TESS first look at evolved compact pulsators

Abstract

Context. Pulsation frequencies reveal the interior structures of white dwarf stars, shedding light on the properties of these compact objects that represent the final evolutionary stage of most stars. Two-minute cadence photometry from the Transiting Exoplanet Survey Satellite (TESS) records pulsation signatures from bright white dwarfs over the entire sky. Aims. As part of a series of first-light papers from TESS Asteroseismic Science Consortium Working Group 8, we aim to demonstrate the sensitivity of TESS data, by measuring pulsations of helium-atmosphere white dwarfs in the DBV instability strip, and what asteroseismic analysis of these measurements can reveal about their stellar structures. We present a case study of the pulsating DBV WD 0158−160 that was observed as TIC 257459955 with the two-minute cadence for 20.3 days in TESS Sector 3. Methods. We measured the frequencies of variability of TIC 257459955 with an iterative periodogram and prewhitening procedure. The measured frequencies were compared to calculations from two sets of white dwarf models to constrain the stellar parameters: the fully evolutionary models from LPCODE and the structural models from WDEC. Results. We detected and measured the frequencies of nine pulsation modes and eleven combination frequencies of WD 0158−160 to ∼0.01 μHz precision. Most, if not all, of the observed pulsations belong to an incomplete sequence of dipole (ℓ = 1) modes with a mean period spacing of 38.1 ± 1.0 s. The global best-fit seismic models from both LPCODE and WDEC have effective temperatures that are ≳3000 K hotter than archival spectroscopic values of 24 100–25 500 K; however, cooler secondary solutions are found that are consistent with both the spectroscopic effective temperature and distance constraints from Gaia astrometry. Conclusions. Our results demonstrate the value of the TESS data for DBV white dwarf asteroseismology. The extent of the short-cadence photometry enables reliably accurate and extremely precise pulsation frequency measurements. Similar subsets of both the LPCODE and WDEC models show good agreement with these measurements, supporting that the asteroseismic interpretation of DBV observations from TESS is not dominated by the set of models used. However, given the sensitivity of the observed set of pulsation modes to the stellar structure, external constraints from spectroscopy and/or astrometry are needed to identify the best seismic solutions.

White-Dwarf Asteroseismology With the Kepler Space Telescope

“…Altogether, the Kepler space telescope has had a strong impact (and will continue to have) on the area of white-dwarf asteroseismology. This first step will be multiplied by current and future space missions, such as the Transiting Exoplanet Survey Satellite (TESS; Ricker et al, 2015), which is already in full operation and is providing the first results on pulsating white dwarfs (Bell et al, 2019;Althaus et al, 2020;Bognár et al, 2020), and other space missions that will become operational in the coming years, such as Cheops (Moya et al, 2018) and Plato (Piotto, 2018).…”

White-dwarf asteroseismology: An update

“…TIC257459955 is a known DBV white dwarf with T eff = 24 100 K and log g = 7.88 (Rolland et al 2018), or alternatively, T eff = 25 500 K and log g = 7.94 (Voss et al 2007). Bell et al (2019) find 9 independent frequencies suitable for asteroseismology plus frequency combinations. The periods of genuine eigenmodes are in the range [245 -866] sec, with a period spacing of ΔΠ = 38.1 ± 1.0 s associated to = 1.…”

“…2012; Bell et al 2019). The quality of the period fits is assessed by means of the average of the absolute period differences, δ = (…”

“…The first pulsating white dwarf analyzed with TESS is the DBV pulsator WD0158-160 (also called EC01585-1600, G272-B2A, TIC257459955; Bell et al 2019). TESS performs extensive time-series photometry that allows to discover pulsating stars, and, in particular, pulsating white dwarfs.…”

A Search for Pulsation in Twenty-One White Dwarfs

“…These objects are too faint for most current automatic ground-based surveys and at the limit of observability even for satellite-based missions such as Kepler (Doyle et al 2017). Among the first published results from the TESS mission was an investigation of the known pulsating DBV WD 0158−160 (Bell et al 2019). With a main period of about 640 s and an amplitude of about 22 mmag, this object is quite typical among pulsating WDs.…”