Observational Constraints on the Formation and Evolution of Binary Stars

We present a high spatial resolution multi-wavelength survey of 44 young binary star systems in Taurus-Auriga with separations of 10 1000 AU. These observations, which were obtained using the Hubble Space Telescope and the NASA Infrared Telescope Facility, quadruple the number of close (< 100 AU) binary stars with spatially resolved measurements from 0.3 to 2.2 μm and are the first 3.6 μmmeasurements for the majority of the companion stars in the sample. Masses and ages are estimated for the components observed at optical wavelengths. The relative ages of binary star components are more similar than the relative ages of randomly paired single stars within the same star forming region. This is the first statistically significant evidence for coeval formation. Only one of the companion masses is substellar, from which we conclude that the apparent overabundance of T Tauri star companions relative to main-sequence star companions is not due to a wealth of substellar secondaries that would have been missed in main-sequence surveys. The circumstellar environments of binary star systems are studied in this work through three diagnostics the infrared color K−L, the ultraviolet excess ∆U , and Hα emission. Several conclusions are drawn. First, the mass accretion rates for primary stars are similar to single stars, which suggests that companions as close as 10 AU have little effect on the mass accretion rate. Second, although most classical T Tauri star binaries retain both a circumprimary and a circumsecondary disk, there are several systems with only a circumprimary disk. Systems with only a circumsecondary disk are rare. This suggests that circumprimary disks survive longer than circumsecondary disks. Third, primary stars accrete at a higher rate, on average, than secondary stars. This is most likely because of their larger stellar mass, since the mass accretion rates for both single and binary T Tauri stars exhibit a moderate mass dependence. Fourth, approximately 10% of T Tauri binary star components have very red near-infrared colors (K − L > 1.4) and unusually high mass accretion rates. This phenomenon does not appear to be restricted to binary systems, however, since a comparable fraction of single T Tauri stars exhibit the same properties. These high accretion stars are probably not at an earlier stage of evolution, as has been proposed. Their semblance of younger protostars at optical and infrared wavelengths is most likely because of their similar Present Address: McDonald Observatory, R.L.M. Hall 15.308, Austin, TX 78712-1083