may differ greatly and be much more highly peaked than in our map. Nevertheless, the $f_{\rm D}$ values derived by us are direct observables and we have therefore used them for the purpose of correlating with parameters such as the observed deuterium fractionation. We have also used them for model comparisons (sections \[ssimple\] and  \[sconcentrated\]) bearing in mind the above discussion.

In Fig. \[fd\_dust\] the $f_{\rm D}$ map is shown (thick contour) overlapped with the smoothed 1.2mm map (grey scale) and the (2–1) map (dashed contours). The CO depletion factor increases between 6 at the lowest contour of the 1.2mm map (${\cal F}_{\rm 1.2mm}$ = 95 mJy/22 beam) to 18 at the peak position (offset \[0,9\]), which is 11 off the millimeter dust emission peak (offset \[-8,+2\]), where ${\cal F}_{\rm 1.2mm}$ = 320 mJy/22 beam). From Fig. \[fd\_dust\] we immediately see that $f_{\rm D}$ correlates with the  emission (and deuterium fractionation, see Sect. \[sdeut\]) and the 1.2 mm dust flux. The good correlation between $f_{\rm D}$ and ${\cal F}_{\rm 1.2mm}$ is also evident in Fig. \[fd\_cd\], where $f_{\rm D}$ is plotted as a function of $N(\MOLH )_{\rm 1.2mm}$ ($\equiv$ 4.25$\times$10$^{20} {\cal F}_{\rm 1.2mm}$ mJy/(22 beam), with the assumptions on $T_{\rm 