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 gri d 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 s tructure. Our analysis of the rotational fine-structure splitting and compa rison of our theoretical periods with observations for L19-2 and GD 165 ena ble us to identify the observed modes as low-order l = 1 and 2 g-modes. Bec ause the period structure of GD 165 is quite similar to that of L19-2, we b elieve that the interior structure of GD 165 is similar. The short period o f the l = 1 118.5 s mode of L19-2 (120.4 s mode of GD 165) implies a hydrog en layer mass of about 10(-4) M-*, independent of constraints from the othe r pulsation modes. Detailed model fitting shows that L19-2 has a hydrogen l ayer mass of 1.0 x 10(-4) M-*, a helium layer mass of 1.0 x 10(-2) M-*, a 2 0:80 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 x 10(-4) M-*, a helium layer mass of 1. 5 to 2.0 x 10(-2) M-*, a 20:80 C/O core that extends out to 0.65 M-*, a ste llar mass of 0.65-0.68 M-., and a rotation period of about 58 hr. In both c ases, the best-fitting models are consistent with the spectroscopic log g-v alue, and the seismological parallax is within 1 sigma of the observed para llax value.