On the [O I] λ6300 line emission from the photoevaporating circumstellar disks in the Orion Nebula

Several of the young stellar objects observed with the Hubble Space Telescope in the Orion Nebula near theta(1)C Ori show disklike structures with sizes 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 propose in this Letter that the [O I] emission is emerging from a H/H-2 photodissociation front that lies close to the disk surface. The H/H-2 front lies inside a photodissociation region between the disk surface and an ionization front that typically has a standoff distance of several disk radii from the disk surface. OH is produced efficiently at the warm H/H-2 front by the endothermic chemical reaction O + H-2 --> OH + H. However, OH is also efficiently destroyed by photodissociation caused by FUV photons. Approximately 50% of the photodissociated OH produces electronically excited atomic oxygen in the upper level of the 6300 Angstrom transition, which radiatively decays as intense [O I] lambda 6300 emission. Essentially, broadband FUV photons are absorbed by OH and efficiently converted to lambda 6300 line photons. The theoretically predicted [O I] lambda 6300 emission agrees well with that observed in 182-413 (HST -10), the best-studied object with a clearly resolved disk. The H/H-2 photodissociation front is close to the disk surface of 182-413, and the [O I] line, which peaks at the photodissociation front, thus traces the disk surface. The [O I] emission provides additional evidence in a number of proplyds for the existence of an extended PDR between the disk surface and the ionization front, and the penetration of OH-dissociating FUV photons from the ionization front to the disk surface.