Structure of the DM Tau outer disk: Probing the vertical kinetic temperature gradient

We use high angular and spectral resolution (similar to1 & 0.1 km s(-1)) images in (CO)-C-12, (CO)-C-13 J = 2 --> 1, J = 1 --> 0 and (CO)-O-18 J = 2 --> 1 coming from the IRAM interferometer to probe the vertical temperature distribution in the disk of DM Tau. We investigate here a new method based on the different opacities of the CO isotopomers to sample the temperature disk structure at various vertical scales inside the disk. Typically, the (CO)-C-12 transitions are sampling at 2-4 scale heights, while the (CO)-C-13 data are more likely tracing 1 scale height for the J = 2-1, and the disk mid-plane for the J = 1-0 line. At the disk scale at which the IRAM observations are sensitive (R greater than or equal to 50-60 AU), the analysis reveals a vertical temperature gradient. The outer layers where tau((CO)-C-12 J = 2 --> 1) similar or equal to 1 are warmer (similar to30 K) than the inner layers probed by (CO)-C-13 and (CO)-O-18 data (similar to13-20 K). These findings are consistent with the structure expected for flared irradiated disks around TTauri stars. We also observe that the outer radius of the disk is smaller in (CO)-C-13 and (CO)-O-18 thanin (CO)-C-12. These differences in truncation radius are in agreement with photodissociation effects. We also note that the dynamical mass determination from CO is weakly affected by the temperature gradient.