On the Evolutionary Status of Class I Stars and Herbig-Haro Energy Sources in Taurus-Auriga

We present high resolution (R ∼ 34,000) optical (6330 8750 Å) spectra obtained with the HIRES spectrograph on the W. M. Keck I telescope of stars in Taurus-Auriga whose circumstellar environment suggests that they are less evolved than optically revealed T Tauri stars. Many of the stars are seen only via scattered light. The sample includes 15 Class I stars and all Class II stars that power Herbig-Haro flows in this region. For 28 of the 36 stars observed, our measurements are the first high dispersion optical spectra ever obtained. Photospheric features are observed in all stars with detected continuum, 11/15 Class I stars (42% of known Taurus Class I stars) and 21/21 Class II stars; strong emission lines (e.g. Hα) are detected in the spectra of all stars. These spectra, in combination with previous measurements, are used to search for differences between stars which power Herbig-Haro flows and stars which do not, and to reassess the evolutionary state of so-called protostars (Class I stars) relative to optically revealed T Tauri stars (Class II stars). The stellar mass distribution of Class I stars is similar to that of Class II stars and includes 3 spectroscopically confirmed Class I brown dwarfs. Class I stars (and brown dwarfs) in Taurus are slowly rotating (vsini < 35 km/s); the angular momentum of a young star appears to dissipate prior to the optically revealed T Tauri phase. The amount of optical veiling and the inferred mass accretion rates of Class I stars are surprisingly indistinguishable from Class II stars. Class I stars do not have accretion dominated luminosities; the accretion luminosity accounts for ∼ 25% of the bolometric luminosity. The median mass accretion rate of Class I and Class II stars of K7-M1 spectral type is 4 × 10 M⊙/yr and the median mass outflow rate is 5% of the mass accretion rate. The large ranges in mass accretion rate (∼ 2 orders of magnitude), mass outflow rate (∼ 3 orders of magnitude) and ratio of these quantities (∼ 2 orders of magnitude) represent real dispersions in young accreting stars of similar mass. We confirm previous results that find larger forbidden-line emission associated with Class I