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

September 2012 • 2012MNRAS.425..950N

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

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.

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Brigitta Sipőcz

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