High-Density CN Filaments in NGC 2023

We report high angular resolution observations of CN, SO, and HCN toward the reflection nebula NGC 2023. The CN data reveal the existence of high-density gas (n greater than 10⁵/cu cm) close to the illuminating star (approximately less than or equal to 0.13 pc) which is not detected in HCN or SO. Since the spatial distribution of the dense gas is similar to that of the vibrationally excited H2 fluorescent emission, we propose that this dense gas is located in the inner walls of the cavity photodissociated by the illuminating star. The CN/HCN abundance ratio is a factor of approximately 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 tracer of enhanced UV fields.