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

Creating He-Core White Dwarfs

C/O WDs Truncation of the

AGB Helium WDs

Truncation of the RGB

(Girardi et al. 2000)

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

Direct Evidence: ZZ Cetis

(courtesy Phil Arras)

Thorough mode period analysis of pulsation spectrum is sensitive to: Total Mass Envelope Mass Core Composition

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)

A Dichotomy in Evolution

(Panei et al. 2007)

(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

A Dichotomy in Timescales

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 transportC

ore

Env

elop

eIn

terf

ace

MWD

MENV

Grey Atmosphere

Key Physics MESA - Bill Paxton - http://mesa.sourceforge.net

EOS, Opacity, numerical algorithms, … Diffusive equilibrium chemical profile

Two component plasma of Hydrogen, Helium and electrons

pp Burning Slows Cooling

0 Gyr

5 Gyr

12 Gyr

0 Gyr

5 Gyr

12 Gyr

pp Burning Slows Cooling

0 Gyr

0 Gyr2 Gyr

12 Gyr

2 Gyr

12 Gyr

Pulsation Probes H Shell

c2kh2

N2

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.

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?