Three New Eclipsing White-dwarf-M-dwarf Binaries Discovered in a Search for Transiting Planets around M-dwarfs

Creators: Law, Nicholas M.; Hillenbrand, Lynne A.; Baranec, Christoph; Davis, Jack T. C.; Dekany, Richard G.; Kulkarni, S. R.; Ofek, Eran O.; Quimby, Robert M.; Riddle, Reed; Tendulkar, Shriharsh P.; Bui, Khanh; Fulton, Benjamin J.

Abstract

We present three new eclipsing white-dwarf/M-dwarf binary systems discovered during a search for transiting planets around M-dwarfs. Unlike most known eclipsing systems of this type, the optical and infrared emission is dominated by the M-dwarf components, and the systems have optical colors and discovery light curves consistent with being Jupiter-radius transiting planets around early M-dwarfs. We detail the PTF/M-dwarf transiting planet survey, part of the Palomar Transient Factory (PTF). We present a graphics processing unit (GPU)-based box-least-squares search for transits that runs approximately 8 × faster than similar algorithms implemented on general purpose systems. For the discovered systems, we decompose low-resolution spectra of the systems into white-dwarf and M-dwarf components, and use radial velocity measurements and cooling models to estimate masses and radii for the white dwarfs. The systems are compact, with periods between 0.35 and 0.45 days and semimajor axes of approximately 2 R_☉ (0.01 AU). The M-dwarfs have masses of approximately 0.35 M_☉, and the white dwarfs have hydrogen-rich atmospheres with temperatures of around 8000 K and have masses of approximately 0.5 M_☉. We use the Robo-AO laser guide star adaptive optics system to tentatively identify one of the objects as a triple system. We also use high-cadence photometry to put an upper limit on the white-dwarf radius of 0.025 R_☉ (95% confidence) in one of the systems. Accounting for our detection efficiency and geometric factors, we estimate that 0.08%^(+0.10%)_(-0.05%) (90% confidence) of M-dwarfs are in these short-period, post-common-envelope white-dwarf/M-dwarf binaries where the optical light is dominated by the M-dwarf. The lack of detections at shorter periods, despite near-100% detection efficiency for such systems, suggests that binaries including these relatively low-temperature white dwarfs are preferentially found at relatively large orbital radii. Similar eclipsing binary systems can have arbitrarily small eclipse depths in red bands and generate plausible small-planet-transit light curves. As such, these systems are a source of false positives for M-dwarf transiting planet searches. We present several ways to rapidly distinguish these binaries from transiting planet systems.

Additional Information

© 2012 American Astronomical Society. Received 2011 December 2; accepted 2012 July 30; published 2012 September 11. We thank Michael Kandrashoff, Jieun Choi, and Peter Blanchard for observations at Lick Observatory, and thank the anonymous referee for their constructive suggestions and swift report. Observations were obtained with the Samuel Oschin Telescope and the 60 inch telescope at the Palomar Observatory as part of the Palomar Transient Factory project, a scientific collaboration between the California Institute of Technology, Columbia University, Las Cumbres Observatory, the Lawrence Berkeley National Laboratory, the National Energy Research Scientific Computing Center, the University of Oxford, and the Weizmann Institute of Science. Some of the data presented herein were obtained at the W.M.Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA; the observatory was made possible by the generous financial support of the W. M. Keck Foundation. We recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community; we are most fortunate to have the opportunity to conduct observations from this mountain. The Byrne Observatory at Sedgwick (BOS) is operated by the Las Cumbres Observatory Global Telescope Network and is located at the Sedgwick Reserve, a part of the University of California Natural Reserve System. The Robo-AO system is supported by collaborating partner institutions, the California Institute of Technology, and the Inter-University Centre for Astronomy and Astrophysics, and by the National Science Foundation under Grant Nos. AST-0906060 and AST-0960343. This paper uses observations obtained with the FTN observatory of the Las Cumbres Observatory Global Telescope. This research has also made use of the SIMBAD database, operated at CDS, Strasbourg, France. The radial velocity analysis in this paper used two existing data analysis pipelines, MAKEE by Tom Barlow and BFall by Slavek Rucinski; we gratefully acknowledge their contribution to the field in developing and supporting this software. N.M.L. is supported by a Dunlap Fellowship at University of Toronto. A.L.K. was supported by NASA through Hubble Fellowship Grant 51257.01 awarded by STScI, which is operated by AURA, Inc., for ANSA, under contract NAS 5-26555. M.M.K. acknowledges support from the Hubble Fellowship and Carnegie-Princeton Fellowship. A.V.F. and his group at UC Berkeley acknowledge generous financial assistance from Gary and Cynthia Bengier, the Richard & Rhoda Goldman Fund, the TABASGO Foundation, and NSF Grant AST-0908886. Facilities: PO:1.2m, Keck:I, LCOGT

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