An Alternative Accurate Tracer of Molecular Clouds: The $X_{rm CI}$-Factor

We explore the utility of CI as an alternative high-fidelity gas mass tracer for Galactic molecular clouds. We evaluate the X$_{rm CI}$-factor for the 609 $mu$m carbon line, the analog of the CO X-factor, which is the ratio of the H$_2$ column density to the integrated $^{12}$CO(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 $6 times 10^{23}$ cm$^{-2}$. Our results hold for both reduced and full chemical networks. For our fiducial Galactic cloud we derive an average $X_{rm CO}$ of $3.0times 10^{20}$ cm$^{-2}$K$^{-1}$km$^{-1}$s and $X_{rm CI}$ of $1.1times 10^{21}$ cm$^{-2}$K$^{-1}$km$^{-1}$s.