Journal Article

Four ultra-short-period eclipsing M-dwarf binaries in the WFCAM Transit Survey

S. V. Nefs, J. L. Birkby, I. A. G. Snellen, S. T. Hodgkin, D. J. Pinfield, B. Sipőcz, G. Kovacs, D. Mislis, R. P. Saglia, J. Koppenhoefer, P. Cruz, D. Barrado, E. L. Martin, N. Goulding, H. Stoev, J. Zendejas, C. del Burgo, M. Cappetta and Y. V. Pavlenko

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 425, issue 2, pages 950-968
Published in print September 2012 | ISSN: 0035-8711
Published online September 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.21338.x
Four ultra-short-period eclipsing M-dwarf binaries in the WFCAM Transit Survey

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Abstract

We report on the discovery of four ultra-short-period (P ≤ 0.18 d) eclipsing M-dwarf binaries in the Wide-Field Camera (WFCAM) Transit Survey. Their orbital periods are significantly shorter than that of any other known main-sequence binary system, and are all significantly below the sharp period cut-off at P ∼ 0.22 d as seen in binaries of earlier-type stars. The shortest-period binary consists of two M4-type stars in a P = 0.112 d orbit. The binaries are discovered as part of an extensive search for short-period eclipsing systems in over 260 000 stellar light curves, including over 10 000 M-dwarfs down to J = 18 mag, yielding 25 binaries with P ≤ 0.23 d. In a popular paradigm, the evolution of short-period binaries of cool main-sequence stars is driven by the loss of angular momentum through magnetized winds. In this scheme, the observed P ∼ 0.22 d period cut-off is explained as being due to time-scales that are too long for lower-mass binaries to decay into tighter orbits. Our discovery of low-mass binaries with significantly shorter orbits implies that either these time-scales have been overestimated for M-dwarfs, e.g. due to a higher effective magnetic activity, or the mechanism for forming these tight M-dwarf binaries is different from that of earlier-type main-sequence stars.

Keywords: binaries: eclipsing; stars: low-mass

Journal Article.  9482 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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