The [C II] 158 micron emission from the Horsehead Nebula

We have mapped the [C II] 158 μm line and observed several rotational lines of CO, 13 CO 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