Optical Spectroscopy of Embedded Young Stars in the Taurus-Auriga Molecular Cloud

This paper describes the first optical spectroscopic survey of class Isources (also known as embedded sources and protostars) in the Taurus-Aurigadark cloud. We detect 10 of the 24 known class I sources in the cloud at5500-9000 A. All detected class I sources have strong H-alpha emission; mostalso have strong [O I] and [S II] emission. These data - together with highquality optical spectra of T Tauri stars in the Taurus-Auriga cloud -demonstrate that forbidden emission lines are stronger and more common in classI sources than in T Tauri stars. Our results also provide a clear discriminantin the frequency of forbidden line emission between weak-emission and classicalT Tauri stars. In addition to strong emission lines, three class I sources haveprominent TiO absorption bands. The M-type central stars of these sourcesmingle with optically visible T Tauri stars in the HR diagram and lie somewhatbelow both the birthline for spherical accretion and the deuterium burningsequence for disc accretion.Comment: 19 pages of text, 4 pages of tables, and 10 pages of figure