Mean period of pulsating white dwarfs as a spectroscopy-independent thermometer

1. Mean period of pulsating white dwarfs as a spectroscopy-independent thermometer Anjum S. Mukadam, University of Washington Collaborators: M. H. Montgomery (UTx), D. E. Winget (UTx), S. O. Kepler (UFRGS, Brasil), J. C. Clemens (UNC), P. Szkody (UW), B. T. Gänsicke (UWr, UK) Animations from whitedwarf.org (T. Metcalfe, HAO)

2. Plan of Talk • Introduction to pulsating white dwarfs and ZZ Ceti stars • Correlating ZZ Ceti pulsation period with temperature • Successful application of this new spectroscopy-independent technique to determine temperature • Can we apply this technique to accreting ZZ Ceti stars?

3. Asteroseismology • Pulsations Only systematic way tostudy the stellar interior • Pulsations are observed in different types of stars in various stages of evolution ZZ Ceti stars

4. White dwarfs show non-radial g-modes due to their high density with periods of 50s to 1400s • Pulsation modes are discrete & characterized by quantum numbers (k,l,m)similar to atomic orbitals • Pulsations reach the inner 99% of a white dwarf star (Montgomery & Winget 1999) Animations from whitedwarf.org (T. Metcalfe, HAO)

5. ZZ Ceti stars (DAVs) Hydrogen atmosphere white dwarf variables

6. Two flavours of ZZ Ceti stars (DAVs) 0.4 Cool ZZ Ceti (cDAV) 0.2 Teff = 11000K P ~ 1000s 0 Fractional Intensity 0 1000 2000 3000 4000 5000 Hot ZZ Ceti (hDAV) 0.05 Teff = 12000K P ~ 200s 0 -0.05 0 1000 2000 3000 Time (s)

7. Mean Period vs. Spectroscopic Temperature

8. Pulsation Period: Means of measuring Teff ? WMP = -0.835 Teff +10060 WMP = -0.830 Teff +10240

9. Spectroscopyvs.WeightedMean Period • Internal uncertainty ~10-60s / 1300s(<5% of the width) • Mass does not affect pulsation period • Relatively simple and model-independent measurement • Internal uncertainty ~200K / 1200K(17% of the width) • Mass & Temperature are not entirely independent • Dependence on model atmosphere & method used to determine Teff from the spectrum.

10. Weighted Mean Period as a temperature scale • We can think of the weighted mean period (WMP) as an effective temperature scale. • If we restrict our Teff determination to units of seconds in the WMP scale, we become completely independent of spectroscopic Teff uncertainties. Average Teff uncertainty reduces from 17% to <5% (Mukadam et al. 2006, ApJ, 640, 956)

11. Mean pulsation amplitude vs.Mean period (serves as temperature) Hot Cool

12. ZZ Ceti stars lose amplitude before pulsations shut down at the red edge! Hot Cool

13. Accreting pulsating white dwarfs found! • A ZZ Ceti star was discovered in a cataclysmic variable (van Zyl et al. 1998). • Interesting systems to study the effect of accretion on pulsations Instability strip for accretors • Use seismology to learn about the pulsating white dwarf in the cataclysmic variable

14. Accreting ZZ Ceti instability strip • Statistically significant sample needed (10 accreting ZZ Ceti stars known to date) • Spectroscopic temperature to the primary white dwarf implies simultaneously fitting: • White dwarf with Balmer absorption lines • Hot spot/ hot belt on the white dwarf • Accretion disk with emission lines

15. Preliminary results from HSTUV time-resolved spectroscopy (Szkody et al. 2006 (in prep); Mukadam et al. 2005, BAAS, 207, 70.01)

16. Conclusion Mean pulsation period seems very promising as an effective temperature scale for the non-interacting white dwarf pulsators. This technique remains to be proven for the accreting pulsators.