Understanding the physics of the X-factor
  We study the relationship between the H2 and CO abundances in simulated
molecular clouds using a fully dynamical model of magnetized turbulence coupled
to a detailed chemical network. We find that the CO-to-H2 conversion factor for
a given molecular cloud, the so-called X-factor, is determined primarily by the
mean extinction of the cloud, rather than by its metallicity. Our results
explain the discrepancy observed in low metallicity systems between cloud
masses derived from CO observations and other techniques such as infrared
emission, and predict that CO-bright clouds in low metallicity systems should
be systematically larger and/or denser than Milky Way clouds.
