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Session 80 - Star Clusters in the Milky Way.
Display session, Friday, January 09
Exhibit Hall,

[80.09] The Multiplicity of the Hyades and its Implications for Binary Star Formation and Evolution

A 2.2 \mum speckle imaging survey of 167 bright (K < 8.5 mag.) Hyades members with the Hale 5m telescope reveals a total of 33 binaries with separations spanning 0\farcs05 to 1\farcs34 and magnitude differences as large as 5.5 Kmag. Of these binaries, 12 are new detections. A statistical analysis of this sample investigates a variety of multiple star formation and evolution theories. Over the binary separation range 0\farcs1 to 1\farcs07 (5 to 53 AU), the sensitivity to companion stars is fairly uniform with <\Delta K_lim>= 4 mag, equivalent to a mass ratio = 0.23. Accounting for the inability to detect high mass ratio binaries in this separation range results in an implied companion star fraction (csf) of 0.30 \pm 0.06. The Hyades csf is intermediate between the values derived from observations of T Tauri stars (csf_TTauri=0.40\pm0.08) and solar neighborhood G-dwarfs(csf_SN=0.14\pm0.03). 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 is independent of the radial distance from the cluster center. The distribution of mass ratios is best fit by a power law q^-1.3\pm0.3 and shows no dependence on the primary mass or the radial distance from the cluster center. Both the csf and the mass ratio distribution provide observational tests of binary formation mechanisms, and the Hyades data are inconsistent with scale-free fragmentation, capture in small clusters, and disk-assisted capture in small clusters. Without testable predictions, scale-dependent fragmentation and disk fragmentation cannot be ruled-out by the Hyades data.

Program listing for Friday