Substellar multiplicity in the Hyades cluster

Aims. We present the first high-angular resolution survey for mult iple systems among very low-mass stars and brown dwarfs in the Hyades open cluster. Methods. Using the Keck II adaptive optics system, we observed a complete sample of 16 objects with estimated masses. 0.1 M⊙. Results. We have identified three close binaries with projected separ ation. 0 ′′ . 11, or. 5 AU. A number of wide, mostly faint candidate companions are also detected in our images, most of which are revealed as unrelated background sources based on astrometric and/or photometric considerations. The derived multiplicity f requency, 19 +13 −6 % over the 2‐350 AU range, and the rarity of systems wider than 10 AU are both consistent with observations of fiel d very low-mass objects. In the limited 3‐50 AU separation range, the companion frequency is essentially constant from brown dwarfs to solar-type stars in the Hyades cluster, which is also i n line with our current knowledge for field stars. Combining the binarie s discovered in this surveys with those already known in the Pleiades cluster reveals that very low-mass binaries in open cluster s, as well as in star-forming regions, are skewed toward lower mass ratios (0.6. q. 0.8) than are their field counterparts, a result that cannot be a ccounted for by selection effects. Although the possibility of severe systematic errors in model-based mass estimates for very low-mass stars cannot be completely excluded, it is unlikely to explain this difference. Conclusions. We speculate that this trend indicates that surveys among very low-mass field stars may have missed a substantial population of intermediate mass ratio systems, implying that these systems are more common and more diverse than previously thought.