  Gas and dust dissipation processes of proto-planetary disks are hardly known.
Transition disks between Class II (proto-planetary disks) and Class III (debris
disks) remain difficult to detect. We investigate the carbon chemistry of the
peculiar CQ Tau gas disk. It is likely a transition disk because it exhibits
weak CO emission with a relatively strong millimeter continuum, indicating that
the disk might be currently dissipating its gas content. We used APEX to
observe the two CI lines at 492GHz and 809 GHz in the disk orbiting CQ Tau. We
compare the observations to several chemical model predictions. We focus our
study on the influence of the stellar UV radiation shape and gas-to-dust ratio.
We did not detect the CI lines. However, our upper limits are deep enough to
exclude high-CI models. The only available models compatible with our limits
imply very low gas-to-dust ratio, of the order of a few, only. These
observations strengthen the hypothesis that CQ Tau is likely a transition disk
and suggest that gas disappears before dust.
