Disks around CQ Tau and MWC 758: Dense PDRs or gas dispersal ?
Résumé
The Herbig Ae stars are the massive analogs of the TTauri stars. Very few disks surrounding these kind of stars have been studied in detail. To better constraint the disks parameters (temperature and density) we observed the disks around CQ Tau and MWC 758 with the IRAM array in continuum and CO line emissions. The disks properties are derived using a standard parametric model. The two sources show a surprising low CO abundance (assuming a standard gas-to-dust ratio). We use the Meudon PDR code to study the chemistry. For CQ Tau we find that photodissociation of CO is a viable mechanism to explain the CO depletion without modifying the gas-to-dust ratio. However, we find in both sources that the temperature of large grains can be low enough to prevent CO from being released from the grain surfaces. In addition the low inclination of the CQ Tau disk challenges the UX Ori classification of this star. We conclude that CO does not appear as a direct tracer of the gas-to-dust ratio.