Emission-Line Studies of Young Stars. IV. The Optical Forbidden Lines

Optical forbidden line strengths and profiles are discussed for a sample of 30 T Tauri stars and 12 Herbig Ae-Be stars. Transitions of (C I), (N II), (O I), (O II), (S II), (Ca II), (Cr II), (Fe II), and (Ni II) are detected. Profile variability occurred in DG Tau and probably other sources. The ensemble profiles can be divided into four generic components that may represent distinct emitting regions; (1) narrow rest-velocity lines, (2) 'low'-velocity lines (peaking at less than or approximately +/- 50 km s^-1), (3) 'high'-velocity (usually greater than or approximately +/- 100 km s^-1) blueshifted peaks or wings, and (4) high-velocity redshifted peaks. Among T Tauri stars, the rest-velocity lines appear most often in sources with weak and narrow permitted lines, such as the Ca II triplet. The low- and high-velocity blueshifted components usually appear together in sources with strong and broad Ca II triplet lines. If the velocity-shifted lines form in jets, the smallest (full) opening angles required by the profiles are less than or approximately 20 deg for the narrow, blueshifted (Ca II) lines of DG Tau and HL Tau. Other lines in DG Tau are much broader, implying larger opening angles or greater velocity dispersions. The variability in DG Tau also implies significant changes in the collimation or velocity coherence on timescales of a few years. RW Aur and AS 353A have blue- and redshifted line peaks that could form in oppositely directed jets. The strong (S II) lambda 6716 and lambda 6731 lines in RW Aur are exclusively redshifted and require opening angles less than or approximately 60 deg. Measurements of different profiles in the same spectrum show that the physical conditions change with the line-of-sight velocities. The most persistent trends are for more (N II) and (O II) and less (O I) lambda 5577 flux at high velocities. Constraints on the physical conditions are derived by modeling the emission lines via multilevel ions in 'coronal ionization equilibrium.' A single temperature and density cannot fully describe the line spectra in any velocity interval. Temperatures in the (O I) region are 9000 less than or approximately T_e less than 14,000 K, and the ionization fraction (of H) is less than 35%. The densities derived from (O I) include n_e less than or approximately 5 x 10⁵ to approximately 10⁷ cm^-3, but n_e greater than or approximately 10⁶ cm^-3 obtains only at low velocities. In the (S II) regions the densities are lower, 10³ less than or approximately n_e less than or approximately 7 x 10⁴ cm^-3, and the temperatures are probably higher, T_e greater than or approximately 13,000 K. At high velocities (only) there is additional hot gas that produces (N II) and (O II), possibly most of the (S II), and little (O I). This region is characterized by T_e greater than or approximately 15,000 K, n_e less than or approximately 10⁵ cm^-3, and an ionization fraction greater than or approximately 50%. <When combined with the spatially segregated emitting regions observed by others by spectral imaging, these results suggest decreasing n_e and increasing T_e away from the star in at least the high velocity gas.