Wandering near the red edge: photometric observations of three cool ZZ Ceti stars

TL;DR

This study presents new photometric observations of three cool ZZ Ceti white dwarfs, identifying multiple pulsation modes and variations, which enhance understanding of pulsations near the empirical ZZ Ceti red edge.

Contribution

It reports the discovery of new pulsation modes and amplitude variations in three cool ZZ Ceti stars, providing insights into their complex pulsational behavior near the red edge.

Findings

Eight independent modes in HS 0733+4119, seven are new

Two new eigenmodes in GD 154 and recurrence of pulsations since 1977

Comparison of Ross 808 pre- and post-Whole Earth Telescope data

Abstract

We summarize our findings on three cool ZZ Ceti type pulsating white dwarfs. We determined eight independent modes in HS 0733+4119, of which seven are new findings. For GD 154, we detected two new eigenmodes, and the recurrence of the pulsational behaviour first observed in 1977. We discuss that GD 154 does not only vary its pulsations between a multiperiodic and a quasi-monoperiodic phase, but there are also differences between the relative amplitudes of the near-subharmonics observed in the latter phase. In the complex pulsator, Ross 808, we compared the pre- and post Whole Earth Telescope campaign measurements, and determined two new frequencies besides the ones observed during the campaign. Studying these stars can contribute to better understanding of pulsations close to the empirical ZZ Ceti red edge. All three targets are in that regime of the ZZ Ceti instability strip where short-term amplitude variations or even outbursts are likely to occur, which are not well-understood theoretically.