Browsing by Subject "gd-358"

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Evidence For Temperature Change And Oblique Pulsation From Light Curve Fits Of The Pulsating White Dwarf GD 358
(2010-06) Montgomery, Michael H.; Provencal, J. L.; Kanaan, A.; Mukadam, Anjum S.; Thompson, Susan E.; Dalessio, J.; Shipman, H. L.; Winget, D. E.; Kepler, S. O.; Koester, Detlev; Montgomery, Michael H.
Convective driving, the mechanism originally proposed by Brickhill for pulsating white dwarf stars, has gained general acceptance as the generic linear instability mechanism in DAV and dbV white dwarfs. This physical mechanism naturally leads to a nonlinear formulation, reproducing the observed light curves of many pulsating white dwarfs. This numerical model can also provide information on the average depth of a star's convection zone and the inclination angle of its pulsation axis. In this paper, we give two sets of results of nonlinear light curve fits to data on the dbV GD 358. Our first fit is based on data gathered in 2006 by the Whole Earth Telescope; this data set was multiperiodic containing at least 12 individual modes. Our second fit utilizes data obtained in 1996, when GD 358 underwent a dramatic change in excited frequencies accompanied by a rapid increase in fractional amplitude; during this event it was essentially monoperiodic. We argue that GD 358's convection zone was much thinner in 1996 than in 2006, and we interpret this as a result of a short-lived increase in its surface temperature. In addition, we find strong evidence of oblique pulsation using two sets of evenly split triplets in the 2006 data. This marks the first time that oblique pulsation has been identified in a variable white dwarf star.
New Pulsating DB White Dwarf Stars From The Sloan Digital Sky Survey
(2009-01) Nitta, Atsuko; Kleinman, S. J.; Krzesinski, J.; Kepler, S. O.; Metcalfe, T. S.; Mukadam, Anjum S.; Mullally, Fergal; Nather, R. E.; Sullivan, Denis J.; Thompson, Susan E.; Winget, D. E.; Nather, R. E.; Winget, D. E.
We are searching for new He atmosphere white dwarf pulsators (dbVs) based on the newly found white dwarf stars from the spectra obtained by the Sloan Digital Sky Survey. dbVs pulsate at hotter temperature ranges than their better known cousins, the H atmosphere white dwarf pulsators (DAVs or ZZ Ceti stars). Since the evolution of white dwarf stars is characterized by cooling, asteroseismological studies of dbVs give us opportunities to study white dwarf structure at a different evolutionary stage than the DAVs. The hottest dbVs are thought to have neutrino luminosities exceeding their photon luminosities, a quantity measurable through asteroseismology. Therefore, they can also be used to study neutrino physics in the stellar interior. So far we have discovered nine new dbVs, doubling the number of previously known dbVs. Here we report the new pulsators' light curves and power spectra.
SDSS J142625.71+575218.3: the First Pulsating White Dwarf With A Large Detectable Magnetic Field
(2008-08) Dufour, P.; Fontaine, G.; Liebert, J.; Williams, K.; Lai, D. K.; Williams, K.
We report the discovery of a strong magnetic field in the unique pulsating carbon- atmosphere white dwarf SDSS J142625.71 + 575218.3. From spectra gathered at the MMT and Keck telescopes, we infer a surface field of B(s) similar or equal to 1.2 MG, based on obvious Zeeman components seen in several carbon lines. We also detect the presence of a Zeeman- splitted He I lambda 4471 line, which is an indicator of the presence of a nonnegligible amount of helium in the atmosphere of this "hot DQ" star. This is important for understanding its pulsations, as nonadabatic theory reveals that some helium must be present in the envelope mixture for pulsation modes to be excited in the range of effective temperature where the target star is found. Out of nearly 200 pulsating white dwarfs known today, this is the first example of a star with a large detectable magnetic field. We suggest that SDSS J142625.71 + 575218.3 is the white dwarf equivalent of a rapidly oscillating Ap star.