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

Abstract

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 (FWHM < 100 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 60μm 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 ∼5 × 10 -7. © ESO 2005.