Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
T. Driebe, T. Blöcker, D. Schönberner,
and F. Herwig:
The evolution of helium white dwarfs:
II. Thermal instabilities
Astronomy and Astrophysics 350, 89-100 (1999)
Abstract.
We calculated a grid of evolutionary models for white dwarfs with
helium cores
(He-WDs) and investigated the occurrence of hydrogen-shell flashes due
to
unstable hydrogen burning via CNO cycling.
Our calculations show that such thermal instabilities
are restricted to a certain mass range
(M=0.21...0.30 Msun),
consistent with earlier studies.
Models within this mass range undergo the more hydrogen shell flashes
the less
massive they are. This is caused by the strong dependence of the
envelope
mass on the white dwarf core mass. The maximum luminosities from
hydrogen burning during the flashes are of the order of 10^5 Lsun.
Because of the development of a pulse-driven
convection zone whose upper boundary temporarily reaches
the surface layers, the envelope's hydrogen content decreases by
0.06 (mass fractions) per flash.
Our study further shows that an additional high mass-loss episode
during a flash-driven
Roche lobe overflow to the white dwarf's companion does not affect the
final cooling behaviour of the models. Independent of hydrogen shell
flashes the evolution along the final white dwarf cooling branch
is determined by hydrogen burning via pp-reactions down to effective
temperatures as low as 8000K.
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