PG 2131+066: A Test of Pre-White Dwarf Asteroseismology

PG 2131+066 is a composite-spectrum binary with a hot pulsating PG 1159-type pre-white dwarf and an early M-type main-sequence star. Analysis of Whole Earth Telescope observations of the pulsating pre-white dwarf component provided an asteroseismological determination of its mass, luminosity, and effective temperature. These determinations allowed Kawaler et al. to determine the distance to this star. In this paper, we refine the asteroseismological distance determination and confirm the distance by an independent measurement to the system via the spectroscopic parallax of the M star. PG 2131+066 was observed by the HST using the original Planetary Camera in 1993 September. Exposures with filters F785LP and F555W both showed the companion at a distance of 0.″3. Photometry of the images provides an apparent magnitude for the main sequence companion of mv = 18.97 ± 0.15, from which we find a distance of 560 pc. We also recalculated the asteroseismological distance to the pre-white dwarf using updated models and new spectroscopic constraints from UV spectra. The new seismological distance is 668 pc, in satisfactory agreement with the distance of the secondary star. These results suggest that this is indeed a physical binary and that the seismological distance determination technique may be the best way to determine the distance to the pulsating hot pre-white dwarf stars.