Photodissociation and the CN:HCN ratio: observations of a `Third Bar' in OMC1

Details Photodissociation and the CN:HCN ratio: observations of a `Third Bar' in OMC1 Photodissociation and the CN:HCN ratio: observations of a `Third Bar' in OMC1

Dec 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996mnras.283.1179g&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 283, Issue 4, pp. 1179-1183.

Astronomy and Astrophysics

Astronomy

11

Ism: Abundances, Ism: Clouds, Ism: Individual: Omc1

Scientific paper

It has recently been proposed that the CN:HCN ratio in molecular clouds is a good tracer of regions affected by UV radiation. Observations of CN N=2-1 and HCN J=3-2 towards part of the OMC1 region support this hypothesis. The CN:HCN ratio is estimated at 1.02 +/- 0.04 in dense clumps, increasing to 2.1 +/- 0.5 in interclump gas. For much of the region, the average CN:HCN ratio is approximately 1.3, but it decreases to ~ 0.3-0.7 at a visual extinction of around 9 mag. These trends in CN:HCN can be explained in the chemical model of Fuente et al. The increased ratio in the diffuse interclump gas is due to rapid destruction of HCN in reactions with H^+ and C^+, while the ratio falls below 1 at moderate A_v where the CH and N needed to form CN are not abundant. The general characteristics of the region have also been studied, using a map of ^13CO J=2-1 emission, and an elongated ridge or `Third Bar' is found to the north-east of the Trapezium stars. This structure links the `Extended Ridge' and `Bright Bar', so that the stars are surrounded by gas on all sides. As in the Bright Bar, evidence is found in this new bar for shocks.

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