CN EMISSION IN ORION - THE HIGH-DENSITY INTERFACE BETWEEN THE H-II REGION AND THE MOLECULAR CLOUD

We present high angular-resolution (12'' - 26'') large-scale mapping ( 19' x 22') of the Orion A molecular complex in the N = 1 --> 0 and N = 2 --> 1 rotational transitions of the CN radical. The CN emission is not only confined to the molecular ridge and the optical bar, but it r eveals filamentary emission toward the north and southwest of the Trap ezium cluster and surrounding M43. The morphology and the kinematics o f the CN emission supports the idea that the CN filaments represent th e interfaces between the molecular cloud and the major ionization fron ts of M 42 and M 43. The CN lines have been used to estimate the physi cal conditions of the molecular gas which confines the ionized materia l in M 42 and M 43. Surprisingly, the largest line intensity ratios be tween the N = 1 --> 0 and the N = 2 --> 1 lines are not observed towar d the molecular ridge or the ionization fronts, but towards the Trapez ium cluster and in the direction of the ionized pas in M 43. Model cal culations for the CN excitation implies a H-2 densities of similar to 10(5) cm(-3) toward the ridge and ionization fronts and > 6 x 10(6) cm (-3) toward the region surrounding the Trapezium stars. This suggests that for the first time we have detected the confining material behind M 42. We estimate that the CN emission arises from a thin (similar to 1.2 x 10(15) cm), dense (> 6 x 10(6) cm(-3)) layer which might have b een compressed by the expansion of the H II region. The morphology of the CN emission, dominated by the ionization fronts of the H II region s, indicates that this molecule is an excellent tracer of regions affe cted by UV radiation. The comparison between the HC3N and the CN abund ance shows dramatic changes (more than 4 orders of magnitude) between the different features observed in OMC 1. The [HC3N]/[CN] abundance ra tio varies from values of similar to 10(-3) for the ionization fronts surrounding the H II regions, to 100 for the hot core in Orion. Our da ta shows that the [HC3N]/[CN] abundance ratio is an excellent tracer o f photon dominated regions (PDRs) and hot cores within regions of mass ive star formation.