Observational Constraints on the Formation and Evolution of Binary Stars

Abstract

We present a high spatial resolution multiwavelength 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 (less than 100 AU) binary stars with spatially resolved measurements from 0.3 to 2.2 μm and are the first 3.6 μm measurements 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 Δ 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 rate 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 high levels of accretion, which are above the norm for T Tauri stars, and not because of similar ages. The stellar and circumstellar properties are also used to trace indirectly the evolution of circumbinary material. In contrast to single T Tauri stars, which have disk dissipation timescales comparable to their ages, the disk dissipation timescales for binary T Tauri stars are ~10 times less than their ages. Replenishment of the inner circumstellar disks may be necessary to explain the continuing disk accretion in these systems. The longer disk lifetimes of circumprimary disks, despite their higher depletion rates, suggest that circumprimary disks are being preferentially replenished, possibly from a circumbinary reservoir with low angular momentum relative to the binary. Further support for circumbinary reservoirs comes from the observed correlated presence of circumprimary and circumsecondary disks for binaries with separations of less than ~200 AU. The presence of disks appears uncorrelated for wider binaries. Additionally, binaries with separations of less than ~100 AU exhibit a higher fraction of high mass ratio ( m s / m p ) pairs than wider binaries. These separation-dependent properties can be explained if the components are being replenished from a common circumbinary reservoir with low angular momentum. The components of the closest pairs are expected to be more equally replenished than the widest pairs, which consequently sustains both disks and drives their mass ratios toward unity. Overall, the results of this study corroborate previous work that suggests that fragmentation is the dominant binary star formation mechanism in Taurus-Auriga; disk instabilities and capture seem unlikely.