ASTEROSEISMOLOGICAL CONSTRAINTS ON THE STRUCTURE OF THE ZZ CETI STARSG117-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 Z Z Ceti white dwarfs G117-B15A and R548 (=ZZ Ceti) in order to constrai n their internal structure. Assuming that the similar periods of the t wo stars is the result of their having nearly identical internal struc tures, then they must have a hydrogen layer mass of similar to 10(-7)M or greater. If we follow Clemens's work and identify the period near 215 s as the e = 1, k = 2 mode, then the hydrogen layer mass is close to 10(-4)M, quite close to the maximum hydrogen layer mass allowed b y stellar evolution theory. Based on this analysis, G117-B15A and R548 probably have masses near 0.60 M. and M-H similar to 10(-4)M, and R5 48 is about 0.04 M. less massive than G117-B15A. Alternatively, if 215 s is the k = 1 mode, then the hydrogen layer mass decreases to about 10(-7)M, but the stellar mass remains near 0.60 M.. In all cases, the helium layer mass is near 10(-2)M, and the C/O core of both stars is oxygen rich. The existence of the 304 s mode of G117-B15A strongly re stricts the allowable core structure. At 0.60 Mo, the core is either 5 0:50 C/O to 0.75M or 20:80 C/O to 0.83M*. The fine-structure splittin g of R548 is due to slow rotation, and we can duplicate the observed s plitting trend and periods if the 213 s mode is k = 2, the model mass is near 0.54 M., and the hydrogen layer mass is near 1.5 x 10(-4)M.