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

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Abstract
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.