Rethinking the N(H2)/I(CO) Conversion Factor
  An improved formulation for the X-factor is proposed. The statement that the velocity-integrated radiation temperature of the $\COone$ line, $I(\CO)$, ``counts'' optically thick clumps is quantified using the formalism of \citet{Martin84} for line emission in a clumpy cloud. Adopting the simplifying assumptions of thermalized $\COone$ line emission and isothermal gas, an effective optical depth, $\tef$, is defined as the product of the clump filling factor within each velocity interval and the clump effective optical depth as a function of the optical depth on the clump's central sightline, $\tau_0$. The clump effective optical depth is well approximated as a power law in $\tau_0$ with power-law index, $\epsilon$, referred to here as the clump ``fluffiness,'' and has values between zero and unity. While the $\COone$ line is optically thick within each clump (i.e., high $\tau_0$), it is optically thin ``to the clumps'' (i.e., low $\tef$). Thus the dependence of $I(CO)$ on $\tef$ is linear, resulting in an X-factor that depends only on clump properties and {\it not} directly on the entire cloud. Assuming virialization of the clumps yields an expression for the X-factor whose dependence on physical parameters like density and temperature is ``softened'' by power-law indices of less than unity that depend on the fluffiness parameter, $\epsilon$. The X-factor provides estimates of gas column density because each sightline within the beam has optically thin gas within certain narrow velocity ranges. Determining column density from the optically thin gas is straightforward and parameters like $\epsilon$ then allow extrapolation of the column density of the optically thin gas to that of all the gas. 
W. F. Wall