Low-Mass Binaries in the Hyades: A Scarcity of Brown Dwarfs

Abstract

We have obtained HST Planetary Carnera observations of a total of 53 low-mass (M<0.3M_sun) members of the Hyades cluster. Nine of these stars are resolved as binaries, with separations between 0.1 and 3.1 arcseconds, while a further three are probably equal-mass systems at smaller separations. Allowing for observational selection effects, this corresponds to an observed binary fraction of 11.3%±4.6% for systems with separations in the range 14 to 825 a.u., consistent with observations of solar neighborhood M-dwarfs. The mass-ratio distribution is only consistent marginally with the secondary stars being drawn from a Ψ(M)αM^-1 mass function, with three equal-mass systems amongst the six binaries with observed separations in the régime where our observations extend below the hydrogen-burning limit. Considering the entire sample, the absence of any brown dwarf companions amongst our sample makes it unlikely that the mass function of stellar/brown dwarf companions rises as steeply as M^-1. If the Hyades has an age of ∼600 Myr, our results are consistent only at the 2α level with a flat (M⁰) mass function.