C I observations in the CQ Tauri proto-planetary disk: evidence of a very low gas-to-dust ratio ?

Context. The 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. Aims. We investigate the carbon chemistry of the peculiar CQTau gas disk. It is likely to be a transition disk because it exhibits weak CO emission with a relatively strong millimeter continuum, indicating that the disk may currently be dissipating its gas content. Methods. We used APEX to observe the two CI transitions P-3(1) -> P-3(0) at 492 GHz and P-3(2) -> P-3(1) at 809 GHz in the disk orbiting CQTau. 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. Results. We did not detect the C I lines. However, our upper limits are deep enough to exclude high-C I models. The only available models compatible with our limits imply very low gas-to-dust ratios, of the order of only a few. Conclusions. These observations strengthen the hypothesis that CQTau is likely to be a transition disk and suggest that gas disappears before dust.