A 15.7-Minute AM CVn Binary Discovered in K2

TL;DR

This paper reports the discovery of a new ultracompact AM CVn binary with the shortest orbital period found by Kepler, providing insights into its properties and potential as a gravitational wave source.

Contribution

The discovery of SDSS J135154.46-064309.0 as the shortest-period AM CVn binary observed by Kepler, with detailed photometric and spectroscopic analysis of its orbital and superhump periods.

Findings

Shortest known orbital period for an AM CVn binary

Identification of multiple photometric periods including orbital, superhump, and precession

Estimated binary mass ratio of approximately 0.111

Abstract

We present the discovery of SDSS J135154.46-064309.0, a short-period variable observed using 30-minute cadence photometry in K2 Campaign 6. Follow-up spectroscopy and high-speed photometry support a classification as a new member of the rare class of ultracompact accreting binaries known as AM CVn stars. The spectroscopic orbital period of $15.65 \pm 0.12$\,minutes makes this system the fourth-shortest period AM CVn known, and the second system of this type to be discovered by the Kepler spacecraft. The K2 data show photometric periods at $15.7306 \pm 0.0003$\,minutes, $16.1121 \pm 0.0004$\,minutes and $664.82 \pm 0.06$\,minutes, which we identify as the orbital period, superhump period, and disc precession period, respectively. From the superhump and orbital periods we estimate the binary mass ratio $q = M_2/M_1 = 0.111 \pm 0.005$, though this method of mass ratio determination may not be well calibrated for helium-dominated binaries. This system is likely to be a bright foreground source of gravitational waves in the frequency range detectable by LISA, and may be of use as a calibration source if future studies are able to constrain the masses of its stellar components.