CARBON RECOMBINATION LINES AS A DIAGNOSTIC OF PHOTODISSOCIATION REGIONS

We have observed the C91alpha radio recombination line toward the Orio n H II region. This narrow (approximately 3-5 km s-1 FWHM) line is spa tially very extended (approximately 8' or 1 pc). These characteristics compare well with the observed characteristics of the C II fine struc ture line at 158 mum. Thus, the C91alpha line originates in the predom inantly neutral photodissociation regions separating the H II region f rom the molecular cloud. We have developed theoretical models for the C ii radio recombination lines from photodissociation regions. The res ults show that the I(C91alpha)/I(C158) intensity ratio is a sensitive function of the temperature and density of the emitting gas. We have a lso extended existing theoretical models for photodissociation regions to include the C II recombination lines. Comparison with these models show that, in the central portion of the Orion region, the C91alpha l ine originates in dense (10(6) cm-3), warm (500-1000 K) gas. Even at l arge projected distances (approximately 1 pc), the inferred density is still high (10(5) cm-3) and implies extremely high thermal pressures. As in the case of the [C II] 158 mum line, the large extent of the C9 1alpha line shows that FUV photons can penetrate to large distances fr om the illuminating source. The decline of the intensity of the incide nt radiation field with distance from THETA1 C seems to be dominated b y geometrical dilution, rather than dust extinction. Finally, we have used our models to calculate the intensity of the 9850 angstrom recomb ination line of C II. The physical conditions inferred from this line are in good agreement with those determined from the radio recombinati on and the far-infrared fine-structure lines. We show that the ratio o f the 9850 angstrom to the C91alpha line is a very good probe of very high density clumps.