THE MULTIPLICITY OF THE HYADES AND ITS IMPLICATIONS FOR BINARY STAR-FORMATION AND EVOLUTION

A 2.2 mu m speckle imaging survey of 167 bright (K < 8.5 mag) Hyades m embers reveals a total of 33 binaries with separations spanning 0 ''.0 44 to 1 ''.34 and magnitude differences as large as 5.5 mag. Of these binaries, 9 are new detections and an additional 20 are now spatially resolved spectroscopic binaries, providing a sample from which dynamic al masses and distances can be obtained. The closest three systems, ma rginally resolved at Palomar Observatory, were reobserved with the 10 m Keck Telescope in order to determine accurate binary star parameters . Combining the results of this survey with previous radial velocity, optical speckle, and direct-imaging Hyades surveys, the detected multi plicity of the sample is 98 singles, 59 binaries, and 10 triples. A st atistical analysis of this sample investigates a variety of multiple s tar formation and evolution theories. Over the binary separation range 0 ''.1-1 ''.07 (5-50 AU), the sensitivity to companion stars is relat ively uniform, with (Delta K-lim) = 4 mag, equivalent to a mass ratio (q(min)) = 0.23. Accounting for the inability to detect high flux rati o binaries results in an implied companion star fraction (CSF) of 0.30 +/- 0.06 in this separation range. The Hyades CSF is intermediate bet ween the values derived from observations of T Tauri stars (CSFTTS = 0 .40 +/- 0.08) and solar neighborhood G dwarfs (CSFSN = 0.14 +/- 0.03). This result allows for an evolution of the CSF from an initially high value for the pre-main sequence to that found for main-sequence stars . Within the Hyades, the CSF and the mass ratio distribution provide o bservational tests of binary formation mechanisms. The CSF is independ ent of the radial distance from the cluster center and the primary sta r mass. The distribution of mass ratios is best fitted by a power law q(-1.3+/-0.3) and shows no dependence on the primary mass, binary sepa ration, or radial distance from the cluster center. Overall, the Hyade s data are consistent with scale-free fragmentation, but inconsistent with capture and disk-assisted capture in small clusters. Without test able predictions, scale-dependent fragmentation and disk fragmentation cannot be assessed with the Hyades data.