A Search for Pulsations in Helium White Dwarfs

TL;DR

This study searched for pulsations in helium-core white dwarfs but found none, suggesting that current theoretical models of pulsation onset in these stars may need revision.

Contribution

The paper presents the first observational search for pulsations in helium white dwarfs, challenging existing theoretical predictions about their pulsation behavior.

Findings

No pulsators detected in the sample

Results imply theoretical models may need refinement

Encourages further observational and theoretical work

Abstract

The recent plethora of sky surveys, especially the Sloan Digital Sky Survey, have discovered many low-mass (M < 0.45 Msun) 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 Msun and H envelopes so thin that they rapidly cool, and those with M < 0.2 Msun (often called extremely low mass, ELM, WDs) that have thick enough H envelopes to sustain 10^9 years of H burning. In both cases, these WDs evolve through the ZZ Ceti instability strip, Teff = 9,000-12,000 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 amongst our sample. Though we have varying amplitude limits, it appears likely that the theoretical expectations regarding the onset of pulsations in these objects requires closer consideration. We close by encouraging additional observations as new He WD samples become available, and speculate on where theoretical work may be needed.