Forbidden emission lines in Herbig Ae/Be stars

The absence of high velocity redshifted forbidden lines in classical T-Tauri stars (Appenzeller et al. 1984; Edwards et al. 1987) has long be taken as evidence of opaque circumstellar 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 literature 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 occluding effects, we present part of a comprehensive study of 56 Herbig Ae/Be stars, 28 of which are observed to possess detectable [OI]λ6300 emission. It was found that those stars with [OI]λ6300 emission can be divided into four distinct groups as determined by line profiles and velocities. Roughly 15% (4) of the sample show both high and low velocity blueshifted forbidden emission lines reminiscent of the line profiles of classical T-Tauri stars with extended outflows. Of the three remaining groups, the first shows low velocity blueshifted emission with centroid velocities in the range -55 kms−1 ≤ vc ≤ -10 kms−1 (14 stars), the second unshifted (|vc|≤ 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 forbidden line emission. The clear tendency towards blueshifted velocities not only implicitly suggests the presence of occluding disks around these stars but there also appears to be a link between the degree of embeddedness and the amount of forbidden line shift. An evolutionary effect may be responsible in the sense that, as the star becomes less enshrouded, the high velocity (jet) component of the forbidden line emission disappears first, followed by a decrease in the velocity of the low velocity component and finally by its disappearance altogether. The low velocity forbidden line emission is most likely a disk wind, the line profile being broadened as a result of the rotation of the disk. It is found that the line widths of the low velocity forbidden line emission are broader than those found in the classical T-Tauri stars. There is also evidence of acceleration in the outflow, traced by an increase in the blueshifted velocities from the [OI]λ6300 to the [SII]λλ6717/6731 lines. Send offprint requests to: M. Corcoran ? Based on observations made at the La Palma Observatory and the ESO/MPI 2.2m Telescope.