Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
T. Driebe, D. Schönberner, T. Blöcker
and F. Herwig:
The evolution of helium white dwarfs:
I. The companion of the millisecond pulsar PSR J1012+5307
Astronomy and Astrophysics 339, 123-133 (1998)
Abstract.
We present a grid of evolutionary tracks for low-mass white dwarfs
with helium cores in the mass range from 0.179 to 0.414 Msol.
The lower mass limit is well-suited for comparison with white dwarf
companions
of millisecond pulsars. The tracks are
based on a 1 Msol model sequence extending from the pre-main
sequence stage up to the tip of the red-giant branch.
Applying large mass loss rates at appropriate positions forced
the models to move off the giant branch. The further
evolution was then followed across the Hertzsprung-Russell diagram
and down the cooling branch. At maximum effective temperature
the envelope masses above the helium cores increase from 0.6 to
5.4 x 10^-3 Msol for decreasing mass.
We carefully checked for the occurrence of
thermal instabilities of the hydrogen shell by adjusting the
computational time steps accordingly. Hydrogen flashes have been
found to take place only in the mass interval 0.21 < M/Msol <
0.3.
The models show that hydrogen shell burning contributes significantly
to
the luminosity budget of white dwarfs with helium cores.
For very low masses the hydrogen shell luminosity remains to be
dominant even down to effective temperatures
well below 10000 K. Accordingly, the
corresponding cooling ages are significantly larger
than those gained from model calculations which neglect
nuclear burning or the white dwarf progenitor evolution.
Using the atmospheric parameters of the white dwarf in the
PSR J1012+5307 system we determined a mass
of M = 0.19 +/- 0.02 Msol and a cooling age
of 6 +/- 1 Gyr, in good agreement with
the spin-down age, 7 Gyr, of the pulsar.
You can get this publication ...