---------------------

 Title: 

            [O I] emission in Herbig Ae/Be stars: signature of disk rotation

 ---------------------

 ---------------------

 Authors:

          Bram Acke, Katholieke Universiteit Leuven, Belgium

          Mario van den Ancker, ESO, Germany

          Kees Dullemond, Max-Planck-Institut f\

 ---------------------

 ---------------------

 Abstract:

          We have analyzed the optical spectra of 49 Herbig Ae/Be stars in search for the [O I] 6300A emission line. The vast majority of the detected features have narrow (FWHM < 100 km/s) profiles, centered around the stellar radial velocity or slightly blueshifted (centroid position down to -70 km/s). In our highest resolution spectra, a blue- and redshifted peak is observed, much alike the CO line profiles. Both peaks are equilly distant to the profile's centroid position. The peak-to-centroid distance is of the order of 5 km/s.

          Most of the sources with strong [O I] 6300A emission display a strong mid-infrared (20-100 micron) flux excess in their spectral energy distribution. According to the models of Dullemond (2002) and Dullemond & Dominik (2004), this excess reflects the flared geometry of the circumstellar disk.

          Both the double-peaked profile and the correlation between the [O I] emission strength and disk geometry indicate that a good candidate for the [O I] emission region are the upper layers of a rotating flared disk, where the stellar flux directly impinges on the disk surface and the photodestruction of OH molecules can lead to excited oxygen atoms in the upper level of the 6300A line. We have modelled the [O I] emission from this region in a flared disk. Assuming Keplerian rotation, we can reconstruct a double-peaked spectral profile that agrees well with the observed line profiles.

 ---------------------