THE [C-II] 158-MICRON EMISSION FROM THE HORSEHEAD NEBULA

We have mapped the [C II] 158 mum line and observed several rotational lines of CO, (CO)-C-13 and CS toward selected positions in the Horsehead extinction region in IC 434. The observations show that the region has a gas density of about 10(4) cm-3 and an external UV flux 20-100 times the average interstellar UV field. Although this is a regime where the C+ emission varies rapidly with UV intensity, fine-structure line emission from gas with this range of physical conditions has not been investigated previously. Comparisons of our results with models of photodissociation regions show that existing plane-parallel photodissociation region models are in general agreement with the observed intensity. It is not necessary to invoke a clumpy structure in the boundary layer to explain the observations, but the overall geometry of the cloud is important in determining the distribution of C+ emission.