Pulsations powered by hydrogen shell burning in white dwarfs

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

Coventry

Workshop; 20th European White Dwarf Workshop; 2016

The Univestity of Warwick

In the absence of a third dredge-up episode during the asymptotic giant branch phase, white dwarfs evolved from low-metallicity progenitors have a thick hydrogen envelope, which makes hydrogen shell burning be the most important energy source. We investigate the pulsational stability of this type of white dwarfs to see whether nonradial g-mode oscillations are triggered by hydrogen burning, with the aim of placing constraints on hydrogen shell burning in cool white dwarfs and on a third dredge-up episode during the asymptotic giant branch evolution of their progenitor stars. For this purpose, we construct white-dwarf sequences from low-metallicity progenitors by means of full evolutionary calculations that take into account the entire history of progenitor stars, including the thermally pulsing and the post-asymptotic giant branch phases, and analyze their pulsational stability by solving the linear, nonadiabatic, nonradial pulsation equations for the models in the range of effective temperatures log Teff ~ 4.17-4.00, which is typical of variable DA white dwarfs (DAV or ZZ Ceti stars). Our calculations demonstrate that, for white-dwarf models with masses M < 0.7 Msun and effective temperatures log Teff ~ 4.17-4.00 that evolved from low-metallicity progenitors (Z= 0.0001, 0.0005, and 0.001), the dipole (l = 1) g_1 mode is excited mostly due to hydrogen burning through the epsilon-mechanism. The epsilon-mechanism is insufficient to drive that mode in white dwarfs evolved from solar-metallicity progenitors. We note that the e-folding time for the dipole g_1 mode excited by the epsilon-mechanism is much shorter than the stellar evolutionary timescale, hence the oscillation can reach observable amplitudes. We suggest that efforts should be made to observe the dipole g_1 mode in white dwarfs associated with low-metallicity environments, such as globular clusters and/or the galactic halo, to place constraints on hydrogen shell burning in cool white dwarfs and the third dredge-up episode during the preceding asymptotic giant branch phase.