CI fine-structure emission from non-equilibrium PDRs

We present computations of [CI] 609 μm and 370 μm fine-structure line emission from non-equilibrium clouds which are suddenly shielded from initially intense far-ultraviolet radiation fields. After the incident UV fluxes are diminished the initially abundant C⁺ ions rapidly recombine, and the neutral atomic carbon is then slowly incorporated into CO molecules. The cloud cooling times are longer than the CI formation times, and intense [CI] emission persists as the clouds cool. The [CI] emission is thus much brighter from cooling clouds than in equilibrium photon dominated regions. We argue that if UV-scattering is inefficient the extended [CI] emission observed in star-forming regions may be better explained by nonequilibrium models than by steady state models.