The pulsation index, effective temperature, and thickness of the hydrogen layer in the pulsating DA white dwarf G117-B15A

Details The pulsation index, effective temperature, and thickness of the hydrogen layer in the pulsating DA white dwarf G117-B15A The pulsation index, effective temperature, and thickness of the hydrogen layer in the pulsating DA white dwarf G117-B15A

Jan 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...438..908r&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 438, no. 2, p. 908-916

Physics

92

Hydrogen, Stellar Atmospheres, Stellar Luminosity, Stellar Oscillations, Stellar Spectra, Stellar Temperature, White Dwarf Stars, Spectrum Analysis, Stellar Evolution, Stellar Physics, Stellar Structure, Thickness

Scientific paper

We have measured the amplitude of the 215 s pulsation of the pulsating DA white dwarf, or ZZ Ceti star, G117-B15A in six passbands with effective wavelengths from 1570 to 6730 A. We find that the index of the pulsation is l = 1 with a high degree of confidence, the first unambiguous determination of l for a pulsation of a ZZ Ceti star. We also find that log g and Teff are tightly correlated for model atmospheres that fit the data, such that at log g = 7.5 the temperature is 11,750 K and at log g = 8.0 the temperature is 12,375 K. Adopting log g = 7.97 +/- 0.06 from published observations of the optical spectrum of G117-B15A, the correlation yields Teff = 12,375 +/- 125 K. This temperature is free of flux calibration errors and should be substantially more reliable than temperatures derived for IUE spectra. Since G117-B15A is thought to lie close to the blue edge of the ZZ Ceti instability strip, this low temperature also implies a low temperature for the blue edge. Using pulsation models calculated by Fontaine et al. (1992) and Bradley (1994), we find that the mass of the hydrogen layer in G117-B15A lies between 1.0 x 10-6 solar mass (for k = 1) and 8 x 10-5 solar mass (for k = 2). This range of masses is (barely) consistent with the masses predicted by recent models for the ejection of planetary nebulae, (8-13) x 10-5 solar mass. The mass is too large to be consistent with models invoking thin hydrogen layers to explain the spectral evolution of white dwarfs.

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