C/O abundance in white dwarf

interior

• The asteroseismological data• Need higher 12C+a ?• Different convective schemes • The effect of 12C+a• So what?

White dwarf pulsationsTYPE TEMPERATURE

RANGESURFACE

COMPOSITION

DOV or GW Vir

80000-180000 K C,He,O

DBV 22400-27800 K He

DAV or ZZ Ceti

11000-13000 K H

Pulsation in WDs are non-radial g modes

The restoring force is gravity

Several pulsations with periods ranging between 100

and 3000 s

The power spectrum

Variable WDs as a tool

Period spacing

Mass, surface temperature

Rotation, magnetic fieldFrequency splitting

PG 2131 Kawaler et al.

1995

Nonuniformities Chemical stratification

dP/dt Cooling rate

The best case: GD 358

Metcalfe et al. 2001: XO = 0.84 ± 0.03

Metcalfe et al. 2002:S300 = 370 ± 40 KeV b

Metcalfe 2003: S300 = 195 ± 15 KeV b

11 low degree (l=1, m=0) modes of consecutiveradial overtone (k=8-18) with periods 400-800 s

A premature determination?Fontaine & Brassard 2002

Not enough information

in the 11 periods availablefor GD358

We don’t need higher 12C()

Straniero et al. 2003

Kunz + semiconvection

XO = 0.79

Different convective schemes

Same nuclear reaction rate (Kunz et al. 2000)

CM: classical model (no overshooting, no semi-convection)

SCM-NoBP: semiconvection without breathing pulses

SCM: semi-convection

MOM: mechanical overshooting

Straniero et al. 2003

Straniero et al. 2003

Bare Schwarzschild Method

Semiconvective Model

High Overshoot Model

XO = 0.56

XO = 0.56

XO = 0.79

The effect of 12C()

A variation within Kunz error range yields a change of about XO= ± 0.15

Straniero et al. 2003

Conclusions

• Asteroseismology will determine with high accuracy XO

• Standard model with Kunz 12C() is in reasonable agreement with seismic value

• Uncertainty on 12C() imply uncertainty of about XO= ± 0.15

• Differences on convection treatment are at the level of 0.20

• Improvement on 12C() can provide significant information on convection.