A new instability domain of CNO-flashing low-mass He-core stars on their early white-dwarf cooling branches

Abstract

Before reaching their quiescent terminal white-dwarf cooling branch, some low-mass helium-core white dwarf stellar models experience a number of nuclear flashes which greatly reduce their hydrogen envelopes. Just before the occurrence of each flash, stable hydrogen burning may be able to drive global pulsations that could be relevant to shed some light on the internal structure of these stars through asteroseismology. We present a pulsational stability analysis applied to low-mass helium-core stars on their early white-dwarf cooling branches going through CNO flashes in order to study the possibility that the $\epsilon$ mechanism is able to excite gravity-mode pulsations. We carried out a nonadiabatic pulsation analysis for low-mass helium-core white-dwarf models going through CNO flashes during their early cooling phases. We found that the $\epsilon$ mechanism due to stable hydrogen burning can excite low-order ($\ell =1,2$) gravity modes with periods between $\sim 80$ and $500$s, for stars with $0.2025\lesssim M_{\star }/M_{\odot }\lesssim 0.3630$ located in an extended region of the $\log g-T_{\mathrm{eff}}$ diagram with effective temperature and surface gravity in the ranges $15000\lesssim T_{\mathrm{eff}}\lesssim 38000$K and $5.8\lesssim \log g\lesssim 7.1$, respectively. Since the timescales required for these modes to reach amplitudes large enough to be observable are shorter than their corresponding evolutionary timescales, the detection of pulsations in these stars is feasible. If a low-mass white dwarf star were found to pulsate with low-order gravity modes in this region of instability, it would confirm our result that such pulsations can be driven by the $\epsilon$ mechanism. In addition, confirming a rapid rate of period change in these pulsations would support that these stars actually experience CNO flashes, as predicted by evolutionary calculations.

9 pages, 6 figures, 1 table. To be published in Astronomy and Astrophysics

References in corpus (22)

• The Transiting Exoplanet Survey Satellite
• Pulsating White Dwarf Stars and Precision Asteroseismology
• The ELM Survey. I. A Complete Sample of Extremely Low Mass White Dwarfs
• White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 14
• The ELM Survey. VIII. 98 Double White Dwarf Binaries
• White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up
• New nonadiabatic pulsation computations on full PG1159 evolutionary models: the theoretical GW Vir instability strip revisited
• Blue large-amplitude pulsators as a new class of variable stars
• Gaia Data Release 2 Catalogue of Extremely-low Mass White Dwarf Candidates
• New binaries among UV-selected, hot subdwarf stars and population properties
• PSR J1738+0333: The First Millisecond Pulsar + Pulsating White Dwarf Binary
• WASP 1628+10 - an EL CVn-type binary with a very-low-mass stripped-red-giant star and multi-periodic pulsations
• Discovery of a Bright, Extremely Low-Mass White Dwarf in a Close Double Degenerate System
• Pruning The ELM Survey: Characterizing Candidate Low-Mass White Dwarfs Through Photometric Variability
• A new class of pulsating hot subdwarfs
• Pulsating low-mass white dwarfs in the frame of new evolutionary sequences: I. Adiabatic properties
• The ELM Survey South. I. An Effective Search for Extremely Low Mass White Dwarfs
• Short-period $g$-mode pulsations in low-mass white dwarfs triggered by H shell burning
• Asteroseismic test of rotational mixing in low-mass white dwarfs
• On the possible existence of short-period g-mode instabilities powered by nuclear burning shells in post-AGB H-deficient (PG1159-type) stars
• Pulsating low-mass white dwarfs in the frame of new evolutionary sequences: V. Asteroseismology of ELMV white dwarf stars
• Pulsations of Pre-White Dwarfs with Hydrogen-dominated Atmospheres

Cited by in corpus (1)

• Hot subdwarfs from the surviving companions of the white dwarf + main-sequence channel of Type Ia supernovae