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Aims. We attempt to understand the presence of gas phase CO below itsfreezing temperature in circumstellar disks. We study two promising mechanismsto explain this phenomenon: turbulent mixing and photodesorption. Methods. Wecompute the chemical evolution of circumstellar disks including grain surfacereactions with and without turbulent mixing and CO photodesorption. Results. Weshow that photodesorption significantly enhances the gas phase CO abundance, byextracting CO from the grains when the visual extinction remains below about 5magnitudes. However the resulting dependence of column density on radialdistance is not consistent with observations so far. We propose that thisinconsistency could be the result of grain growth. On the other hand, theinfluence of turbulent mixing is not found to be straightforward. Theefficiency of turbulent mixing depends upon a variety of parameters, includingthe disk structure. For the set of parameters we chose, turbulent mixing is notfound to have any significant influence on the CO column density.

Cold CO in circumstellar disks: On the effects of photodesorption and vertical mixing