Structure of the DM Tau Outer Disk: Probing the vertical kinetic temperature gradient

We use high angular and spectral resolution (${\sim} 1''$& 0.1 km s-1) images in $^{12}{\rm CO}$, $^{13}{\rm CO}$$J=2\!\rightarrow\! 1$, $J=1\!\rightarrow\! 0$and ${\rm C}^{18}{\rm O}$$J=2\!\rightarrow\! 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 $^{12}{\rm CO}$transitions are sampling at 2–4 scale heights, while the $^{13}{\rm CO}$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 ($R\geq 50$–60 AU), the analysis reveals a vertical temperature gradient. The outer layers where τ($^{12}{\rm CO}$$J=2\!\rightarrow\! 1$$) \simeq 1$are warmer (${\sim} 30$K) than the inner layers probed by $^{13}{\rm CO}$and ${\rm C}^{18}{\rm O}$data (${\sim} 13$–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 $^{13}{\rm CO}$and ${\rm C}^{18}{\rm O}$than in $^{12}{\rm CO}$. 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.