comment12 pages, 11 figures, 5 tables. Accepted for publication in Astronomy & Astrophysics. The full version of this work, with more analyzed stars, is available upon request

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