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Asteroseismological Constraints on the Structure of the ZZ Ceti Stars L19-2 and GD 165

This study compares the theoretical pulsation periods from an extensive grid of evolutionary DA white dwarf models with the observed periods of the ZZ Ceti white dwarfs L19-2 and GD 165, in order to constrain their internal structure. Our analysis of the rotational fine-structure splitting and comparison of our theoretical periods with observations for L19-2 and GD 165 enable us to identify the observed modes as low-order ℓ = 1 and 2 g-modes. Because the period structure of GD 165 is quite similar to that of L19-2, we believe that the interior structure of GD 165 is similar. The short period of the ℓ = 1 118.5 s mode of L19-2 (120.4 s mode of GD 165) implies a hydrogen layer mass of about 10-4 M*, independent of constraints from the other pulsation modes. Detailed model fitting shows that L19-2 has a hydrogen layer mass of 1.0 × 10-4 M*, a helium layer mass of 1.0 × 10-2 M*, a 20 : 80 C/O core that extends out to 0.60 M*, a stellar mass of 0.72 M☉, and a rotation period of about 13 hr. The best-fitting models for GD 165 have a hydrogen layer mass of 1.5 to 2.0 × 10-4 M*, a helium layer mass of 1.5 to 2.0 × 10-2 M*, a 20 : 80 C/O core that extends out to 0.65 M*, a stellar mass of 0.65-0.68 M☉, and a rotation period of about 58 hr. In both cases, the best-fitting models are consistent with the spectroscopic log g-value, and the seismological parallax is within 1 σ of the observed parallax value.