Gas density drops inside dust cavities of transitional disks around young stars observed with ALMA

Context. Transitional disks with large dust cavities are important laboratories in which to study planet formation and disk evolution. Cold gas may still be present inside these cavities, but quantying this gas is challenging. The gas content is important for constraining the origin of the dust cavity. Aims. We use Atacama Large Millimeter/submillimeter Array (ALMA) observations of 12CO 6–5 and 690 GHz (Band 9) continuum of five well-studied transitional disks. In addition, we analyze previously published Band 7 observations of a disk in the 12CO 3–2 line and 345 GHz continuum. The observations are used to set constraints on the gas and dust surface density profiles, in particular, the drop δgas of the gas density inside the dust cavity. Methods. The physical-chemical modeling code DALI was used to simultaneously analyze the gas and dust images. We modeled SR21, HD 135344B, LkCa15, SR24S, and RX J1615-3255 (Band 9) and J1604-2130 (Band 7). The spectral energy distribution and continuum visibility curve constrain the dust surface density. Then we used the same model to calculate the 12CO emission, which we compared with the observations through spectra and intensity cuts. The amount of gas inside the cavity was quantified by varying the δgas parameter. Results. Model fits to the dust and gas indicate that gas is still present inside the dust cavity for all disks, but at a reduced level. The gas surface density drops inside the cavity by at least a factor 10, while the dust density drops by at least a factor 1000. Disk masses are comparable with previous estimates from the literature, cavity radii are found to be smaller than in the data obtained with the 345 GHz SubMillimeter Array. Conclusions. The derived gas surface density profiles suggest that the cavity was cleared by one or more companions in all cases, which trapped the millimeter-sized dust at the edge of the cavity