Two New Tidally Distorted White Dwarfs

TL;DR

This paper reports the discovery and analysis of two extremely low mass, tidally distorted white dwarfs in close binary systems, revealing their large radii, strong gravitational wave signals, and potential for orbital decay detection.

Contribution

The study presents the identification and detailed characterization of two new tidally distorted white dwarfs, including indirect radius measurements and analysis of their gravitational wave emissions.

Findings

Both WDs show ellipsoidal variations and Doppler beaming.

J1741 has one of the strongest Doppler beaming signals observed.

The inferred radii are among the largest for white dwarfs.

Abstract

We identify two new tidally distorted white dwarfs (WDs), SDSS J174140.49+652638.7 and J211921.96-001825.8 (hereafter J1741 and J2119). Both stars are extremely low mass (ELM, < 0.2 Msun) WDs in short-period, detached binary systems. High-speed photometric observations obtained at the McDonald Observatory reveal ellipsoidal variations and Doppler beaming in both systems; J1741, with a minimum companion mass of 1.1 Msun, has one of the strongest Doppler beaming signals ever observed in a binary system (0.59 \pm 0.06% amplitude). We use the observed ellipsoidal variations to constrain the radius of each WD. For J1741, the star's radius must exceed 0.074 Rsun. For J2119, the radius exceeds 0.10 Rsun. These indirect radius measurements are comparable to the radius measurements for the bloated WD companions to A-stars found by the Kepler spacecraft, and they constitute some of the largest radii inferred for any WD. Surprisingly, J1741 also appears to show a 0.23 \pm 0.06% reflection effect, and we discuss possible sources for this excess heating. Both J1741 and J2119 are strong gravitational wave sources, and the time-of-minimum of the ellipsoidal variations can be used to detect the orbital period decay. This may be possible on a timescale of a decade or less.