1998A&A...329.1097R

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 reveals filamentary emission toward the north and southwest of the Trapezium cluster and surrounding M43. The morphology and the kinematics of the CN emission supports the idea that the CN filaments represent the interfaces between the molecular cloud and the major ionization fronts of M42 and M43. The CN lines have been used to estimate the physical conditions of the molecular gas which confines the ionized material in M42 and M43. Surprisingly, the largest line intensity ratios between the N=1->0 and the N=2->1 lines are not observed toward the molecular ridge or the ionization fronts, but towards the Trapezium cluster and in the direction of the ionized gas in M43. Model calculations for the CN excitation implies H₂ densities of ∼10⁵cm^–3 toward the ridge and ionization fronts and >6x10⁶cm^–3 toward the region surrounding the Trapezium stars. This suggests that for the first time we have detected the confining material behind M42. We estimate that the CN emission arises from a thin (∼1.2x10¹⁵cm), dense (>6x10⁶cm^–3) layer which might have been compressed by the expansion of the HII region. The morphology of the CN emission, dominated by the ionization fronts of the HII regions, indicates that this molecule is an excellent tracer of regions affected by UV radiation. The comparison between the HC₃N and the CN abundance shows dramatic changes (more than 4 orders of magnitude) between the different features observed in OMC1. The [HC₃N]/[CN] abundance ratio varies from values of ∼10^–3 for the ionization fronts surrounding the HII regions, to 100 for the hot core in Orion. Our data shows that the [HC₃N]/[CN] abundance ratio is an excellent tracer of photon dominated regions (PDRs) and hot cores within regions of massive star formation.