An alternative accurate tracer of molecular clouds: the 'XCI-factor'

We explore the utility of CI as an alternative high-fidelity gas-mass tracer for galactic molecular clouds. We evaluate the 'X-CI-factor' for the 609 mu m carbon line, the analogue of the CO 'X-factor', which is the ratio of the H-2 column density to the integrated (CO)-C-12(1-0) line intensity. We use 3D-PDR to post-process hydrodynamic simulations of turbulent, star-forming clouds. We compare the emission of CI and CO for model clouds irradiated by 1 and 10 times the average background and demonstrate that CI is a comparable or superior tracer of the molecular gas distribution for column densities up to 6 x 10(23) cm(-2). Our results hold for both reduced and full chemical networks. For our fiducial Galactic cloud, we derive an average X-CO of 3.0 x 10(20) cm(-2) K-1 km(-1) s and X-CI of 1.1 x 10(21) cm(-2) K-1 km(-1) s.