1994A&A...285..929H

We have carried out long-slit spectroscopy of the forbidden emission line region of the T Tauri star CW Tau with both medium (FWHM ≃ 50 km/s) and high spectral resolution (FWHM ≃ 20 km/s), and with a typical spatial resolution of 1.4"-1.8". From the spectra obtained at various slit positions we found that CW Tau has a bipolar outflow which can be traced over 4"- 6" along both outflow directions. The blueshifted flow is orientated at a position angle of about 160deg. The deduced high-resolution position-velocity diagrams of the [OI]λλ 6300, 6363, [NII]λ 6583 and the [SII]λλ 6716, 6731 lines show two velocity components which differ strongly in their spatial and spectroscopic properties (e.g. spatial extent along the outflow direction, degree of excitation and electron density). These results and similar ones from other T Tauri stars support the model of Kwan & Tademaru (1988) in which the two velocity components observed in the forbidden emission lines of T Tauri stars originate in two different flow components, namely in a jet and in a disk wind (and/or disk corona) in the case of the high-velocity and low-velocity component, respectively. All other models for the formation of the forbidden emission lines in T Tauri stars proposed so far encounter severe difficulties in explaining the observations. The model of Kwan and Tademaru can also explain qualitatively various new data on the low-velocity component, in particular its relatively large line width and high-velocity red wing. We suggest that rotation in the disk atmosphere or disk magnetosphere may explain these line profile data. Furthermore we show that the differences in the kinematical properties of the low-velocity component in various forbidden lines having different critical densities (e.g. [OI]λλ 6300, 6363, [SII]λλ 6716, 6731) might be due to acceleration effects in the presumed disk wind. We also briefly discuss whether the hypothesis of Kwan and Tademaru might be adopted to explain recently obtained [OI]λλ 6300, 6363 line profile data of Herbig Ae/Be stars.