{#scofreeze}
-------------

The comparison between the dust continuum emission and the  integrated intensity map allows the determination of the amount of CO freeze–out onto dust grain surfaces, integrated along the line of sight. This is possible because the millimeter continuum data furnish $N(\MOLH )_{\rm 1.2mm}$, the column density of molecular hydrogen across the core, assuming optically thin conditions. The same quantity is obtained from  lines ($N(\MOLH )_{\rm CO}$), again under the assumption of optically thin conditions, and adopting the relation between CO and  valid in undepleted conditions (\[CO\]/\[\] $\simeq$ 9.5$\times$10$^{-5}$ $\equiv$ $x({\rm CO})_{\rm can}$, the “canonical” CO abundance value; Frerking et al. [@flw82]). From the $N(\MOLH )_{\rm 1.2mm}$/$N(\MOLH )_{\rm CO}$ column density ratio, the integrated CO depletion factor ($f_{\rm D}$), or the ratio between the canonical and the observed CO abundance ($x({\rm CO})_{\rm can}$/$x({\rm CO})_{\rm obs}$), is easily derived.

In practice, this process requires the division of the 1.2mm dust continuum emission (${\cal F}_{\rm 1.2mm}$\[mJy/beam\] = $S_{\rm 1.2mm}$\[mJy\]/$\Omega_{\rm beam}$) map by the  integrated intensity ($W_{\CEIO}$) map. The map–division has been carried out using the IRAM image manipulation software GRAPHIC, after degrading the continuum map to the angular resolution of the (1–0) observations (22) so that the integrated depletion factor can 