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

A survey of 30 nearby M6.0-M7.5 dwarfs with Ks<12 mag utilizing the Hokupa'aadaptive optics system at the Gemini North Telescope has discovered 3 newbinary systems. All 3 systems have separations between 0.12-0.29 arcsec (3-10AU) with similar mass ratios (q>0.8, Delta Ks<0.7). This result gives furthersupport to the suggestion that wide (a>20 AU) very low mass (M_{tot}<0.185M_sun) binary systems are exceedingly rare or perhaps even non-existant. Thesemimajor axis distribution of these systems peaks at ~5 AU, tighter than moremassive 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 of5_{-2}^{+4}% for M6.0-M7.5 stars with separations a>3 AU. This binary frequencyis less than the ~32% measured among early M dwarfs over the same separationrange. Two of the low-mass binaries are probable Hyades open cluster membersbased on proper motions, cluster membership probabilities, radial velocities,and near-IR photometry. LP415-20 has the distinction of being the tightest (3.6AU) multiple system ever spatially resolved in the cluster and the companionsof LP415-20 and LP475-855 are among the least massive objects ever resolved inthe Hyades with estimated masses of 0.081_{-0.010}^{+0.009} and0.082_{-0.009}^{+0.009} M_{\sun}.Comment: 24 pages, 9 figures, to appear in ApJ,v598, n2, December 1, 2003 issu