Low gas to dust ratio in protoplanetary disks: the Carbon content of CQ Tau comma MWC 758 and MWC 480

The study of the transition from gasrich protoplanetary disk togaspoor debris disk is crucial to constrain the planetary formationmechanisms. Although it is a key parameter for big gaseous planetformation, the evolution of the gastodust ratio with time and starproperties is not yet known. One of the first steps to observationallyconstrain it is to determine independently the gas mass and the dustmass of disks. The dust content, determined from continuum emission, isbetter known than the gas content. As molecular hydrogen is notobservable at the low temperatures of disks, the gas mass is usuallyderived from CO observation. However, CO may not be always the maincarbon reservoir: it should freeze on grain mantles in the coldmidplane of T Tauri disks, and be photodissociated in the upper layersby the UV field, leading to CI and CII, especially in disks surroundingA stars.We propose here to characterize the gaseous Carbon content in threedisks (CQ Tau, MWC 758 and MWC 480) using the three main C carriers: CO,CI and C+. Previous CO observations indicates warm disks (thetemperature being well above the CO freezeout temperature). Two ofthem, CQ Tau and MWC 758, have very low CO content and may be in thetransition stage between gasrich and gas poor disks. A low CI contentwas also found for CQ Tau using APEX. We propose to take advantage ofsensitivity of Hershel at 1900 GHz (157 um) and high spectral resolutionprovided by the HIFI instrument to observe C+ in these disks.This proposal was ranked B for the OT1 period. We resubmit an updated version. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]