Forbidden emission lines in Herbig Ae/Be stars ?

The absence of high velocity redshifted forbidden linesinclassicalT-Tauristars(Appenzelleretal.1984;Edwards et al. 1987) has long be taken as evidence of opaque circumstel- lar disks: disks which occlude the receding component of the stellar wind or outflow, allowing only the blueshifted emission to be observed. There has been some controversy in the liter- ature recently as to whether a disk model is appropriate to the higher mass counterparts of the T-Tauri stars: the Herbig Ae/Be stars. With this controversy in mind, and a search for such oc- cluding effects, we present part of a comprehensive study of 56 Herbig Ae/Be stars, 28 of which are observed to possess de- tectable (OI)6300 emission. It was found that those stars with (OI)6300 emission can be divided into four distinct groups as determined by line proles and velocities. Roughly 15% (4) of the sample show both high and low velocity blueshifted forbid- den emission lines reminiscent of the line proles of classical T-Tauri stars with extended outflows. Of the three remaining groups, the rst shows low velocity blueshifted emission with centroid velocities in the range -55 kms 1 vc -10 kms 1 (14 stars), the second unshifted (jvcj 5 kms 1 ) symmetrical forbidden emission lines (7 stars) and the third group of 3 stars low velocity (10 kms 1 vc 15 kms 1 ) redshifted emission. No Herbig Ae/Be star was found to possess strongly redshifted forbiddenlineemission.Thecleartendencytowardsblueshifted velocities not only implicitly suggests the presence of occlud- ing disks around these stars but there also appears to be a link between the degree of embeddedness and the amount of forbid- den line shift. An evolutionary effect may be responsible in the sense that, as the star becomes less enshrouded, the high veloc- ity (jet) component of the forbidden line emission disappears rst, followed by a decrease in the velocity of the low velocity component and nally by its disappearance altogether. The low velocity forbidden line emission is most likely a disk wind, the line prole being broadened as a result of the rotation of the disk. It is found that the line widths of the low velocity forbid- den line emission are broader than those found in the classical T-Tauri stars. There is also evidence of acceleration in the out- flow, traced by an increase in the blueshifted velocities from the (OI)6300 to the (SII)6717/6731 lines.