Download Asteroseismology of white dwarf stars provides important constraints

Asteroseismology of white dwarf stars provides important constraints
on our models of stellar evolution and Galactic history. Recently,
theoretical advances by Montgomery (2005) have allowed us to use
stellar pulsations to probe the physics of convection through
nonlinear light curve fitting.
We have chosen R808 (mv=14.2) as our second target for XCOV26 for
its importance to both asteroseismology and our understanding of
convection across the DAV instability strip. R808 was discovered to be
a pulsating DA white dwarf in 1976 (McGraw & Robinson 1976). It is a
large amplitude, multiperiodic pulsator with an extremely nonlinear
light curve (Figure 1), making it a perfect candidate for Montgomery's
technique. With Teff=10300 K (Mullaly et al. 2007), it is
also one of the coolest DAV pulsators, and offers the opportunity to
study the characteristics of white dwarfs at the red edge of the DAV
instability strip. However, despite its importance and its bright
magnitude, this star has not been observed extensively since its
discovery and little is known about its pulsation characteristics.
Our first goal is to measure an accurate set of frequencies in this
complex pulsator, and for this the coverage provided by the Whole
Earth Telescope is essential. These frequencies will allow us to use
asteroseismology to constrain its total mass, core composition,
rotation rate, and the mass of its hydrogen surface layer. The
hydrogen layer mass is determined in the highly uncertain final stages
of mass loss, and in principle can range from 10-11 M* to
$10-11 Mo. This layer plays an important role in the cooling
of DA white dwarfs and such a large range can lead to uncertainties of
up to a Gyr in the ages of the coolest white dwarfs. Our second,
complementary goal is to determine R808's viability as a candidate for
Montgomery's light curve fitting technique. We will examine the
stability of the pulsations in phase and amplitude, resolve the
multiplets, and attempt to assign l and m values to the modes
in order to assess the suitability of R808 as a WET candidate.
References:
McGraw, J.T. & Robinson, E. L., 1976, ApJL, 205, L155
Montgomery, M. 2005, ApJ, 633, 1142
Mullaly, F. et al. 2007, ApJS, 206