DETECTION OF THE LAMBDA-609 MU-M LINE OF NEUTRAL ATOMIC CARBON IN THERING NEBULA

We report the detection of the 609 mum (492 GHz) (P-3(1)-->P-3(0)) fin e-structure line of neutral atomic carbon (CI) in the Ring Nebula (NGC 6720). The emission is spatially extended, and we have been able to m ake a preliminary strip map along the minor axis of the nebula. The pe ak observed emission is approximately 500 mK, and the line width is ap proximately 40 km s-1, characteristic of the expanding nebula. The CI column densities at the positions where the line is detected are 4 x 1 0(16) - 10(17) cm-2. Comparison of the CI data with high-sensitivity C O observations shows that the CI and CO emissions have roughly similar spatial distributions. The CI/CO column density ratio is >1, and may be much higher at particular positions of the nebula. The total number of CI atoms in the Ring nebula exceeds that of CO molecules by a fact or of about 10. Our observations confirm that the mass of the neutral shell (greater than or similar to 0.1 M.) exceeds that of the ionized nebula, and indicate that most the neutral gas probably resides in the photo-dissociation regions of rather dense (greater than or equal to 10(5) cm-3) neutral clumps exposed to the ultraviolet radiation field of the central star.