[O I] 6300 Å emission in Herbig Ae/Be systems: Signature of Keplerian rotation

We present high spectral-resolution optical spectra of 49 Herbig Ae/Be stars in a search for the [O i] 6300 Å line. The vast majority of the stars in our sample show narrow ( km s-1) emission lines, centered on the stellar radial velocity. In only three sources is the feature much broader (~400 km s-1), and strongly blueshifted (-200 km s-1) compared to the stellar radial velocity. Some stars in our sample show double-peaked line profiles, with peak-to-peak separations of ~10 km s-1. The presence and strength of the [O i] line emission appears to be correlated with the far-infrared energy distribution of each source: stars with a strong excess at 60 μm have in general stronger [O i] emission than stars with weaker 60 μm excesses. We interpret these narrow [O i] 6300 Å line profiles as arising in the surface layers of the protoplanetary disks surrounding Herbig Ae/Be stars. A simple model for [O i] 6300 Å line emission due to the photodissociation of OH molecules shows that our results are in quantitative agreement with that expected from the emission of a flared disk if the fractional OH abundance is ~.