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 th e cloud at 5500-9000 Angstrom. All detected Class I sources have stron g 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 ar e stronger and more common in Class I sources than in T Tauri stars. O ur results also provide a clear discriminant in the frequency of forbi dden line emission between weak-emission and classical T Tauri stars. In addition to strong emission lines, three Class I sources have promi nent TiO absorption bands. The M-type central stars of these sources m ingle with optically visible T Tauri stars in the H-R diagram and lie somewhat below both the birth line for spherical accretion and the deu terium burning sequence for disk accretion.