Pulsations in Hydrogen Burning Low Mass Helium White Dwarfs

TL;DR

This paper predicts that low-mass helium white dwarfs with stable hydrogen burning can exhibit g-mode pulsations with longer periods than typical ZZ Ceti stars, enabling seismological probing of their core structure.

Contribution

It introduces models of helium core white dwarfs with stable hydrogen burning that show pulsations similar to ZZ Ceti stars but with longer periods, providing a new way to study their internal structure.

Findings

Predicted g-mode periods are nearly twice as long as in canonical ZZ Ceti stars.

Core composition and structure can be probed through seismology of these pulsations.

Identified specific low-mass helium WDs likely to exhibit pulsations for observational follow-up.

Abstract

Helium core white dwarfs (WDs) with mass M <~ 0.20 Msun undergo several Gyrs of stable hydrogen burning as they evolve. We show that in a certain range of WD and hydrogen envelope masses, these WDs may exhibit g-mode pulsations similar to their passively cooling, more massive carbon/oxygen core counterparts, the ZZ Cetis. Our models with stably burning hydrogen envelopes on helium cores yield g-mode periods and period spacings longer than the canonical ZZ Cetis by nearly a factor of 2. We show that core composition and structure can be probed using seismology since the g-mode eigenfunctions predominantly reside in the helium core. Though we have not carried out a fully nonadiabatic stability analysis, the scaling of the thermal time in the convective zone with surface gravity highlights several low-mass helium WDs that should be observed in search of pulsations: NLTT 11748, SDSS J0822+2753, and the companion to PSR J1012+5307. Seismological studies of these He core WDs may prove especially fruitful, as their luminosity is related (via stable hydrogen burning) to the hydrogen envelope mass, which eliminates one model parameter.