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The Extent and Cause of the Pre-White Dwarf Instability Strip
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The Extent and Cause of the Pre-White Dwarf Instability Strip
M. S. O’Brien1
Received ; accepted
1Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218;
obrien@stsci.edu.
– 2 –
ABSTRACT
One of the least understood aspects of white dwarf evolution is the process
by which they are formed. The initial stages of white dwarf evolution are
characterized by high luminosity, high effective temperature, and high surface
gravity, making it difficult to constrain their properties through traditional
spectroscopic observations. We are aided, however, by the fact that many H-
and He-deficient pre-white dwarfs (PWDs) are multiperiodic g-mode pulsators.
These stars fall into two classes, the variable planetary nebula nuclei (PNNV)
and the “naked” GW Vir stars. Pulsations in PWDs provide a unique
opportunity to probe their interiors, which are otherwise inaccesible to direct
observation. Until now, however, the nature of the pulsation mechanism, the
precise boundaries of the instability strip, and the mass distribution of the
PWDs were complete mysteries. These problems must be addressed before we
can apply knowledge of pulsating PWDs to improve understanding of white
dwarf formation.
This paper lays the groundwork for future theoretical investigations of these
stars. In recent years, Whole Earth Telescope observations led to determination
of mass and luminosity for the majority of the GW Vir pulsators. With these
observations, we identify the common properties and trends PWDs exhibit as a
class.
We find that pulsators of low mass have higher luminosity, suggesting the
range of instability is highly mass-dependent. The observed trend of decreasing
periods with decreasing luminosity matches a decrease in the maximum
(standing-wave) g-mode period across the instability strip. We show that the
red edge can be caused by the lengthening of the driving timescale beyond
the maximum sustainable p
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