Pulsations powered by hydrogen shell burning in white dwarfs

CAMISASSA, M. E.; CÓRSICO, A. H.; L. G. ALTHAUS; H. SHIBAHASHI

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2016 vol. 595 p. 45 - 51

0004-6361

Context. In the absence of a third dredge-up episode during theasymptotic giant-branch phase, white dwarf models evolved fromlow-metallicity progenitors have a thick hydrogen envelope, which makeshydrogen shell burning be the most important energy source. Aims:We investigate the pulsational stability of white dwarf models withthick envelopes to see whether nonradial g-mode pulsations are triggeredby hydrogen burning, with the aim of placing constraints on hydrogenshell burning in cool white dwarfs and on a third dredge-up during theasymptotic giant-branch evolution of their progenitor stars. Methods: We construct white-dwarf sequences from low-metallicityprogenitors by means of full evolutionary calculations that take intoaccount the entire history of progenitor stars, including the thermallypulsing and the post-asymptotic giant-branch phases, and analyze theirpulsation stability by solving the linear, nonadiabatic, nonradialpulsation equations for the models in the range of effectivetemperatures Teff 15 000-8000 K. Results: Wedemonstrate that, for white dwarf models with masses M⋆≲ 0.71 M⊙ and effective temperatures 8500 ≲Teff ≲ 11 600 K that evolved from low-metallicityprogenitors (Z = 0.0001, 0.0005, and 0.001), the dipole (ℓ = 1) andquadrupole (ℓ = 2) g1-modes are excited mostly as aresult of the hydrogen-burning shell through the ɛ-mechanism, inaddition to other g-modes driven by either the κ - γ or theconvective driving mechanism. However, the ɛ mechanism isinsufficient to drive these modes in white dwarfs evolved fromsolar-metallicity progenitors. Conclusions: We suggest thatefforts should be made to observe the dipole g1-mode in whitedwarfs associated with low-metallicity environments, such as globularclusters and/or the galactic halo, to place constraints on hydrogenshell burning in cool white dwarfs and the third dredge-up episodeduring the preceding asymptotic giant-branch phase.