Dispersed T Tauri stars and galactic star formation

Existing samples of low-mass T Tauri stars from nearby star forming regions are very deficient in stars older than 2 Myr. We argue that this arises from the dispersal of stars outside well-surveyed regions and is not due to a burst of star formation, erroneous theoretical isochrones, or survey flux limits. Evidence is accumulating, most dramatically from the ROSAT All-Sky Survey, that a large population of weak-lined T Tauri (WTT) stars is widely dispersed within and around star forming complexes. The spatial distribution, age distribution, and kinematics of T Tauri stars, both close to and widely distributed around active clouds, are studied using simple models of T Tauri dispersal. Models are compared to observations of the Chamaeleon and Taurus-Auriga cloud complexes. The dispersal of T Tauri stars appears to have two major causes: slow isotropic drifting of stars away from long-lived star forming clouds, and star formation in short-lived rapidly moving cloudlets. The first mechanism is determined by the Δν ≃ 1 km s^-1 thermal velocity dispersion of gas within molecular cloud cores. The second mechanism is determined by the large-scale turbulent motions of molecular cloud complexes. A third mechanism for dispersal, dynamical ejection of high-velocity T Tauri stars, appears to be less important. The results have a number of implications for star formation in the Galaxy: star formation in at least one cloud (Chamaeleon I) has been continuous for ≃20 Myr; star formation efficiencies of clouds may often be 20% or higher; a large fraction of low-mass stars may form in small short-lived cloudlets each producing no more than a few stars; and T Tauri kinematics support molecular evidence for large-scale turbulence in molecular clouds.