Gijs H. A. Roelofs1, Arne Rau2,3, Tom R. Marsh4, Danny Steeghs4, Paul J. Groot5 and Gijs Nelemans5
1
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
2
Astronomy Department, California Institute of Technology, Pasadena, CA 91125, USA
3
Max-Planck Institute for Extra-Terrestrial Physics, Giessenbachstr. 1, 85748 Garching, Germany
4
Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
5
Department of Astrophysics, Radboud University Nijmegen, P.O. Box 9010, 6500 GL, Nijmegen, The Netherlands
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Gijs H. A. Roelofs et al 2010 ApJ 711 L138
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 LISA.
binaries: close; gravitational waves; stars: individual (HM Cancri, V407 Vulpeculae); X-rays: binaries
97.10.Wn Proper motions and radial velocities (line-of-sight velocities); space motions
97.10.Ri Luminosities; magnitudes; effective temperatures, colors, and spectral classification
Issue 2 (2010 March 10)
Received 2009 November 30 , accepted for publication 2010 February 5
Published 2010 February 23
Gijs H. A. Roelofs et al 2010 ApJ 711 L138