Effects of molecular cloud properties on the CO-to-H-2 conversion factor

Effects of molecular cloud properties on the CO-to-H-2 conversion factor are quantitatively examined on the basis of a simple model of radiative transfer and excitation of the CO (J = 1-0) emission for clumpy molecular clouds. This model deals with effects of cloud structure, gas density, kinetic temperature, and the CO abundance. Since most of the molecular gas mass in molecular clouds in galaxies is low density, the CO-to-H-2 conversion factor is strongly dependent on the degree of photon trapping, or the CO abundance. Properties of molecular clouds other than the CO abundance have a minor effect on the conversion factor. Molecular gas mass in metal-poor galaxies will be severely underestimated if the Galactic value of the conversion factor is assumed.