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, including 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 similar to R(-2) out to 3.25 pc. The CO abundance is constant. Within the framework of the basic cloud structure it is possible to predict line shapes for the 492-GHz P-3(1)-P-3(0) CI line, for different assumed radial behaviour of the CI abundance. These are compared with observations. The best fit is found with CI proportional to r(0.9) from the perimeter of the cloud into r = 0.24 pc where A(v)= 55. The column-density-averaged [CI]/[CO] ratio for G 34.3 + 0.2 is very similar to that for the rho Ophiuchi molecular cloud, notwithstanding the very different structures of the two clouds. Models for producing high carbon abundances deep in clouds that may have difficulty in explaining the results include those with external ultraviolet penetration around clumps, or dynamic mixing, or in which the initial [C]/[O] ratio is greater than unity.