Seismology of an Ensemble of ZZ Ceti Stars

TL;DR

This paper combines observational data of ZZ Ceti stars to compare with theoretical models, revealing insights into their internal structure and highlighting discrepancies in current evolutionary models.

Contribution

It provides the first ensemble seismology analysis of ZZ Ceti stars using a large dataset and theoretical model grids to infer stellar layer properties.

Findings

Hydrogen layer masses are consistent with thick limits from nuclear burning.

Models with spectroscopic masses and temperatures fail to reproduce observed periods.

Helium layer masses in models may be overestimated.

Abstract

We combine all the reliably-measured eigenperiods for hot, short-period ZZ Ceti stars onto one diagram and show that it has the features expected from evolutionary and pulsation theory. To make a more detailed comparison with theory we concentrate on a subset of 16 stars for which rotational splitting or other evidence gives clues to the spherical harmonic index (l) of the modes. The suspected l=1 periods in this subset of stars form a pattern of consecutive radial overtones that allow us to conduct ensemble seismology using published theoretical model grids. We find that the best-matching models have hydrogen layer masses most consistent with the canonically thick limit calculated from nuclear burning. We also find that the evolutionary models with masses and temperatures from spectroscopic fits cannot correctly reproduce the periods of the k=1 to 4 mode groups in these stars, and speculate that the mass of the helium layer in the models is too large.