Resolved gas cavities in transitional disks inferred from CO isotopologs with ALMA

Context Transitional disks around young stars with large dust cavities are promising candidates to look for recently formed, embedded planets. Models of planet-disk interaction predict that young planets clear a gap in the gas while trapping dust at larger radii. Other physical mechanisms might also be responsible for cavities. Previous observations have revealed that gas is still present inside these cavities, but the spatial distribution of this gas remains uncertain. Aims. We present high spatial resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of (CO)-C-13 and (CO)-O-18 3-2 or 6-5 lines of four well-studied transitional disks around pre-main-sequence stars with large dust cavities. The line and continuum observations are used to set constraints on the the gas surface density, specifically on the cavity size and density drop inside the cavity. Methods. The physical-chemical model DALI was used to analyze the gas images of SR21, HD 135344B (also known as SAO 206462), DoAr44, and IRS 48. The main parameters of interest are the size, depth and shape of the gas cavity in each of the disks. CO isotope-selective photodissociation is included to properly constrain the surface density in the outer disk from C18O emission. Results. The gas cavities are up to three times smaller than those of the dust in all four disks. Model fits indicate that the surface density inside the gas cavities decreases by a factor of 100 to 10 000 compared with the surface density profile derived from the outer disk. The data can be fit by either introducing one or two drops in the gas surface density or a surface density profile that increases with radius inside the cavity. A comparison with an analytical model of gap depths by planet-disk interaction shows that the disk viscosities are most likely low, between between 10(-3) and 10(-4), for reasonable estimates of planet masses of up to 10 Jupiter masses. Conclusions. The resolved measurements of the gas and dust in transition disk cavities support the predictions of models that describe how planet-disk interactions sculpt gas disk structures and influence the evolution of dust grains. These observed structures strongly suggest the presence of giant planetary companions in transition disk cavities, although at smaller orbital radii than is typically indicated from the dust cavity radii alone.