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arXiv · 1003.0658

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

G. H. A. Roelofs·A. Rau·T. R. Marsh·D. Steeghs·P. J. Groot·G. Nelemans
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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).

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G. H. A. Roelofs, A. Rau, T. R. Marsh, D. Steeghs, P. J. Groot, G. Nelemans. 2010-03-02. Spectroscopic Evidence for a 5.4-Minute Orbital Period in HM Cancri. https://doi.org/10.1088/2041-8205%2F711%2F2%2Fl138

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