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.