Optical spectroscopy of embedded young stars in the Taurus-Auriga molecular cloud

This paper describes the first optical spectroscopic survey of Class I sources (also known as embedded sources and protostars) in the Taurus-Auriga dark cloud. We detect 10 of the 24 known Class I sources in the cloud at 5500-9000 Angstrom. All detected Class I sources have strong Ha emission; most also have strong [O I] and [S II] emission. These data - together with high-guality optical spectra of T Tauri stars in the Taurus-Auriga cloud - demonstrate that forbidden emission lines are stronger and more common in Class I sources than in T Tauri stars. Our results also provide a clear discriminant in the frequency of forbidden line emission between weak-emission and classical T Tauri stars. In addition to strong emission lines, three Class I sources have prominent TiO absorption bands. The M-type central stars of these sources mingle with optically visible T Tauri stars in the H-R diagram and lie somewhat below both the birth line for spherical accretion and the deuterium burning sequence for disk accretion.