THE CI CO RATIO IN THE MOLECULAR CLOUD G-34.3+0.2/

A model for the structure of the massive molecular cloud G 34.3 + 0.2 is derived from extensive observations of carbon monoxide lines, inclu ding J = 1-0, J = 2-1, J = 3-2 and J = 6-5 (CO)-C-13, and J = 2-1 (CO) -O-18. The model consists of a slowly collapsing turbulent cloud with density constant out to a radius of 0.15 pc, but falling off as simila r to R(-2) out to 3.25 pc. The CO abundance is constant. Within the fr amework of the basic cloud structure it is possible to predict line sh apes for the 492-GHz P-3(1)-P-3(0) CI line, for different assumed radi al behaviour of the CI abundance. These are compared with observations . The best fit is found with CI proportional to r(0.9) from the perime ter of the cloud into r = 0.24 pc where A(v)= 55. The column-density-a veraged [CI]/[CO] ratio for G 34.3 + 0.2 is very similar to that for t he rho Ophiuchi molecular cloud, notwithstanding the very different st ructures of the two clouds. Models for producing high carbon abundance s deep in clouds that may have difficulty in explaining the results in clude those with external ultraviolet penetration around clumps, or dy namic mixing, or in which the initial [C]/[O] ratio is greater than un ity.