ASTRONOMY AND ASTROPHYSICS 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′× 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 ∼ 105 cm−3 toward the ridge and ionization fronts and > 6 × 106 cm−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 (∼ 1.2 × 1015 cm), dense (> 6 × 106 cm−3) layer which might have been compressed by the expansion of the H II region. The morphology of the CN emission, dominated by the ionization fronts of the H II 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 H II 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. Send offprint requests to: A. Rodrı́guez-Franco to the IGN address