Observational Constraints on the Formation and Evolution of Binary Stars

We present a high spatial resolution UV to NIR survey of 44 young binarystars in Taurus with separations of 10-1000 AU. The primary results include:(1) The relative ages of binary star components are more similar than therelative ages of randomly paired single stars, supporting coeval formation. (2)Only one of the companion masses is substellar, and hence the apparentoverabundance of T Tauri star companions relative to main-sequence starcompanions can not be explained by a wealth of substellar secondaries thatwould have been missed in main-sequence surveys. (3) Roughly 10% of T Tauribinary star components have very red NIR colors (K-L > 1.4) and unusually highmass accretion rates. This phenomenon does not appear to be restricted tobinary systems, however, since a comparable fraction of single T Tauri starsexhibit the same properties. (4) Although the disk lifetimes of single starsare roughly equal to their stellar ages, the disk lifetimes of binary stars arean order of magnitude less than their ages. (5) The accretion rates for bothsingle and binary T Tauri stars appear to be moderately mass dependent. (6)Although most classical T Tauri star binaries retain both a circumprimary and acircumsecondary disk, there are several systems with only a circumprimary disk.Together with the relative accretion rates, this suggests that circumprimarydisks survive longer, on average, than circumsecondary disks. (7) The disklifetimes, mass ratios, and relative accretion signatures of the closestbinaries (10-100 AU) suggest that they are being replenished from acircumbinary reservoir with low angular momentum. Overall, these resultssupport fragmentation as the dominant binary star formation mechanism.Comment: 67 pages including 11 figures, LaTeX2e, accepted for publication in Ap