Discovery, TESS Characterization, and Modeling of Pulsations in the Extremely Low Mass White Dwarf GD 278

TL;DR

This paper reports the discovery and analysis of pulsations in the extremely low mass white dwarf GD 278 using TESS data, revealing its rotation period and providing insights into its internal structure through asteroseismology.

Contribution

It is the first ELM white dwarf with extended space-based photometry, providing detailed pulsation data and initial asteroseismic modeling insights.

Findings

Detected 19 pulsation periods in GD 278, including the longest ever in a white dwarf.

Measured a stellar rotation period of roughly 10 hours for GD 278.

Identified two main asteroseismic solutions consistent with spectroscopic data.

Abstract

We report the discovery of pulsations in the extremely low mass (ELM), likely helium-core white dwarf GD 278 via ground- and space-based photometry. GD 278 was observed by the Transiting Exoplanet Survey Satellite (TESS) in Sector 18 at a 2-min cadence for roughly 24 d. The TESS data reveal at least 19 significant periodicities between 2447-6729 s, one of which is the longest pulsation period ever detected in a white dwarf. Previous spectroscopy found that this white dwarf is in a 4.61 hr orbit with an unseen >0.4 solar-mass companion and has Teff = 9230 +/- 100 K and log(g) = 6.627 +/- 0.056, which corresponds to a mass of 0.191 +/- 0.013 solar mass. Patterns in the TESS pulsation frequencies from rotational splittings appear to reveal a stellar rotation period of roughly 10 hr, making GD 278 the first ELM white dwarf with a measured rotation rate. The patterns inform our mode identification for asteroseismic fits, which unfortunately do not reveal a global best-fit solution. Asteroseismology reveals two main solutions roughly consistent with the spectroscopic parameters of this ELM white dwarf, but with vastly different hydrogen-layer masses; future seismic fits could be further improved by using the stellar parallax. GD 278 is now the tenth known pulsating ELM white dwarf; it is only the fifth known to be in a short-period binary, but is the first with extended, space-based photometry.