Rethinking the N(H2)/I(CO) conversion factor

Abstract

Consequences of an improved formulation, proposed in an earlier paper, for the N(H2)/I(CO) conversion factor are examined. That formulation quantified the statement that the velocity-integrated radiation temperature of the 12CO J= 1 0 line, I(12CO), ‘counts’ optically thick clumps. The clump effective optical depth was approximated as a power law of the clump's optical depth on its central sightline; the power-law index, e, had values between zero and unity. Assuming virialization of the clumps yielded an expression for the N(H2)/I(CO) conversion factor, or X-factor, whose dependence on physical parameters like density and temperature was ‘softened’ by power-law indices of less than unity that depend on the parameter e. One important suggestion of this formulation is that virialization of entire clouds is irrelevant. The densities required to give reasonable values of Xf are consistent with those found in cloud clumps (i.e. ∼103 H2 cm−3). Thus virialization of clumps, rather than of entire clouds, is consistent with the observed values of Xf. 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 subcloud structure. The properties of real clumps in real molecular clouds can be used to estimate the X-factor within these clouds and then be compared with the observationally determined X-factor. This yields X-factor values that are within a factor of 2 of the observed values. This is acceptable for the first attempt, but reducing this discrepancy will require improving the formulation. While this formulation improves upon the earlier explanation given about 20 yr ago, it has shortcomings of its own. These include uncertainties as to why e seems to be constant from cloud to cloud, uncertainties in defining the average clump density and neglecting certain complications, such as non-local thermodynamic equilibrium effects, magnetic fields, turbulence, etc. Despite these shortcomings, the proposed formulation represents the first major improvement in understanding the X-factor because it is the first formulation to include radiative transfer.