On the Evolutionary Status of Class I Stars and Herbig-Haro Energy   Sources in Taurus-Auriga
  [abridged] We present high resolution optical spectra 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. These spectra 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 Class I brown dwarfs. Class I stars in Taurus are slowly rotating; the angular momentum of a young star appears to dissipate prior to the optically revealed T Tauri phase. The mass accretion rates of Class I stars are surprisingly indistinguishable from those of Class II stars; they do not have accretion dominated luminosities. We confirm previous results that find larger forbidden-line emission associated with Class I stars than Class II stars. We suggest that this is caused by an orientation bias that allows a more direct view of the somewhat extended forbidden emission line regions than the obscured stellar photospheres, rather than because of larger mass outflow rates. Overall, the similar masses, luminosities, rotation rates, mass accretion rates, mass outflow rates, and millimeter flux densities of Class I and Class II stars are best explained by a scenario in which most Class I stars are no longer in the main accretion phase and are older than traditionally assumed. Similarly, although stars which power Herbig-Haro flows appear to have larger mass outflow rates, their stellar and circumstellar properties are generally indistinguishable from those of stars that do not power these flows. 
Russel White and Lynne Hillenbrand (Caltech)