Pulsations in Hydrogen Burning Low-mass Helium White Dwarfs
Abstract
Helium core white dwarfs (WDs) with mass M <~ 0.20 M sun undergo several Gyr 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2010
- DOI:
- 10.1088/0004-637X/718/1/441
- arXiv:
- arXiv:1005.5423
- Bibcode:
- 2010ApJ...718..441S
- Keywords:
-
- stars: oscillations;
- white dwarfs;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 6 pages, 3 figures. Published ApJ version