THE CARBON-MONOXIDE TO MOLECULAR-HYDROGEN CONVERSION FACTOR REVISITED

The demonstration that the CO, H-2 conversion factor had been previous ly overestimated was one of the early successes of gamma ray astronomy . There is still argument, however, about its precise value and whethe r or not it varies from place to place in the galaxy and beyond. We co nclude that the case for 'low' values of the factor, variations from p lace to place and a slow upward trend of the mean value with increasin g Galactocentric distance, is still strong.