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.