extinctions $A_{\rm V}$ greater than 6 magnitudes). Thus we conclude that in particular cases such as L1521F, it is quite possible that we are using a value of $x({\rm CO})_{\rm can}$ which is a factor of order 2 too large or small thus influencing the values of $f_{\rm D}$ which we derive but not the trends over our map.

Another problem is that the values of $f_{\rm D}$ which we derive are integrated along the line of sight in a situation where the observed  emission forms in an outer shell whereas the dust emission mainly emanates from the dense core nucleus. As a consequence, we observe  mainly from foreground and background gas which (see Fig. \[fig1b\]) has an essentially flat distribution over the region mapped by us. One concludes that our values of $f_{\rm D}$ are strict lower limits to the CO depletion [*in the core nucleus*]{} from which dust emission is observed. It is also the case that in this situation, the form of our map of $f_{\rm D}$ is essentially that of the dust emission (as we indeed find, see eq. \[efdh2\]). The [*local*]{} distribution of the CO depletion factor (which we call ${\cal F}_{\rm D}$, see below) 