PHOTODISSOCIATION AND THE CN HCN RATIO - OBSERVATIONS OF A 3RD BAR INOMC1/

It has recently been proposed that the CN:HCN ratio in molecular cloud s is a good tracer of regions affected by UV radiation. Observations o f 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 cl umps, increasing to 2.1 +/- 0.5 in interclump gas. For much of the reg ion, the average CN:HCN ratio is approximately 1.3, but it decreases t o similar to 0.3-0.7 at a visual extinction of around 9 mag. These tre nds 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 dest ruction of HCN in reactions with H+ and C+, while the ratio falls belo w 1 at moderate A(V) where the CH and N needed to form CN are not abun dant. The general characteristics of the region have also been studied , using a map of (CO)-C-13 J=2-1 emission, and an elongated ridge or ' Third Bar' is found to the north-east of the Trapezium stars. This str ucture 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.