Modelling [CI] emission from turbulent molecular clouds
  We use detailed numerical simulations of the coupled chemical, thermal and
dynamical evolution of the gas in a turbulent molecular cloud to study the
usefulness of the [CI] 609 micron and 370 micron fine structure emission lines
as tracers of cloud structure. Emission from these lines is observed throughout
molecular clouds, and yet the question of what we can learn from them about the
physics of the clouds remains largely unexplored.
  We show that the fact that [CI] emission is widespread within molecular
clouds is a simple consequence of the fact that the clouds are dominated by
turbulent motions. Turbulence creates large density inhomogeneities, allowing
radiation to penetrate deeply into the clouds. As a result, [CI] emitting gas
is found throughout the cloud, rather than being concentrated at the edges. We
examine how well we can use [CI] emission to trace the structure of the cloud,
and show that the integrated intensity of the 609 micron line traces column
density accurately over a wide range of visual extinctions. For extinctions
greater than a few, [CI] and 13CO both perform well, but [CI] becomes a
superior tracer of column densities for visual extinctions A_V <= 3
  We have also studied the distribution of [CI] 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 T_ex from observations of the
[CI] 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.
