Cold CO Gas in Protoplanetary Disks

In a disk around DM Tau, previous observation of 13CO (J=2-1 and 1-0transitions) derived the 13CO gas temperature of \sim 13-20K, which is lowerthan the sublimation temperature of CO (20 K). We argue that the existence ofsuch cold CO can be explained by a vertical mixing of disk material. As the gasis transported from a warm layer to a cold layer, CO is depleted onto dustgrains with a timescale of \sim 10^3 yr. Because of the steep temperaturegradient in the vertical direction, an observable amount of CO is still in thegas phase when the fluid parcel reaches the layer of \sim 13 K. Apparenttemperature of CO decreases as the maximum grain size increases frommicron-size to mm-size.Comment: 11 pages, 2 figures, accepted to ApJ