A high angular resolution multiplicity survey of the open clusters alpha Persei and Praesepe

Two hundred forty-two members of the Praesepe and alpha Persei clusters have been surveyed with high angular resolution 2.2 mum speckle imaging on the 3 m Infrared Telescope Facility, the 5 m Hale, and the 10 m Keck telescopes, along with direct imaging using the near-infrared camera (NICMOS) aboard the Hubble Space Telescope. The observed stars range in spectral type from B (similar to5 M-circle dot) to early M(similar to0.5 M-circle dot), with the majority of the targets more massive than similar to0.8 M-circle dot. The one quadruple and 39 binary systems detected encompass separations from 0.053 to 7.28; 28 of the systems are new detections, and there are nine candidate substellar companions. The results of the survey are used to test binary star formation and evolution scenarios and to investigate the effects of companion stars on X-ray emission and stellar rotation. The main results are as follows: 1. Over the projected separation range of 26 to 581 AU and magnitude differences of DeltaK < 4.0 (comparable to mass ratios q = M-sec/M-prim > 0.25), the companion-star fraction (CSF) for alpha Per is 0.09 +/- 0.03, and that for Praesepe is 0.10 +/- 0.03. This fraction is consistent with the field G dwarf value, implying that there is not a systematic decline in multiplicity with age at these separations on timescales of a few times 10(7) yr. The combination of previous spectroscopic work and the current cluster survey results in a cluster binary separation distribution that peaks at 4(-1.5)(+1) AU, a significantly smaller value than the peaks of both the field G dwarf and the nearby T Tauri distributions. If the field G dwarf distribution represents a superposition of distributions from the populations that contributed to the field, then the data imply that similar to30% of field binaries formed in dark clouds like the nearby T Tauri stars and the remaining similar to70% formed in denser regions. 2. An exploration of the binary star properties reveals a cluster CSF that increases with decreasing target mass, and a cluster mass ratio distribution that rises more sharply for higher mass stars but is independent of binary separation. These observational trends are consistent with several models of capture in small clusters and simulations of accretion following fragmentation in a cluster environment. Other types of capture and fragmentation are either inconsistent with these data or currently lack testable predictions. 3. Among the cluster A stars, there is a higher fraction of binaries in the subset with X-ray detections, consistent with the hypothesis that lower mass companions are the true source of X-ray emission. 4. Finally, in the younger cluster alpha Per, the rotational velocities for solar-type binaries with separations less than 60 AU are significantly higher than those of wider systems. This suggests that companions may critically affect the rotational evolution of young stars.