ATOMIC CARBON IN A DARK CLOUD - TMC-1

We report the first observations of the P-3(1)-P-3(0) line of atomic carbon in the dark cloud TMC-1. Additionally, to compare with CO, we observed (CO)-C-13(2-1), (CO)-O-18(2-1) and C-17 O(2-1). The lines were observed at five positions perpendicular to the molecular ridge close to the cyanopolyyne peak. Optical depth effects render the determination of atomic carbon column densities difficult, so only lower limits of about 10(17) cm(-2) are reported. However, the widths and shapes of the carbon lines strongly suggest high optical depths, which would be an argument in favor of the existence of atomic carbon deep inside molecular clouds. A comparison of the observed intensities with the intensities predicted by a photochemical model of the cloud shows a reasonably good agreement, giving confidence in our understanding of the physical and chemical processes at the edges of dark clouds.