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)

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?

Asteroseismology

• Pulsations Only systematic way to study the stellar interior

• Pulsations are observed in different types of stars in various stages of evolution

ZZ Ceti stars

• 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)

ZZ Ceti stars (DAVs)Hydrogen atmospherewhite dwarf variables

Two flavours of ZZ Ceti stars (DAVs)

Teff = 11000K P ~ 1000s

Teff = 12000K P ~ 200s

s/s10 ~P -15

0 1000 2000 3000 Time (s)

0.4

0.2

0

0.05

-0.05

Frac

tiona

l Int

ensi

ty

0

0 1000 2000 3000 4000 5000

Cool ZZ Ceti (cDAV)

Hot ZZ Ceti (hDAV)

Mean Period vs. Spectroscopic Temperature

Pulsation Period: Means of measuring Teff ?

WMP = -0.830 Teff +10240

WMP = -0.835 Teff +10060

Spectroscopy vs. Weighted Mean Period

• 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.

• Internal uncertainty ~10-60s / 1300s (<5% of the width)

• Mass does not affect pulsation period

• Relatively simple and model-independent measurement

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)

Mean pulsation amplitude vs. Mean period (serves as temperature)

Hot Cool

ZZ Ceti stars lose amplitude before pulsations shut down at the red edge!

Hot Cool

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

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

Preliminary results from HSTUV time-resolved spectroscopy

Accreting ZZ Ceti Teff (K)(Spec)

Period, Amplitude (~1250 -1800 Å)

SDSSJ013132.39-090122.2 14500 213.72 s, 78 mma1

SDSSJ161033.64-010223.2 14500 220.81 s, 23.4 mma1

304.10 s, 48.3 mma1

608.22 s, 186.1 mma1

SDSSJ220553.98+115553.6 15000 576.2 s, 46 mma1

(Szkody et al. 2006 (in prep); Mukadam et al. 2005, BAAS, 207, 70.01)

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.