dust}$ and $\kappa_{\rm 1.2mm}$ as described above). Thus, in L1521F, and with the caveats discussed above, $f_{\rm D}$ is linearly dependent on the  column density ($N(\MOLH )$); the best fit to the data in Fig. \[fd\_cd\] (dashed curve) gives: $$\begin{aligned}
f_{\rm D} & = & 1.5 \, 
\left[ \frac{N(\MOLH )}{10^{22} {\rm cm}^{-2}} \right]^{0.9} .
\label{efdh2}\end{aligned}$$ In retrospect, this is not surprising as $N(\CEIO )$ does not vary greatly over the map. We note that $f_{\rm D}$ is always $\ga$ 2 in the region traced by the present observations, suggesting that even at the core edges (where $N(\MOLH )$ $\sim$ 10$^{22}$ cm$^{-2}$) CO molecules are partially ($\simeq$ 30%) frozen onto grain mantles. This result is consistent with the average value of CO depletion ($\simeq$ 25%) gauged from observations of solid CO features in the direction of background stars in the Taurus complex (Chiar et al. [@cak95]). The thick curve in Fig. \[fd\_cd\] is the result of a simple chemical model of a centrally concentrated cloud, where CO depletion is taken into account, and where the CO abundance has been integrated along the line of sight to obtain the observed $f_{\rm D}$ ($f_{\rm D}$ = $\int {F}_{\rm D}(r) \, dl / \int 