Emission-line studies of young stars. 4: The optical forbidden lines

Abstract

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 n], [O i], [On], [S n], [Ca n], [Cr n], [Fe n], and [Ni n] 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 ^ ±50 km s-1), (3) "high"-velocity (usually ^ ± 100 km s-1) blueshifted peaks or wings, and (4) high-velocity redshifted peaks. Among T Tauri stars, the rest-velocity fines appear most often in sources with weak and narrow permitted fines, such as the Ca n triplet. The low-and high-velocity blueshifted components usually appear together in sources with strong and broad Ca n triplet fines. If the velocity-shifted fines form in jets, the smallest (full) opening angles required by the profiles are ^20° for the narrow, blueshifted [Ca n] fines of DG Tau and HL Tau. Other fines 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 fine peaks that could form in oppositely directed jets. The strong [S il] X6716 and X6731 fines in RW Aur are exclusively redshifted and require opening angles ^60°. 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 n] and [O n] and less [O i] X5577 flux at high velocities. Constraints on the physical conditions are derived by modeling the emission fines via multilevel ions in "coronal ionization equilibrium." A single temperature and density cannot fully describe the fine spectra in any velocity interval. Temperatures in the [O i] region are 9000 ^T e < 14,000 K, and the ionization fraction (of H) is <35%. The densities derived from [O i] include n e ^5x 10 5 to ~ 10 7 cm-3 , but n e ^ 10 6 cm-3 obtains only at low velocities. In the [S n] regions the densities are lower, 10 3 ^n e ^l X 10 4 cm-3 , and the temperatures are probably higher, T e