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J. Patience, A. M. Ghez (UCLA), I. N. Reid (University of Pennsylvania)
In order to test binary star formation and evolution scenarios, we have conducted a high angular resolution multiplicity survey of 410 members of 3 young open clusters - the Hyades, \alpha Persei, and Praesepe - using 2.2 \mum speckle imaging on the IRTF 3-m, the Hale 5-m, and the Keck 10-m along with direct imaging with NICMOS aboard HST. The 62 detected binaries encompass separations from 0".027 to ~5'' and flux ratios up to 40; 22 are newly resolved. Over the limited separation range directly studied, the measured companion star fraction (CSF5-50AU) for the Hyades (0.16 ±0.03) is intermediate between the younger T Tauri stars and older G-dwarfs, but the CSF25-561AU of \alpha Persei (0.10 ±0.03) and Praesepe (0.10 ±0.03) are both consistent with the G-dwarf value, suggesting that there is not a systematic decline in multiplicity with age. The higher CSF of the Hyades is explained by the overall separation distribution of cluster binaries. Merging the current cluster surveys with previous spectroscopic work results in a cluster binary distribution that peaks at 5 AU, a significantly smaller value than the peaks of both the G-dwarf (~30AU) and T Tauri (~60AU) distributions. If the field G-dwarf distribution represents a superposition of distributions from the populations which contributed to the field, then the data implies that ~ 33% of field binaries formed in dark clouds like the nearest T Tauri stars and the remaining ~67% formed in the dense progenitors of open clusters. An exploration of the binary star properties reveals a cluster CSF that increases with decreasing target mass and a cluster mass ratio distribution power law that has a shallower slope for lower mass stars. These observational trends are inconsistent with several binary formation models involving capture, fragmentation, and accretion. Finally, in the youngest cluster - \alpha Persei - the rotational velocities for binaries with separations less than 55 AU are significantly higher than that of wider systems. This suggests that companions may critically affect the rotational evolution of young stars.
Support for this work was provided by grant no. NAG5-6975 through the Origins of Solar Systems Project, no. GO-07833.01-96A from the Space Telescope Science Institute, and a dissertation year fellowship from the American Association of University Women.