Cold CO gas in protoplanetary disks

In a disk around DM Tau, previous observation of CO ( and transitions) derived a (CO)-C-13 gas temperature of similar to 13 - 20 K, which is lower than the sublimation temperature of CO (20 K). We argue that the existence of such cold CO can be explained by a vertical mixing of disk material. As the gas is transported from a warm layer to a cold layer, CO is depleted onto dust grains with a timescale of similar to 10(3) yr . Because of the steep temperature gradient in the vertical direction, an observable amount of CO is still in the gas phase when the fluid parcel reaches the layer of similar to 13 K. Apparent temperature of CO decreases as the maximum grain size increases from micron to millimeter size.