A Search for Pulsations in Helium White Dwarfs

The recent plethora of sky surveys, especially the Sloan Digital Sky Survey, have discovered many low-mass (M < 0.45 M_⊙) white dwarfs that should have cores made of nearly pure helium. These WDs come in two varieties: those with masses 0.2 < M < 0.45 M_⊙ and H envelopes so thin that they rapidly cool and those with M < 0.2 M_⊙ (often called extremely low mass [ELM] WDs) that have thick enough H envelopes to sustain 10^9 yr of H burning. In both cases, these WDs evolve through the ZZ Ceti instability strip, T_(eff) ≈ 9000–12,000 K, where g-mode pulsations always occur in carbon/oxygen WDs. This expectation, plus theoretical work on the contrasts between C/O and He-core WDs, motivated our search for pulsations in 12 well-characterized helium WDs. We report here on our failure to find any pulsators among our sample. Though we have varying amplitude limits, it appears likely that the theoretical expectations regarding the onset of pulsations in these objects require closer consideration. We close by encouraging additional observations as new He WD samples become available, and we speculate on where theoretical work may be needed