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

Abstract

We explore the utility of CI as an alternative high-fidelity gas mass tracer for Galactic molecular clouds. We evaluate the X${}_{\mathrm{CI}}$-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 $X_{\mathrm{CO}}$ of $3.0\times 10^{20}$ cm${}^{-2}$K${}^{-1}$km${}^{-1}$s and $X_{\mathrm{CI}}$ of $1.1\times 10^{21}$ cm${}^{-2}$K${}^{-1}$km${}^{-1}$s.

5 pages, 4 figures, 1 table, accepted to MNRAS Letters

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