Pulsations powered by hydrogen shell burning in white dwarfs

Abstract

In the absence of a third dredge-up episode during the asymptotic giant branch phase, white dwarf models 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 white dwarf models with thick envelopes to see whether nonradial $g$-mode pulsations 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 during the asymptotic giant branch evolution of their progenitor stars. We construct white-dwarf sequences from low-metallicity progenitors by means of full evolutionary calculations, and analyze their pulsation stability for the models in the range of effective temperatures $T_{\mathrm{eff}}\sim 15000-8000$ K. We demonstrate that, for white dwarf models with masses $M_{\star} \lesssim 0.71\,\rm M_{\sun}$ and effective temperatures $8500\lesssim T_{\mathrm{eff}}\lesssim 11600$ K that evolved from low-metallicity progenitors ($Z=0.0001$, $0.0005$, and $0.001$) the dipole ($\ell =1$) and quadrupole ($\ell =2$) $g_1$ modes are excited mostly due to the hydrogen-burning shell through the $\epsilon$-mechanism, in addition to other $g$ modes driven by either the $\kappa -\gamma$ or the convective driving mechanism. However, the $\epsilon$ mechanism is insufficient to drive these modes in white dwarfs evolved from solar-metallicity progenitors. 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.

6 pages, 4 figures, 1 table. To be published in Astronomy and Astrophysics

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• A Large Catalog of DA White Dwarf Characteristics Using SDSS and Gaia Observations
• The effects of unresolved double-degenerates in the white dwarf luminosity function
• LISA Galactic Binaries in the Roman Galactic Bulge Time-Domain Survey
• A new instability domain of CNO-flashing low-mass He-core stars on their early white-dwarf cooling branches