Modelling [C i] emission from turbulent molecular clouds

We use detailed numerical simulations of the coupled chemical, thermal and dynami-cal evolution of the gas in a turbulent molecular cloud to study the usefulness of the [C i] 609 µm and 370 µm fine structure emission lines as tracers of cloud structure. Emission from these lines is observed throughout molecular clouds, and yet the ques-tion of what we can learn from them about the physics of the clouds remains largely unexplored. We show that the fact that [C i] emission is widespread within molecular clouds is a simple consequence of the fact that the clouds are dominated by turbulent mo-tions. Turbulence creates large density inhomogeneities, allowing radiation to pene-trate deeply into the clouds. As a result, [C i] emitting gas is found throughout the cloud, rather than being concentrated at the edges. We examine how well we can use [C i] emission to trace the structure of the cloud, and show that the integrated inten-sity of the 609 µm line traces column density accurately over a wide range of visual extinctions. For extinctions greater than a few, [C i] and 13CO both perform well, but [C i] becomes a superior tracer of column densities for visual extinctions AV 6 3. We have also studied the distribution of [C i] excitation temperatures in the gas, and show that these are typically smaller than the kinetic temperature, indicating that most of the carbon atoms are not in local thermodynamic equilibrium. We discuss how best to determine Tex from observations of the [C i] lines, and how to use these values to estimate the column density of neutral atomic carbon. We show that even in the best case, we tend to systematically underestimate the atomic carbon content of the gas. Our results suggest that observationally-derived estimates of the atomic carbon content of real GMCs could be in error by as much as a factor of two