A New Brown Dwarf Desert A Scarcity of Wide Ultracool Binaries.pdf

a r X i v : a s t r o - p h / 0 6 1 0 7 6 3 v 1 2 5 O c t 2 0 0 6 February 5, 2008 A New Brown Dwarf Desert? A Scarcity of Wide Ultracool Binaries Peter R. Allen Pennsylvania State University, 525 Davey Lab, University Park PA 16802; pallen@ David W. Koerner Northern Arizona University, Dept. of Physics and Astronomy, PO Box 6010, Flagstaff, AZ 86011-6010; koerner@ Michael W. McElwain Department of Physics and Astronomy, UCLA, Los Angeles, CA 90095-1592; mcelwain@ Kelle L. Cruz American Museum of Natural History, Dept. of Astrophysics, Central Park West at 79th Street, New York, New York, 10024-5192; kelle@ I. Neill Reid Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; inr@ ABSTRACT We present the results of a deep-imaging search for wide companions to low- mass stars and brown dwarfs using NSFCam on IRTF. We searched a sample of 132 M7-L8 dwarfs to magnitude limits of J ～ 20.5 and K ～ 18.5, corresponding to secondary-primary mass ratios of ～ 0.5. No companions were found with separations between 2′′ to 31′′ (～40 AU to ～1000 AU). This null result implies a wide companion frequency below 2.3% at the 95% confidence level within the sensitivity limits of the survey. Preliminary modeling efforts indicate that we could have detected 85% of companions more massive than 0.05 M⊙ and 50% above 0.03 M⊙. Subject headings: stars: (low-mass, brown dwarfs, binaries: general) – 2 – 1. Introduction Low-mass stars and brown dwarfs are likely to be the most numerous constituents of the solar neighborhood. Over the last few years, several major surveys for these objects have been undertaken, aimed at measuring their numbers and discovering their origins. More than 350 L dwarfs and as many late-type M dwarfs have been discovered as a result of these projects (Delfosse et al. 1999; Kirkpatrick et al. 2000; Fan et al. 2000; Cruz et al. 2003; Phan-Bao et al. 2003). The current consensus is that the increasing number of objects per unit mass seen in high