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 inTaurus-Auriga whose circumstellar environment suggests that they are lessevolved than optically revealed T Tauri stars. Many of the stars are seen onlyvia scattered light. These spectra are used to search for differences betweenstars which power Herbig-Haro flows and stars which do not, and to reassess theevolutionary state of so-called protostars (Class I stars) relative tooptically revealed T Tauri stars (Class II stars). The stellar massdistribution of Class I stars is similar to that of Class II stars and includes3 Class I brown dwarfs. Class I stars in Taurus are slowly rotating; theangular momentum of a young star appears to dissipate prior to the opticallyrevealed T Tauri phase. The mass accretion rates of Class I stars aresurprisingly indistinguishable from those of Class II stars; they do not haveaccretion dominated luminosities. We confirm previous results that find largerforbidden-line emission associated with Class I stars than Class II stars. Wesuggest that this is caused by an orientation bias that allows a more directview of the somewhat extended forbidden emission line regions than the obscuredstellar photospheres, rather than because of larger mass outflow rates.Overall, the similar masses, luminosities, rotation rates, mass accretionrates, mass outflow rates, and millimeter flux densities of Class I and ClassII stars are best explained by a scenario in which most Class I stars are nolonger in the main accretion phase and are older than traditionally assumed.Similarly, although stars which power Herbig-Haro flows appear to have largermass outflow rates, their stellar and circumstellar properties are generallyindistinguishable from those of stars that do not power these flows.Comment: 84 pages, including 21 figures; accepted for publication in Ap