Pulsating low-mass white dwarfs in the frame of new evolutionary sequences. III. The pre-ELM white dwarf instability strip

CÓRSICO, A. H.; ALTHAUS, L. G.; SERENELLI, A. M.; KEPLER, S. O.; JEFFERY, C. S.; CORTI, M. A.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2016

0004-6361

Context. Many low-mass (M⋆/M⊙ ≲0.45) and extremely low-mass (ELM, M⋆/M⊙≲ 0.18-0.20) white-dwarf stars are currently being found in thefield of the Milky Way. Some of these stars exhibit long-periodgravity-mode (g-mode) pulsations, and constitute the class of pulsatingwhite dwarfs called ELMV stars. In addition, two low-mass pre-whitedwarfs, which could be precursors of ELM white dwarfs, have beenobserved to show multiperiodic photometric variations. They couldconstitute a new class of pulsating low-mass pre-white dwarf stars. Aims: Motivated by this finding, we present a detailed nonadiabaticpulsation study of such stars, employing full evolutionary sequences oflow-mass He-core pre-white dwarf models. Methods: Our pulsationstability analysis is based on a set of low-mass He-core pre-white dwarfmodels with masses ranging from 0.1554 to 0.2724 M⊙,which were derived by computing the nonconservative evolution of abinary system consisting of an initially 1 M⊙ ZAMS starand a 1.4 M⊙ neutron star companion. We have consideredmodels in which element diffusion is accounted for and also models inwhich it is neglected. Results: We confirm and explore in detaila new instability strip in the domain of low gravities and low effectivetemperatures of the Teff - log g diagram, where low-masspre-white dwarfs are currently found. The destabilized modes are radialand nonradial p and g modes excited by the κ - γ mechanismacting mainly at the zone of the second partial ionization of He, withnon-negligible contributions from the region of the first partialionization of He and the partial ionization of H. The computations withelement diffusion are unable to explain the pulsations observed in thetwo known pulsating pre-white dwarfs, suggesting that element diffusionmight be inhibited at these stages of the pre-white dwarf evolution. Ournonadiabatic models without diffusion, on the other hand, naturallyexplain the existence and range of periods of the pulsating pre-whitedwarf star WASP J1628+10B, although they fail to explain the pulsationsof WASP J0247-25B, the other known member of the class, indicating thatthe He abundance in the driving region of this star might besubstantially higher than predicted by our models. Conclusions:Discoveries of additional members of this new class of pulsating starsand their analysis in the context of the theoretical backgroundpresented in this paper will shed new light on the evolutionary historyof their progenitor stars.