Pulsational Instabilities in Accreting White Dwarfs

Abstract

The detection of g-mode pulsations in accreting white dwarfs (WDs) in cataclysmic variables (CVs) with a large range of effective temperatures (Teff) has shown these WDs to be a more diverse class than their isolated counterparts, the ZZ Ceti and DB pulsators. The simplest contrast of CV to isolated pulsators is an envelope of solar-like composition (of various helium enrichments if the donor is evolved) rather than pristine hydrogen or helium. A range of WD masses is expected, from low-mass He core WDs to massive WDs. We investigate the impact of this diversity on the range of Teff for which g-modes are unstable. Motivated by earlier theoretical studies, we compare a fiducial g-mode period to the thermal time at the base of the convection zones created by H and first He (H/He I) ionization or second He (He II) ionization zones. We find that (for solar composition envelopes), relative to a fiducial WD mass 0.6 M☉, the blue edge for a 0.4 M☉ He core WD shifts downward by ≈1000 K, while that for a massive ≈1.2 M☉ WD shifts upward by ≈2000 K. Surprisingly, increasing Y by only 10% relative to solar creates an intermediate instability strip near 15,000 K.