Low-mass Binaries in the Hyades - A scarcity of brown dwarfs

Abstract

We have obtained HST Planetary Camera observations of a total of fifty-three low-mass (M < 0.3 M_solar) 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 plus or minus 4.6% for systems with separations in the range 14 to 825 a.u., consistent with observations of solar neighbourhood M-dwarfs. The mass-ratio distribution is only consistent marginally with the secondary stars being drawn from a Psi(M) proportioal to M^{-1} mass function, with three equal-mass systems amongst the six binaries with observed separations in the regime 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 Myrs, our results are consistent only at the 2 sigma level with a flat (M^0) mass function.