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 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 13 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. In particular, the absence of pulsations in the bright (m_g' = 15.2) object SDSS J1300+5904 requires that the "blue edge" of the ZZ Ceti instability strip move to Teff = 11,000 K at log g = 7.2, potentially confirming the log g dependence implied by the Brickhill mechanism of mode excitation. We close by encouraging additional observations as new He WD samples become available, and speculate on where theoretical work may be needed.