Carbon recombination lines as a diagnostic of photodissociation regions

Abstract

We have observed the C91 alpha radio recombination line toward theOrion H II region. This narrow (approximately 3-5 km per sec fullwidth at half maximum (FWHM)) line is spatially very extended (approximately8 arcmin or 1 pc). These charateristics compare well with the observedcharacteristics of the C II fine structure line at 158 microns. Thus,the C91 alpha line originates in the predominantly neutral photodissociationregions separating the H II region from the molecular cloud. We havedeveloped theoretical models for the C II radio recombination linesfrom photodissociation regions. The results show that the I(C91 alpha)/I(C158)intensity ratio is a sensitive function of the temperature and densityof the emitting gas. We have also extended theoretical models forphotodissociation regions to include the C II recombination lines.Comparison with these models show that, in the central portion ofthe Orion region, the C91 alpha line originates in dense (10^{6}per cu cm), warm (500-1000 K) gas. Even at large projected distances(approximately 1 pc), the inferred density is still high (10^{5}per cu cm) and implies extremely high thermal pressures. As in thecase of the (C II) 158 microns line, the large extent of the C91alpha line shows that (FUV) photons can penetrate to large distancesfrom the illuminating source. The decline of the intensity of theincident radiation field with distance from Theta^{1} C seems tobe dominated by geometrical dilution, rather than dust extinction.Finally, we have used our models to calculate the intensity of the9850 A recombination line of C II. The physical conditions inferredfrom this line are in good agreement with those determined from theradio recombination and the far-infrared fine-structure lines. Weshow that the ratio of the 9850 A to the C91 alpha lines is a verygood probe of very high density clumps.