摘要：Context. 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.
Aims. 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.
Methods. 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 pulsation stability by solving the linear,
nonadiabatic, nonradial pulsation equations for the models in the range of effective
temperatures Teff
~ 15 000−8000 K.
Results. We demonstrate that, for white dwarf models with masses
M⋆ ≲ 0.71
M⊙ and effective temperatures
8500 ≲ Teff ≲ 11
600 K that evolved from low-metallicity progenitors (Z = 0.0001, 0.0005, and 0.001), the dipole (ℓ = 1) and quadrupole
(ℓ = 2)
g1-modes are excited mostly as a result of
the hydrogen-burning shell through the ε-mechanism, in addition to other g-modes driven by either
the κ −
γ or the convective driving mechanism. However, the
ε mechanism
is insufficient to drive these modes in white dwarfs evolved from solar-metallicity
progenitors.
Conclusions. We suggest that efforts should be made to observe the
dipole g1-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.
