2002AJ....123.1570P

Over the projected separation range of 26 to 581 AU and magnitude differences of ΔK<4.0 (comparable to mass ratios q=M_sec/M_prim>0.25), the companion-star fraction (CSF) for α 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 10⁷ yr. The combination of previous spectroscopic work and the current cluster survey results in a cluster binary separation distribution that peaks at 4⁺¹_–1.5 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 ∼30% of field binaries formed in dark clouds like the nearby T Tauri stars and the remaining ∼70% formed in denser regions.

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.

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.

Finally, in the younger cluster α 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.