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

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 T(sub eff) 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 T(sub eff) = 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(exp -6) solar mass (for k = 1) and 8 x 10(exp -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(exp -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.