EPIC 228782059: Asteroseismology of What Could Be the Coolest Pulsating Helium-atmosphere White Dwarf (DBV) Known

Abstract

We present analysis of a new pulsating helium-atmosphere (DB) white dwarf, EPIC 228782059, discovered from 55.1 days of K2 photometry. The long-duration, high-quality light curves reveal 11 independent dipole and quadruple modes, from which we derive a rotational period of 34.1 ± 0.4 hr for the star. An optimal model is obtained from a series of grids constructed using the White Dwarf Evolution Code, which returns M * = 0.685 ± 0.003M ⊙, T eff = 21,910 ± 23 K, and log g = 8.14 ± 0.01 dex. These values are comparable to those derived from spectroscopy by Koester & Kepler (20,860 ± 160 K, and 7.94 ± 0.03 dex). If these values are confirmed or better constrained by other independent works, it would make EPIC 228782059 one of the coolest pulsating DB white dwarf stars known, and would be helpful for testing different physical treatments of convection, and to further investigate the theoretical instability strip of DB white dwarf stars.

On Trapped Modes in Variable White Dwarfs as Probes of the 12C(α, γ)16O Reaction Rate

“…The g-mode periods of observed variable WD are derived from a Fourier analysis of the photometric light curves and are typically given to 6−7 significant figures of precision (e.g., Duan et al 2021). Usually WD composition profile templates are fit to the extracted g-mode period spectrum and other observed constraints (e.g., T eff , g log ) of a specific WD.…”

Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS

“…Finally, in recent years, the area has received a strong boost driven by the uninterrupted observations from space made by the Kepler telescope, including both the main mission (Borucki et al 2010) and K2 mode (Howell et al 2014). Indeed, these efforts paved the way for the analysis of 32 ZZ Ceti stars and three DBV stars (Østensen et al 2011;Greiss et al 2014;Bell et al 2015Hermes et al 2017a,b;Duan et al 2021), until the mission was terminated due to a lack of fuel in 2018. The successor to Kepler is the Transiting Exoplanet Survey Satellite (TESS, Ricker et al 2015).…”

“…Due to the high-quality Kepler and K2 observations, three DBV stars, KIC 8626021 (Østensen et al 2011), PG 0112+104 (Hermes et al 2017b), and EPIC 228782059 (Duan et al 2021), were intensively studied with space data. In particular, KIC 8626021 has been repeatedly modeled by several independent research groups (Bischoff-Kim & Østensen 2011;Córsico et al 2012;Bischoff-Kim et al 2014;Giammichele et al 2018;Charpinet et al 2019), who have explored its internal structure with unprecedented precision.…”

Pulsating hydrogen-deficient white dwarfs and pre-white dwarfs observed with TESS

“…At higher temperatures (T eff ∼ 21 000 − 30 000 K), and with helium (He)-dominated atmospheres, we find the pulsating WDs called V777 Her or DBVs. Although not many DBVs are known at present (47 objects; see Córsico et al 2019a;Duan et al 2021;Vanderbosch et al 2022), these compact pulsating stars are very interesting because the origin of DB WDs is not entirely clear. They are probably descendants of the PG 1159 stars (oxygen (O)-, carbon (C)-, and He-rich atmospheres) after going through the DO WD (with very hot He-rich atmospheres) stage (Dehner & Kawaler 1995;Althaus et al 2005;Battich et al 2020;Bédard et al 2022).…”

“…Later on, discoveries of pulsat-ing WDs increased enormously thanks to the identification of candidates from the Sloan Digital Sky Survey (SDSS, York et al 2000;Kleinman et al 2013;Kepler et al 2015Kepler et al , 2016Kepler et al , 2019a, and in recent years, with space missions. In particular, the Kepler/K2 mission (Borucki et al 2010;Howell et al 2014) resulted in the study of 32 ZZ Ceti stars and three DBV stars (Østensen et al 2011;Bell et al 2015;Hermes et al 2017a,b;Bell et al 2017;Córsico 2020;Duan et al 2021). That mission ended in 2018.…”