Unveiling the properties of pulsating low-mass helium-core white dwarfs through TESS asteroseismology I. First results

Abstract

Recent space-based photometry, particularly from TESS, has opened new possibilities for probing the internal structure of low-mass (LM) helium (He)-core white dwarfs (WDs). We present a homogeneous asteroseismological analysis of six pulsating LM WD stars, based on new and updated TESS photometry. We processed short- and ultra-short-cadence TESS observations of TIC 290904838 (J1112), TIC 156064657, TIC 33717565, TIC 344130696, TIC 72637474, and TIC 188087204, and analyzed the resulting pulsation spectra. We then carried out a detailed asteroseismological analysis using fully evolutionary models of LM He-core WDs that allow for varying hydrogen (H)-envelope thicknesses. We also estimated spectroscopic/photometric stellar masses when atmospheric parameters are available. We report the first TESS-based frequencies for J1112 and provide revised or expanded frequency solutions for the remaining targets. The asteroseismological analysis yields relatively well-constrained solutions for three stars, a representative but more tentative solution for one target, and constrained ranges for the remaining two. The inferred solutions span a broad range of H-envelope thicknesses, although some of the asteroseismological inferences remain tentative because of the limited number of observed periods available for the analysis. For most objects, the derived spectroscopic/photometric stellar masses are broadly compatible with the asteroseismological values. This is the first homogeneous TESS-based asteroseismological study of a small sample of pulsating LM WDs. Our results suggest that LM WDs can harbor H envelopes with a range of thicknesses, from canonical (thick) to very thin, as in average-mass H-rich pulsating WDs. They also provide a useful reference point for future studies of larger samples, which will hopefully benefit from richer mode sets and improved mode identification.