1. Hydrogen Burning and Pulsations on Low Mass Helium Core White Dwarfs Justin D. R. Steinfadt Lars Bildsten UCSB Wild Stars in the Wild West II 14th North American Workshop on Cataclysmic Variables and Related Objects March 16, 2009

2. Creating He-Core White Dwarfs C/O WDs Truncation of the AGB Helium WDs Truncation of the RGB (Girardi et al. 2000)

3. Channel must prevent the core mass from growing beyond 0.45-0.48 M  Binary interaction via common envelope Rapid mass loss via stellar winds induced by high metallicity Two Possible Channels

4. Direct Evidence: ZZ Cetis (courtesy Phil Arras) Thorough mode period analysis of pulsation spectrum is sensitive to: Total Mass Envelope Mass Core Composition

5. He-core ZZ Cetis Low Mass WDs PSR J0218+4232, PSR J1019+5307, PSR J1911-5958A SDSS J0022+00, J0022-10, J0822+27, J0849+04, J1053+52, J1056+65, J1234-02, J1426+01, J1436+50, J1630+42, J2252-00, J2345-01 LP 400-22 (Steinfadt et al. 2008)

6. A Dichotomy in Evolution (Panei et al. 2007) 0.18 M  represents a boundary between two very different classes of models Highly dependent upon metallicity Seen in the recent time- dependent evolution codes Panei et al. 2001, 2007 Serenelli et al. 2002 Driebe et al. 1998

7. A Dichotomy in Timescales

8. Building a Hydrostatic Model He Core No energy generation Isothermal Pure Helium composition Envelope Nuclear Energy Generation PP - Chain Pure Hydrogen composition Radiative and convective heat transport Core Envelope Interface M WD M ENV Grey Atmosphere Key Physics MESA - Bill Paxton - http://mesa.sourceforge.nethttp://mesa.sourceforge.net EOS, Opacity, numerical algorithms, … Diffusive equilibrium chemical profile Two component plasma of Hydrogen, Helium and electrons

9. pp Burning Slows Cooling 0 Gyr 5 Gyr 12 Gyr 0 Gyr 5 Gyr 12 Gyr

10. pp Burning Slows Cooling 0 Gyr 2 Gyr 12 Gyr 2 Gyr 12 Gyr

11. Pulsation Probes H Shell c2kh2c2kh2 N2N2 p-modes g-modes (courtesy Phil Arras) Non-adiabatic driving will determine where the ZZ Ceti Strip resides (ala Brickhill) Epsilon mechanism may be possible with the pp burning shell Period spacing would constrain the WD envelope mass.

12. Implications of a He-core WD Pulsator New extension to the ZZ Ceti instability strip New insights into the driving mechanisms Constraints on He-core models through seismic modeling Is there really a thick Hydrogen envelope? Do low mass He-core WDs have nuclear burning envelopes?