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

Abstract

This study compares the theoretical pulsation periods from an extensivegrid of evolutionary DA white dwarf models with the observed periods ofthe ZZ Ceti white dwarfs L19-2 and GD 165, in order to constrain theirinternal structure. Our analysis of the rotational fine-structuresplitting and comparison of our theoretical periods with observationsfor L19-2 and GD 165 enable us to identify the observed modes aslow-order l=1 and 2 g-modes. Because the period structure of GD 165 isquite similar to that of L19-2, we believe that the interior structureof 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 about10^{-4} M_{*}, independent of constraints from the otherpulsation modes. Detailed model fitting shows that L19-2 has a hydrogenlayer mass of 1.0{\times}10^{-4} M_{*}, a helium layermass of 1.0{\times}10^{-2} M_{*}, a 20:80 C/O core thatextends out to 0.60 M_{*}, a stellar mass of 0.72M_{solar}, and a rotation period of about 13 hr. Thebest-fitting models for GD 165 have a hydrogen layer mass of 1.5 to2.0{\times}10^{-4} M_{*}, a helium layer mass of 1.5 to2.0{\times}10^{-2} M_{*}, a 20:80 C/O core that extendsout to 0.65 M_{*}, a stellar mass of 0.65-0.68M_{solar}, and a rotation period of about 58 hr. In both cases,the best-fitting models are consistent with the spectroscopic logg-value, and the seismological parallax is within 1 {$σ$} of theobserved parallax value.