Multimode pulsation of the ZZ Ceti star GD 154

TL;DR

This study analyzes the pulsation modes of the ZZ Ceti star GD 154, discovering new frequencies, amplitude variability, and inferring stellar rotation and hydrogen layer thickness through detailed modeling.

Contribution

It reports the first detection of two previously unreported normal modes and provides new insights into mode variability and stellar structure modeling for GD 154.

Findings

Six normal pulsation modes identified.

Two new normal modes at 807.62 and 861.56 microHz.

Rotational period of 2.1 days inferred.

Abstract

We present the results of a comparative period search on different time-scales and modelling of the ZZ Ceti (DAV) star GD 154. We determined six frequencies as normal modes and four rotational doublets around the ones having the largest amplitude. Two normal modes at 807.62 and 861.56 microHz have never been reported before. A rigorous test revealed remarkable intrinsic amplitude variability of frequencies at 839.14 and 861.56 microHz over a 50 d time-scale. In addition, the multimode pulsation changed to monoperiodic pulsation with an 843.15 microHz dominant frequency at the end of the observing run. The 2.76 microHz average rotational split detected led to a determination of a 2.1 d rotational period for GD 154. We searched for model solutions with effective temperatures and log g close to the spectroscopically determined ones. The best-fitting models resulting from the grid search have M_H between 6.3 x 10^-5 and 6.3 x 10^-7 M*, which means thicker hydrogen layer than the previous studies suggested. Our investigations show that mode trapping does not necessarily operate in all of the observed modes and the best candidate for a trapped mode is at 2484 microHz.