The CO-H2 conversion factor of diffuse ISM: Bright 12CO emission also   traces diffuse gas
  We show that the XCO factor, which converts the CO luminosity into the column density of molecular hydrogen has similar values for dense, fully molecular gas and for diffuse, partially molecular gas. We discuss the reasons of this coincidence and the consequences for the understanding of the interstellar medium. 
J\'er\^ome Pety (IRAM, OP), Harvey S. Liszt (NRAO), Robert Lucas (JAO)
astro-ph.GA