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<sub>CI</sub>-factor' for the 609 μm carbon line, the analogue of the CO `X-factor', which is the ratio of the H₂ column density to the integrated ¹²CO(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 × 10²³ 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 × 10²⁰ cm^-2 K^-1 km^-1 s and X_CI of 1.1 × 10²¹ cm^-2 K^-1 km^-1 s. L1