HIGH-DENSITY CN FILAMENTS IN NGC-2023

We report high angular resolution observations of CN, SO, and HCN towa rd the reflection nebula NGC 2023. The CN data reveal the existence of high-density gas (n > 10(5) cm(-3)) close to the illuminating star (l ess than or similar to 0.13 pc) which is not detected in HCN or SO. Si nce the spatial distribution of the dense gas is similar to that of th e vibrationally excited H-2 fluorescent emission, we propose that this dense gas is located in the inner walls of the cavity photodissociate d by the illuminating star. The CN/HCN abundance ratio is a factor of similar to 25 larger in these dense gas layers than in the bulk of the molecular cloud, suggesting that CN is especially abundant in regions exposed to UV radiation. Thus, the CN/HCN abundance ratio is a good t racer of enhanced UV fields.