mean period of pulsating white dwarfs as a spectroscopy-independent thermometer anjum s. mukadam,...
TRANSCRIPT
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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)
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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?
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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
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• 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)
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ZZ Ceti stars (DAVs)Hydrogen atmospherewhite dwarf variables
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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
Fra
ctio
nal
In
ten
sity
0
0 1000 2000 3000 4000 5000
Cool ZZ Ceti (cDAV)
Hot ZZ Ceti (hDAV)
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Mean Period vs. Spectroscopic Temperature
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Pulsation Period: Means of measuring Teff ?
WMP = -0.830 Teff +10240
WMP = -0.835 Teff +10060
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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
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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)
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Mean pulsation amplitude vs. Mean period (serves as temperature)
Hot Cool
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ZZ Ceti stars lose amplitude before pulsations shut down at the red edge!
Hot Cool
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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
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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
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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)
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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.