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 of 15 class I stars (42% of known Taurus class I stars) and 21 of 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 that power Herbig-Haro flows and stars that 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 three spectroscopically confirmed class I brown dwarfs. Class I stars (and brown dwarfs) in Taurus are slowly rotating (v sin i < 35 km s^(-1)); 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^(-8) M_☉ yr^(-1), 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 the 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 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 of the obscured stellar photospheres, rather than being caused by larger mass outflow rates. Overall, the similar masses, luminosities, rotation rates, mass accretion rates, mass outflow rates, and millimeter flux densities of class I stars 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 much older than traditionally assumed. Similarly, although stars that power Herbig-Haro flows appear to have larger mass outflow rates, their stellar and circumstellar properties are generally indistinguishable from those of similar mass stars that do not power these flows