CO line ratios in molecular clouds: the impact of environment
  Line emission is strongly dependent on the local environmental conditions in
which the emitting tracers reside. In this work, we focus on modelling the CO
emission from simulated giant molecular clouds (GMCs), and study the variations
in the resulting line ratios arising from the emission from the $J=1-0$,
$J=2-1$ and $J=3-2$ transitions. We perform a set of smoothed particle
hydrodynamics (SPH) simulations with time-dependent chemistry, in which
environmental conditions -- including total cloud mass, density, size, velocity
dispersion, metallicity, interstellar radiation field (ISRF) and the cosmic ray
ionisation rate (CRIR) -- were systematically varied. The simulations were then
post-processed using radiative transfer to produce synthetic emission maps in
the 3 transitions quoted above. We find that the cloud-averaged values of the
line ratios can vary by up to $\pm 0.3$ dex, triggered by changes in the
environmental conditions. Changes in the ISRF and/or in the CRIR have the
largest impact on line ratios since they directly affect the abundance,
temperature and distribution of CO-rich gas within the clouds. We show that the
standard methods used to convert CO emission to H$_2$ column density can
underestimate the total H$_2$ molecular gas in GMCs by factors of 2 or 3,
depending on the environmental conditions in the clouds.
