On the [O I] lambda 6300 line emission from the photoevaporating circumstellar disks in the Orion Nebula

Several of the young stellar objects observed with the Hubble Space Telesco pe in the Orion Nebula near theta(1)C Ori show disklike structures with siz es r similar to 100 AU, similar to our own planetary system. These disklike shapes appear as dark silhouettes in [O III], [S II], [N II], H alpha, and the continuum but are seen in emission in the [O I] lambda 6300 line. We p ropose in this Letter that the [O I] emission is emerging from a H/H-2 phot odissociation front that lies close to the disk surface. The H/H-2 front li es inside a photodissociation region between the disk surface and an ioniza tion front that typically has a standoff distance of several disk radii fro m the disk surface. OH is produced efficiently at the warm H/H-2 front by t he endothermic chemical reaction O + H-2 --> OH + H. However, OH is also ef ficiently destroyed by photodissociation caused by FUV photons. Approximate ly 50% of the photodissociated OH produces electronically excited atomic ox ygen in the upper level of the 6300 Angstrom transition, which radiatively decays as intense [O I] lambda 6300 emission. Essentially, broadband FUV ph otons are absorbed by OH and efficiently converted to lambda 6300 line phot ons. The theoretically predicted [O I] lambda 6300 emission agrees well wit h that observed in 182-413 (HST -10), the best-studied object with a clearl y resolved disk. The H/H-2 photodissociation front is close to the disk sur face of 182-413, and the [O I] line, which peaks at the photodissociation f ront, thus traces the disk surface. The [O I] emission provides additional evidence in a number of proplyds for the existence of an extended PDR betwe en the disk surface and the ionization front, and the penetration of OH-dis sociating FUV photons from the ionization front to the disk surface.