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The Solar Neighborhood. XXVIII. The Multiplicity Fraction of Nearby Stars from 5 to 70 AU and the Brown Dwarf Desert around M Dwarfs
We report on our analysis of Hubble Space Telescope/NICMOS snapshothigh-resolution images of 255 stars in 201 systems within ~10 pc of theSun. Photometry was obtained through filters F110W, F180M, F207M, andF222M using NICMOS Camera 2. These filters were selected to permit clearidentification of cool brown dwarfs through methane contrast imaging.With a plate scale of 76 mas pixel-1, NICMOS can easilyresolve binaries with subarcsecond separations in the19farcs5×19farcs5 field of view. We previously reported fivecompanions to nearby M and L dwarfs from this search. No new companionswere discovered during the second phase of data analysis presented here,confirming that stellar/substellar binaries are rare. We establishmagnitude and separation limits for which companions can be ruled outfor each star in the sample, and then perform a comprehensivesensitivity and completeness analysis for the subsample of 138 M dwarfsin 126 systems. We calculate a multiplicity fraction of0.0+3.5 -0.0% for L companions to M dwarfsin the separation range of 5-70 AU, and 2.3+5.0-0.7% for L and T companions to M dwarfs in theseparation range of 10-70 AU. We also discuss trends in thecolor-magnitude diagrams using various color combinations and presentastrometry for 19 multiple systems in our sample. Considering theseresults and results from several other studies, we argue that theso-called brown dwarf desert extends to binary systems with low-massprimaries and is largely independent of primary mass, mass ratio, andseparations. While focusing on companion properties, we discuss how thequalitative agreement between observed companion mass functions andinitial mass functions suggests that the paucity of brown dwarfs ineither population may be due to a common cause and not due to binaryformation mechanisms.
| Target selection for the SUNS and DEBRIS surveys for debris discs in the solar neighbourhood
Debris discs - analogous to the asteroid and Kuiper-Edgeworth belts inthe Solar system - have so far mostly been identified and studied inthermal emission shortward of 100?m. The Herschel space observatoryand the Submillimetre Common-User Bolometer Array-2 (SCUBA-2) camera onthe James Clerk Maxwell Telescope will allow efficient photometricsurveying at 70 to 850?m, which allows for the detection of coolerdiscs not yet discovered, and the measurement of disc masses andtemperatures when combined with shorter wavelength photometry. TheSCUBA-2 Unbiased Nearby Stars survey (SUNS) and the Disc Emission via aBias-free Reconnaissance in the Infrared/Submillimetre (DEBRIS) HerschelOpen Time Key Project are complementary legacy surveys observing samplesof ~500 nearby stellar systems. To maximize the legacy value of thesesurveys, great care has gone into the target selection process. Thispaper describes the target selection process and presents the targetlists of these two surveys.
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