Cold CO Gas in Protoplanetary Disks

In a disk around DM Tau, previous observation of CO (J = 2− 1 and 1− 0 transitions) derived the CO gas temperature of ∼ 13−20K, 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 ∼ 10 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 ∼ 13 K. Apparent temperature of CO decreases as the maximum grain size increases from μm-size to mm-size. Subject headings: stars: planetary systems: protoplanetary disks — stars: premain-sequence — ISM: molecules