Spectroscopic Evidence for a 5.4-Minute Orbital Period in HM Cancri

TL;DR

This paper provides spectroscopic evidence confirming that HM Cancri is an ultracompact binary white dwarf system with a 5.4-minute orbital period, making it the shortest known binary star and a key gravitational wave source.

Contribution

The study presents the first spectroscopic confirmation of the 5.4-minute orbital period in HM Cancri, establishing it as the shortest-period binary star and a significant gravitational wave source.

Findings

Confirmed 5.4-minute orbital period through spectroscopy

Measured mass ratio q=0.50+/-0.13

Identified emission lines from different stellar faces

Abstract

HM Cancri is a candidate ultracompact binary white dwarf with an apparent orbital period of only 5.4 minutes, as suggested by X-ray and optical light-curve modulations on that period, and by the absence of longer-period variability. In this Letter we present Keck-I spectroscopy which shows clear modulation of the helium emission lines in both radial velocity and amplitude on the 5.4-minute period and no other. The data strongly suggest that the binary is emitting He I 4471 from the irradiated face of the cooler, less massive star, and He II 4686 from a ring around the more massive star. From their relative radial velocities, we measure a mass ratio q=0.50+/-0.13. We conclude that the observed 5.4-minute period almost certainly represents the orbital period of an interacting binary white dwarf. We thus confirm that HM Cnc is the shortest-period binary star known: a unique test for stellar evolution theory, and one of the strongest known sources of gravitational waves for the Laser Interferometer Space Antenna (LISA).