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Discovery of Two Very Low-Mass Binaries Final Results of an Adaptive Optics Survey of Nearb.pdf

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Discovery of Two Very Low-Mass Binaries Final Results of an Adaptive Optics Survey of Nearb

a r X i v : a s t r o - p h / 0 4 1 2 2 4 0 v 1 1 0 D e c 2 0 0 4 Discovery of Two Very Low-Mass Binaries: Final Results of an Adaptive Optics Survey of Nearby M6.0-M7.5 Stars Nick Siegler1, Laird M. Close1, Kelle L. Cruz2,3, Eduardo L. Mart??n4, I. Neill Reid2,5 ABSTRACT We present updated results of a high-resolution, magnitude limited (Ks 12mag) imaging survey of nearby low-mass M6.0-M7.5 field stars. The obser- vations were carried out using adaptive optics at the Gemini North, VLT, Keck II, and Subaru telescopes. Our sample of 36 stars consists predominantly of nearby (. 30 pc) field stars, 5 of which we have resolved as binaries. Two of the binary systems, 2MASSI J0429184-312356 and 2MASSI J1847034+552243, are presented here for the first time. All 5 discovered binary systems have separations between 0.′′08 ? 0.′′53 (2 - 9AU) with similar mass ratios (q 0.8, ?Ks 1mag). This result supports the hypothesis that wide (a 20AU) very low-mass (Mtot 0.19M⊙) binary systems are rare. The projected semimajor axis distribution of these systems peak at ～ 5AU and we report a sensitivity- corrected binary fraction of 9+4 ?3% for stars with primaries of spectral type M6.0- M7.5 with separations 3AU and mass ratios q 0.6. Within these instrumen- tal sensitivities, these results support the overall trend that both the semimajor axis distribution and binary fraction are a function of the mass of the primary star and decrease with decreasing primary mass. These observations provide important constraints for low-mass binary star formation theories. Subject headings: instrumentation: adaptive optics—binaries: general—stars: low mass—stars: individual () 1Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA 2Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104, USA 3American Museum of Natural History, Department of Astrophysics, Central Park West at 79th St., New York, NY, 10023 USA