On the size of the CO-depletion radius in the IRDC G351.77-0.51

We present a study of the depletion of CO in the IRDC G351.77-0.51. Knowledge of the degree of CO-depletion provides information on the physical conditions occurring in the innermost and the densest regions of molecular clouds. A key issue is the radius within which the CO is depleted (Rdep). To estimate the depletion map on the cloud scale, we use the dust continuum (Hi-Gal), combined with APEX C18O(2-1) line observations. We built a simple model to investigate the size of the CO-depleted region in G351. The model suggests that Rdep is lower than 0.12 pc where n(H2) is larger than 3 x 10**4 cm**-3. These results provide crucial information on the spatial scales on which different chemical processes operate in high-mass star forming regions and also presents a warning for using CO for kinematical studies in IRDCs.