Young Binary Stars and Associated Disks

The typical product of the star formation process is a multiple star system, most commonly a binary star. Binaries have provided the first dynamical measures of the masses of pre-main-sequence (PMS) stars. These measurements have established that T Tauri-like stars are indeed of solar mass or less, and have provided preliminary support for the mass calibrations of theoretical PMS evolutionary tracks. Surprisingly, in some star-forming regions PMS binary frequencies have been found to be higher than among main-sequence solar-type stars. The binary frequency in the Taurus star-forming region is a factor two in excess of the field, although other regions show no excess. Observations suggest that the difference in PMS and main-sequence binary frequencies is not an evolutionary effect, and thus recent attention has focussed on correlations between binary frequency and initial conditions (e.g., stellar density or cloud temperatures). Accretion disks are common among young binary stars. Binaries with separations between 1 AU and 100 AU have substantially less submillimeter emission than closer or wider binaries, suggesting that such binaries have dynamically truncated their associated disks. Direct evidence of dynamical clearing has been seen in several binaries, most notably GG Tau. Remarkably, PMS binaries of all separations show evidence of long-lived circumstellar disks and continued accretion at stellar surfaces. This strongly suggests that the circumstellar disks are replenished from circumbinary disks or envelopes, perhaps through recently hypothesized accretion streams across dynamically cleared gaps. The frequent presence of either circumstellar or circumbinary disks suggests that planet formation can occur in binary environments. That planets may form around stars in wide binaries is already established by their discovery. Circumbinary disk masses around very short period binaries are ample to form planetary systems such as our own. The nature of planetary systems among the most frequent binaries, with separations between 10 AU and 100 AU, is less clear given the observed reduction in disk mass. However, even these