AN ASTEROSEISMOLOGICAL DETERMINATION OF THE STRUCTURE OF THE DBV WHITE-DWARF GD-358

We determine the structure of the pulsating DB white dwarf GD 358 thro ugh the technique of asteroseismology-where we compare our theoretical period distributions with the observational data. The observed period s are based on over 150 hours of nearly continuous high-speed photomet ric data obtained with the Whole Earth Telescope. We find that GD 358 has a mass of 0.61 +/- 0.03 M. with a helium layer mass of log M(He)/M = -5.70(+0.18/-0.30)). Our results also constrain the structure of t he helium/carbon transition zone and may have important consequences f or our theoretical understanding of mixing and diffusion in white-dwar f stars. Our best-fit models have a luminosity of log L/L. = -1.30(+0. 09/-0.12); we use this to derive a distance of 42 +/- 3 pc, consistent with the parallax value. This distance determination is completely in dependent of the parallax determination, and our errors are dominated by the uncertainty in effective temperature. In the future, accurate d istances to white dwarfs derived from asteroseismology will offer us t he opportunity to probe properties of the interstellar medium as a fun ction of distance and allow us to use white dwarfs as tracers of the d ynamics of the local Galactic disk.