A High Angular Resolution Multiplicity Survey of the Open Clusters alpha Persei and Praesepe

Abstract

Two hundred and forty-two members of the Praesepe and alpha Persei clusters ranging from B (~5Msun) to early-M (~0.5 Msun) have been surveyed with high angular resolution imaging. The 39 binary and 1 quadruple systems detected encompass separations from 0".053 to 7".28; 28 of the systems are new detections and there are 9 candidate substellar companions. The main results from the survey are: * Over the projected separation range of 26-581 AU and magnitude differences of DeltaK < 4.0 mag, the companion star fraction (CSF) for alpha Persei is 0.09 +/- 0.03 and 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 on timescales of at 4 +1/-1.5 AU, significantly smaller value than both the field G-dwarf and the nearby T Tauri distributions. A simple population synthesis model suggests that the G-dwarf binary population is a combination of ~30% dark cloud and ~70% giant molecular cloud systems. * 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. * 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. * In alpha Persei, the rotational velocities for solar-type binaries with separations less that 60 AU the rotational evolution of young stars.