Comparison of 13CO Line and Far-Infrared Continuum as a Diagnostic of Dust and Molecular Gas Physical Conditions --Implications for the N(H2)/I(CO) Conversion Factor

Far-infrared continuum data from the COBE/DIRBE instrument were combined with Nagoya 4-m 13CO J=1-0 spectral line data to infer the multiparsec-scale physical conditions in the Orion A and B molecular clouds, using 140mic/240mic dust color temperatures and the 240mic/13CO J=1-0 intensity ratios. Two-component models fit the Orion data best. The models require that the dust-gas temperature difference is 0+/-2 K. If this surprising result applies to much of the Galactic ISM, except in unusual regions such as the Galactic Center, then there are a number implications. These include dust-gas thermal coupling that is commonly factors of 5 to 10 stronger than previously believed and an improved explanation for the N(H2)/I(CO) conversion factor or X_factor.

This improved formulation for the X-factor quantifies the statement that the velocity-integrated radiation temperature of the 12CO J=1-0 line, I(CO), "counts" optically thick clumps using the formalism of Martin et al. (1984). One important suggestion of this formulation is that virialization of entire clouds is irrelevant. The densities required to give reasonable values of the X-factor are consistent with those found in cloud clumps (i.e. roughly 10³ H₂ cm^-3). Thus virialization of clumps, rather than of entire clouds, is consistent with the observed values of the X-factor. And even virialization of clumps is not strictly required; only a relationship between clump velocity width and column density similar to that of virialization can still yield reasonable values of the X-factor. The underlying physics is now at the scale of cloud clumps, implying that the X-factor can probe sub-cloud structure.

While this formulation improves upon that of Dickman et al. (1986), it has shortcomings of its own. These include uncertainties in defining the average clump density and neglecting certain complications, such as non-LTE effects, magnetic fields, turbulence, etc.