1. C/O abundance in white dwarf interior The asteroseismological data Need higher 12 C+a ? Different convective schemes The effect of 12 C+a So what?

2. White dwarf pulsations TYPETEMPERATURE RANGE SURFACE COMPOSITION DOV or GW Vir 80000-180000 KC,He,O DBV22400-27800 KHe DAV or ZZ Ceti 11000-13000 KH Pulsation in WDs are non-radial g modes The restoring force is gravity

3. Several pulsations with periods ranging between 100 and 3000 s

4. The power spectrum

5. Variable WDs as a tool Period spacing Mass, surface temperature Rotation, magnetic field Frequency splitting PG 2131 Kawaler et al. 1995 Nonuniformities Chemical stratification dP/dtCooling rate

6. The best case: GD 358 Metcalfe et al. 2001:X O = 0.84 ± 0.03 Metcalfe et al. 2002: S 300 = 370 ± 40 KeV b Metcalfe 2003: S 300 = 195 ± 15 KeV b 11 low degree (l=1, m=0) modes of consecutive radial overtone (k=8-18) with periods 400-800 s

7. A premature determination? Fontaine & Brassard 2002 Not enough information in the 11 periods available for GD358

8. We don’t need higher 12 C(  ) Straniero et al. 2003 Kunz + semiconvection X O = 0.79

9. 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

10. Bare Schwarzschild Method Semiconvective Model High Overshoot Model X O = 0.56 X O = 0.79

11. The effect of 12 C(  ) A variation within Kunz error range yields a change of about  X O = ± 0.15 Straniero et al. 2003

12. Conclusions Asteroseismology will determine with high accuracy X O Standard model with Kunz 12 C(  ) is in reasonable agreement with seismic value Uncertainty on 12 C(  ) imply uncertainty of about  X O = ± 0.15 Differences on convection treatment are at the level of 0.20 Improvement on 12 C(  ) can provide significant information on convection.