AN IRAS SURVEY OF THE TAURUS-AURIGA MOLECULAR CLOUD

We describe a search for young pre-main-sequence stars not previously associated with molecular cloud cores in the Taurus-Auriga region using data collected with the Infrared Astronomical Satellite (IRAS). Near-infrared photometry and optical spectroscopy indicate that many of these objects are young stars; the six new embedded sources in this sample have luminosities comparable to that of the average T Tauri star. These data suggest that surveys for pre-main-sequence stars in the Taurus-Auriga cloud are essentially complete for luminosities L≳0.5L⊙. The relative numbers of embedded objects and T Tauri stars indicate that the duration of the embedded phase is τemb ∼ 1.2 × 105 yr if the age of a typical T Tauri star is τTTS ∼ 106 yr. However, embedded objects generally are not more luminous than classical T Tauri stars in Taurus-Auriga; the derived bolometric luminosities imply script M sign ∼ 10-7 script M sign⊙ yr-1 if the observed flux is produced solely by accretion onto a stellar-sized object. The apparent disagreement between the accretion rates inferred from τemb and Lbol can be reconciled if a star accretes most of its mass in a time that is short compared to τemb or if the ages of T Tauri stars have been underestimated. If either hypothesis is correct, the mass accretion rates of embedded pre-main-sequence stars are not simply related to the infall rates from their parent clouds, probably because infall occurs first to circumstellar disks.