C i 492 GHz mapping towards Cas A

We have mapped the [CI] 3 P 1 - 3 P 0 emission at 492 GHz towards the supernova remnant Cas A. We detect [C i] emission from the periphery of the diffuse photon-dominated region (PDR) covering the disc of Cas A, as traced by the carbon recombination lines, as well as from the denser PDRs associated with the molecular clouds towards the south-east. [C i] emission is detected from both the Perseus and Orion arm molecular clouds, with the -47 km s -1 Perseus arm feature being strong enough to be detected at all positions. We estimate the C/CO relative abundance to be 0.2 at the position of the identified CO clouds and > 1 for most of the cloud. Here we show that the distribution of [C I] emitting regions compared to the C + region and molecular cloud is consistent with a scenario involving PDRs. Using physical models for PDRs, we constrain the physical properties of the [C I] line-forming regions. We estimate the densities of the [C I] emitting regions to be between 10 2 and 10 3 cm -3 . Based on rather high volume filling factors (∼50 per cent), we conclude that [C i] emission mainly arises from diffuse neutral gas in the Perseus arm.