Gravity-Mode Instabilities in Models of Post-Extreme Horizontal Branch Stars: Another Class of Pulsating Stars?

Abstract

We present new results of a stability analysis of realistic models of post-extreme horizontal branch stars. We find that g-mode instabilities develop in some of these models as a result of a potent epsilon-mechanism associated with the presence of an active H-burning shell. The epsilon-process drives low order and low degree g-modes with typical periods in the range 40-125 s. The unstable models populate a broad instability strip covering the interval 76,000 K > ~ T_eff >~ 44,000 K, and are identified with low-mass DAO white dwarfs. They descend from stars that reach the zero-age "extended" horizontal branch with H-rich envelope masses M_env >~ 0.0010a solar masses. Our computations indicate that some DAO stars should show luminosity variations resulting from pulsational instabilities. We suggest looking for brightness variations in six particularly promising candidates.

LaTeX, 10pp text plus 2 Postscript figures (included) and one table (included). Submitted for publication to Astrophysical Journal (Letters). PS file also available at http://www.astro.virginia.edu/~bd4r