Forbidden Emission Lines in the Winds of Classical T Tauri Stars

Abstract

We present a model for the origin of the forbidden emission lines (FELs), especially [OI]λλ 6300, 6363 and [SII]λλ 6716, 6731, which are observed in the winds of classical T Tauri stars (CTTSs). This model is based on numerical calculations of the wind structure so that for the first time a physically motivated scenario of the FEL formation is given. Our calculations suggest that the low-velocity component (LVC) and the high-velocity component (HVC) of the double-peaked FELs are formed in spatially different regions of the young stellar outflow with the LVCs being formed near the star. The differences between their profiles can be explained by their different critical densities and the rotation properties of the wind in the formation region. The HVC of the lines is formed in a shock which occurs in the collimation region probably due to intersecting flow characteristics. Furthermore, the steepening of the field lines in that region leads to a deceleration of the wind with increasing distance from the star, as also indicated by observations.