OPTICAL CHARACTERIZATION OF A NEW YOUNG STELLAR POPULATION IN THE SERPENS MOLECULAR CLOUD

We report on the results of an optical spectroscopic survey designed to confirm the youth and determine the spectral types among a sample of young stellar object (YSO) candidates in the Serpens molecular cloud. We observed 150 infrared (IR) excess objects previously discovered by the Spitzer Legacy Program From Molecular Cores to Planet-Forming Disks (c2d), bright enough for subsequent Spitzer/InfraRed Spectrograph (IRS) spectroscopy. We obtained 78 optical spectra of sufficient signal-to-noise for analysis. Extinctions, effective temperatures, and luminosities are estimated for this sample, and used to construct Hertzsprung-Russell (H-R) diagrams for the population. We identified 20 background giants contaminating the sample, based on their relatively high extinction, position in the H-R diagram, the lack of H alpha emission and relatively low IR excess. Such strong background contamination (25%) is consistent with the location of Serpens being close to the Galactic plane (5 degrees Galactic latitude). The remaining 58 stars (75%) were all confirmed to be young, mostly K- and M-type stars that are presumed to belong to the cloud. Individual ages and masses for the YSOs are inferred based on theoretical evolutionary models. The models indicate a spread in stellar ages from 1 Myr to 15 Myr, peaking at 2-6 Myr, and a mass distribution of 0.2-1.2M(circle dot) with median value around 0.8 M(circle dot). Strong H alpha emission lines (EW[H alpha] > 3 degrees angstrom) have been detected in more than half of the sample (35 stars). The mass accretion rates as derived from the Ha line widths span a broad distribution over four orders of magnitude with median accretion rate of 10(-8) M(circle dot) yr(-1). Our analysis shows that the majority of the IR excess objects detected in Serpens are actively accreting, young T-Tauri stars.