Asteroseismological Constraints on the Structure of the ZZ Ceti Stars G117-B15A and R548

This study compares the theoretical pulsation periods from a new grid of evolutionary DA white dwarf models to the observed periods of the ZZ Ceti white dwarfs G117-B15A and R548 (≡ZZ Ceti) in order to constrain their internal structure. Assuming that the similar periods of the two stars is the result of their having nearly identical internal structures, then they must have a hydrogen layer mass of ∼10-7Mandbstarf; or greater. If we follow Clemens's work and identify the period near 215 s as the ell=1, k = 2 mode, then the hydrogen layer mass is close to 10-4Mandbstarf;, quite close to the maximum hydrogen layer mass allowed by stellar evolution theory. Based on this analysis, G117-B15A and R548 probably have masses near 0.60 Mcircle-dot and MH ∼ 10-4Mandbstarf;, and R548 is about 0.04 Mcircle-dot less massive than G117-B15A. Alternatively, if 215 s is the k = 1 mode, then the hydrogen layer mass decreases to about 10-7Mandbstarf;, but the stellar mass remains near 0.60 Mcircle-dot. In all cases, the helium layer mass is near 10-2Mandbstarf;, and the C/O core of both stars is oxygen rich. The existence of the 304 s mode of G117-B15A strongly restricts the allowable core structure. At 0.60 Mcircle-dot, the core is either 50:50 C/O to 0.75Mandbstarf; or 20:80 C/O to 0.83Mandbstarf;. The fine-structure splitting of R548 is due to slow rotation, and we can duplicate the observed splitting trend and periods if the 213 s mode is k = 2, the model mass is near 0.54 Mcircle-dot, and the hydrogen layer mass is near 1.5 x 10-4Mandbstarf;. copyright copyright 1998. The American Astronomical Society