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 Horseh ead 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 whe re the C+ emission varies rapidly with UV intensity, fine-structure li ne emission from gas with this range of physical conditions has not be en investigated previously. Comparisons of our results with models of photodissociation regions show that existing plane-parallel photodisso ciation region models are in general agreement with the observed inten sity. It is not necessary to invoke a clumpy structure in the boundary layer to explain the observations, but the overall geometry of the cl oud is important in determining the distribution of C+ emission.