[C I] 492 GHz mapping observations of the high-latitude translucent cloud MCLD 123.5+24.9

We present the first map of the [C I] P-3(1) --> P-3(0) fine-structure transition of neutral carbon made toward a translucent molecular cloud (MCLD 123.5+ 24.9, located in the Polaris Flare). The [C I] observations were made with the Submillimeter Wave Astronomy Satellite and are supplemented by ground-based observations of (CO)-C-12 and (CO)-C-13 rotational transitions. We find that the [C I] emission is spatially extended following the region bright in (CO)-C-12. The [C I] to CO line ratios observed throughout the MCLD 123.5+ 24.9 cloud are relatively low, and the [C I] line flux density is only similar to50% of the emission by the three lowest CO rotational transitions. However, the ratios are still within the range observed along selected lines of sight toward other diffuse and translucent molecular clouds. Assuming LTE conditions for the neutral atomic carbon with an excitation temperature of 8 K derived from the (CO)-C-12 spectra, we derive a total carbon column density of (0.25-1) x 10(17) cm(-2) and a C to CO column density ratio between 0.2 and 1.1. Comparison with a photon-dominated region model shows that the model consistently would require uncomfortably high values for the gas volume density in order to reproduce the low [C I] to CO line ratios observed (n > 10(5) cm(-3)), unless we assume that the line-emitting clumps are embedded in an interclump medium with a density of n < 10(3) cm(-3). The low-density interclump medium does not significantly contribute to the observed [C I] and CO line emission, but the molecular hydrogen in the gas provides an effective shielding for the CO in the embedded clumps by blocking the FUV photons at the frequencies of CO line transition to the predissociation states. This reduces the photodissociation of CO and, thus, the abundance of neutral and ionized carbon in the denser clumps.