[Previous] | [Session 64] | [Next]
M.D. Reed, S.D. Kawaler (Iowa State Univ.)
Asteroseismology of pre-white dwarfs (PWDs) provides fresh insights into their interior structure and bulk properties. However, because of their extreme temperatures and surface gravities, seismic analysis for these stars has had only limited spectroscopic verification. Also, since the results of seismic analysis include distances, we would like to compare these values with distances obtained from other methods. Unfortunately, there are no pulsating PG~1159 (GW Vir) stars nearby, so methods other than trigonometric parallax must be used.
Luckily, one GW Vir star, PG~2131+066 (IR Peg, hereafter PG~2131), has a main sequence companion. We report on efforts to determine the binary nature of this PWD + late-type pair which has a separation of 0.3 arc seconds. Initial work by Wesemael et al (1985) deconvolved visual and UV spectra to obtain a spectral type of K6V for the secondary. This led to a distance estimate of d=1047+1000-500pc. Analysis of time-series photometry of the pulsating component led to an asteroseismological distance of d=470+180-130pc (Kawaler et al. 1995). The distance discrepancy is puzzling if they are indeed a physical pair.
Bond (1992) obtained HST WFPC1 images through the F555W and F785LP filters that clearly resolve the pair. However, the uncorrected spherical aberration complicates efforts to obtain accurate photometry. The 0.3" separation in the pair produces significant overlap in the images. We have analyzed the WFPC1 data by using core aperture photometry. We compare the results with model spectra of the late-type star from Kurucz (1993) and of the PWD from Dreizler (1998). By convolving the model spectra with the system response of the WFPC1 CCD and filters, we obtain theoretical colors for comparison with the observed colors. Using the colors to constrain the spectral type of the companion, we then compare observed apparent magnitudes to absolute magnitudes to derive the distances of the two stars.
The author(s) of this abstract have provided an email address for comments about the abstract: mreed@iastate.edu