Conversion of carbon monoxide intensities tomolecular hydrogen abundances

We present results of theoretical models (static spherical clouds with a microturbulent velocity field) to study the conversion of carbon monoxide (CO) line parameters into molecular hydrogen (H2) column densities, N2. The three potential H2 tracers that we investigate are the integrated 12CO and 13CO intensities, I12and I13, and the 13CO LTE column density, N(13. We find that I12 may be a reasonable tracer of N2 under conditions appropriate to the envelopes of giant molecular clouds and for studies involving cloud ensembles of different cloud sizes and velocity dispersions. However, it saturates under higher density conditions. It is important that empirical conversion factors be set using the types of objects to which they will be applied. For this reason, our analysis suggests that the conversion factor N2/I12 for giant molecular clouds in the molecular ring of our galaxy may be a factor of 2 lower than the average used by many observers. This lower value is supported by some recent empirical determinations. The quantity I13 is a good tracer of N2 over a wide range of densities but it is more sensitive to the actual 13CO abundance. The quantity N(13 is similar to I13 as a good tracer of N2 except at low densities and temperatures. The ratio I12/I13 may be used to delineate temperature and column density effects. Finally, we find a strong temperature dependence in the various conversion factors, with N2/I12 scaling with gas temperature (T/sub k/ approximately as (T/sub k/)/sup -1.3/