Pulsating low-mass white dwarfs in the frame of new evolutionary sequences. II. Nonadiabatic analysis

CÓRSICO, A. H.; ALTHAUS, L. G.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2016 vol. 585 p. 1 - 16

0004-6361

Context. Low-mass (M⋆/M⊙ ≲ 0.45)white dwarfs, including the so-called extremely low-mass white dwarfs(ELM, M⋆/M⊙ ≲ 0.18-0.20), are beingcurrently discovered in the field of our Galaxy through dedicatedphotometric surveys. That some of them pulsate raises the unparalleledchance to investigate their interiors. Aims: We present adetailed nonadiabatic pulsational analysis of such stars, employing fullevolutionary sequences of low-mass He-core white dwarf models derivedfrom binary star evolution computations. The main aim of this study isto provide a detailed description of the pulsation stability propertiesof variable low-mass white dwarfs during the terminal cooling branch. Methods: Our nonadiabatic pulsation analysis is based on a newset of He-core white-dwarf models with masses ranging from 0.1554 to0.4352 M⊙, which were derived by computing thenonconservative evolution of a binary system consisting of an initially1 M⊙ ZAMS star and a 1.4 M⊙ neutron star.We computed nonadiabatic radial (ℓ = 0) and nonradial (ℓ = 1,2)g and p modes to assess the dependence of the pulsational stabilityproperties of these objects with stellar parameters such as the stellarmass, the effective temperature, and the convective efficiency. Results: We found that a dense spectrum of unstable radial modes andnonradial g and p modes are driven by the κ-γ mechanism dueto the partial ionization of H in the stellar envelope, in addition tolow-order unstable g modes characterized by short pulsation periods thatare significantly excited by H burning via the ɛ mechanism ofmode driving. In all the cases, the characteristic times required forthe modes to reach amplitudes large enough to be observable (thee-folding times) are always shorter than cooling timescales. We explorethe dependence of the ranges of unstable mode periods (the longest andshortest excited periods) with the effective temperature, the stellarmass, the convective efficiency, and the harmonic degree of the modes.We also compare our theoretical predictions with the excited modesobserved in the seven known variable low-mass white dwarfs (ELMVs) andfound excellent agreement.