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Astron. Astrophys. 329, 1097-1110 (1998)

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CN emission in Orion. The high density interface
between the H II region and the molecular cloud

A. Rodríguez-Franco 1, 2, J. Martín-Pintado 2 and A. Fuente 2

1 Departamento de Matemática Aplicada II, Sección departamental de Optica, Escuela Universitaria de Optica,
Universidad Complutense de Madrid, Av. Arcos de Jalón s/n, E-28037 Madrid, Spain
2 Observatorio Astronómico Nacional (IGN), Campus Universitario, Apdo. 1143, E-28800 Alcalá de Henares, Spain

Received 11 February 1997 / Accepted 24 July 1997


We present high angular-resolution ([FORMULA]) large-scale mapping ([FORMULA]) of the Orion A molecular complex in the [FORMULA] and [FORMULA] 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 M 43. 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 M [FORMULA] and M [FORMULA].

The CN lines have been used to estimate the physical conditions of the molecular gas which confines the ionized material in M [FORMULA] and M [FORMULA]. Surprisingly, the largest line intensity ratios between the [FORMULA] and the [FORMULA] 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 M [FORMULA]. Model calculations for the CN excitation implies H2 densities of [FORMULA] cm-3 toward the ridge and ionization fronts and [FORMULA] cm-3 toward the region surrounding the Trapezium stars. This suggests that for the first time we have detected the confining material behind M [FORMULA]. We estimate that the CN emission arises from a thin ([FORMULA] cm), dense ([FORMULA] 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 OMC [FORMULA]. The [HC3N]/[CN] abundance ratio varies from values of [FORMULA] 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.

Key words: ISM: clouds – ISM: Orion A; Orion clouds – ISM: molecules – radiolines: ISM

Send offprint requests to: A. Rodríguez-Franco to the IGN address

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997