Gas-Phase CO in Protoplanetary Disks: A Challenge for Turbulent Mixing

This is the first paper in a series in which we study the influence of turbulent diffusion and advective transport on the chemical evolution of protoplanetary disks, using a 2D flared-disk model and a 2D-mixing gas-grain chemical code with surface reactions. A first interesting result concerns the abundance of gas-phase CO in the outer regions of protoplanetary disks. In this Letter we argue that the gas-phase CO concentration in the disk regions, where the temperature is lower than ~25 K, can be significantly enhanced due to the combined effect of vertical and radial mixing. This finding has a potential implication for the current observational data on the DM Tau disk chemistry.