The GW Vir instability strip in the light of new observations of PG 1159 stars. Discovery of pulsations in the central star of Abell 72 and variability of RX J0122.9-7521

TL;DR

This study presents new observations of PG 1159 stars, discovering pulsations in Abell 72 and variability in RX J0122.9-7521, refining the understanding of the GW Vir instability strip and the nature of these pre-white dwarf stars.

Contribution

It provides the first comprehensive luminosity-based placement of PG 1159 stars on the Hertzsprung-Russell diagram and challenges traditional classifications of GW Vir pulsators.

Findings

Detected pulsations in Abell 72's central star.

Identified variability in RX J0122.9-7521 possibly due to pulsations, binarity, or rotation.

Estimated that 36% of PG 1159 stars are pulsators within the GW Vir instability strip.

Abstract

We present the results of new time series photometric observations of 29 pre-white dwarf stars of PG 1159 spectral type, carried out in the years 2014-2022. For the majority of stars, a median noise level in Fourier amplitude spectra of 0.5-1.0 mmag was achieved. This allowed the detection of pulsations in the central star of planetary nebula Abell 72, consistent with g-modes excited in GW Vir stars, and variability in RX J0122.9-7521 that could be due to pulsations, binarity or rotation. For the remaining stars from the sample that were not observed to vary, we placed upper limits for variability. After combination with literature data, our results place the fraction of pulsating PG 1159 stars within the GW Vir instability strip at 36%. An updated list of all known PG 1159 stars is provided, containing astrometric measurements from the recent Gaia DR3 data, as well as information on physical parameters, variability, and nitrogen content. Those data are used to calculate luminosities for all PG 1159 stars to place the whole sample on the theoretical Hertzsprung-Russell diagram for the first time in that way. The pulsating stars are discussed as a group, and arguments are given that the traditional separation of GW Vir pulsators in "DOV" and "PNNV" stars is misleading and should not be used.