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An Adaptive Optics Survey of M6.0-M7.5 Stars: Discovery of Three Very Low Mass Binary Systems Including Two Probable Hyades Members

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Nick Siegler1, Laird M. Close1, Eric E. Mamajek1 and Melanie Freed1

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A survey of 30 nearby M6.0-M7.5 dwarfs with Ks < 12 mag utilizing the Hokupa`a adaptive optics system at the Gemini North Telescope has discovered three new binary systems. All three systems have separations between 0farcs12 and 0farcs29 (3-10 AU) with similar mass ratios (q > 0.8, ΔKs < 0.7). This result gives further support to the suggestion that wide (a > 20 AU), very low mass (Mtot < 0.185 Msun) binary systems are exceedingly rare or perhaps even nonexistent. The semimajor axis distribution of these systems peaks at ~5 AU, tighter than more massive M and G binary distributions, which have a broad peak at separations of ~30 AU. We find a sensitivity-corrected binary fraction in the range 5img1.gif% for M6.0-M7.5 stars with separations a > 3 AU. This binary frequency is less than the ~32% measured among early M dwarfs over the same separation range. Two of the low-mass binaries are probable Hyades open cluster members based on proper motions, cluster membership probabilities, radial velocities, and near-IR photometry. LP 415-20 has the distinction of being the tightest (3.6 AU) multiple system ever spatially resolved in the cluster, and the companions of LP 415-20 and LP 475-855 are among the least massive objects ever resolved in the Hyades, with estimated masses of 0.081img2.gif and 0.082img3.gif Msun.


Subject headings

binaries: general; instrumentation: adaptive optics; open clusters and associations: individual (Hyades); stars: individual (LP 415-20, LP 475-855, 2MASSW J1750129+442404); stars: low-mass, brown dwarfs


Dates

Issue 2 (2003 December 1)

Received 2003 January 9, accepted for publication 2003 August 4



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