Can We Trust CO as a Probe of the Densities and Temperatures of Molecular Clouds?

Abstract

We select a chemical magnetohydrodynamic simulation of molecular cloud formation and evolution as a typical example of a Galactic molecular cloud. Its analysis helps us understand how to interpret temperatures and densities inferred from CO line emission maps. We find that the kinetic temperature is always underestimated if it is inferred only from the excitation temperature, T _ex, of the¹²CO(1–0) emission line. We find also that CO primarily traces material at densities above the mean cloud density. In addition, we show that if one assumes a fixed value for the CO–H₂ conversion factor, then one will underestimate the density (and hence the mass) of H₂ at low column densities. In this scenario, the total H₂ mass of the cloud inferred from the emission map is only 60 % of the true mass.