2016MNRAS.460...82S

We present numerical simulations of molecular clouds (MCs) with self-consistent CO gas-phase and isotope chemistry in various environments. The simulations are post-processed with a line radiative transfer code to obtain ^12CO and ^13CO emission maps for the J=1 - 0 rotational transition. The emission maps are analysed with commonly used observational methods, i.e. the ^13CO column density measurement, the virial mass estimate and the so-called X_CO (also CO-to-H_2) conversion factor, and then the inferred quantities (i.e. mass and column density) are compared to the physical values. We generally find that most methods examined here recover the CO-emitting H_2 gas mass of MCs within a factor of 2 uncertainty if the metallicity is not too low. The exception is the ^13CO column density method. It is affected by chemical and optical depth issues, and it measures both the true H_2 column density distribution and the molecular mass poorly. The virial mass estimate seems to work the best in the considered metallicity and radiation field strength range, even when the overall virial parameter of the cloud is above the equilibrium value. This is explained by a systematically lower virial parameter (i.e. closer to equilibrium) in the CO-emitting regions; in CO emission, clouds might seem (sub-)virial, even when, in fact, they are expanding or being dispersed. A single CO-to-H_2 conversion factor appears to be a robust choice over relatively wide ranges of cloud conditions, unless the metallicity is low. The methods which try to take the metallicity dependence of the conversion factor into account tend to systematically overestimate the true cloud masses.