7 Summary

Using high angular (1'') resolution PdBI millimeter interferometry we have mapped the circumstellar disk of the T Tauri star DM Tau in $^{12}{\rm CO}$, $^{13}{\rm CO}$ and ${\rm C}^{18}{\rm O}$ in their $J=1\!\rightarrow\! 0$ and $J=2\!\rightarrow\! 1$ transitions. The combination of isotopes imaging allows to sample the entire line opacity range in the disk and therefore to separate and constrain the parameters describing its physical state. The analysis of the data allow to draw several conclusions:

• The observed disk external radius is lower in ${\rm C}^{18}{\rm O}$ (600 AU) and $^{13}{\rm CO}$ (640 AU) than in $^{12}{\rm CO}$ (800 AU). This difference is in agreement with the selective photodissociation of CO isotopes.
• The vertical behavior of the temperature distribution varies with the radius. We have direct observational evidence of the presence of a positive vertical temperature gradient in the DM Tau circumstellar disk. The region concerned corresponds to the transition from the totally thick to totally thin dust disk. In this regime the midplane exhibits a "plateau'' of nearly constant temperature of about $\sim$13 K.
• The existence of such a plateau put limits on the disk mass (related to the dust opacity). Comparison with current models suggest that grain growth must have occured to explain the observations.
• The measurement of CO dynamical mass is weakly affected by the existence of the temperature gradient.

This method is likely to become more general and very powerful with the next generation interferometer ALMA. Particularly, it will allow us to probe the temperature gradient not only in the outer disk but also closer to the star, at $r \simeq 10$-30 AU, a domain which is currently impossible to study due to limited sensitivity and resolution.

Acknowledgements

We acknowledge all the PdBI IRAM staff for their help during the observations. Particular thanks to D. Nürnberger who pointed out a spurious problem. Most of the calculations were performed using the LAOG computer facilities we thank Ginette Buisson for assisting us in the installation of the software.