CI FINE-STRUCTURE EMISSION FROM NONEQUILIBRIUM PDRS

We present computations of [CI] 609 mu m and 370 mu m fine-structure l ine emission from non-equilibrium clouds which are suddenly shielded f rom initially intense far-ultraviolet radiation fields. After the inci dent UV fluxes are diminished the initially abundant C+ ions rapidly r ecombine, and the neutral atomic carbon is then slowly incorporated in to CO molecules. The cloud cooling times are longer than the CI format ion times, and intense [CI] emission persists as the clouds cool. The [CI] emission is thus much brighter from cooling clouds than in equili brium photon dominated regions. We argue that if UV-scattering is inef ficient the extended [CI] emission observed in star-forming regions ma y be better explained by non-equilibrium models than by steady state m odels.