Gas Density Perturbations Induced by One or More Forming Planets in the AS 209 Protoplanetary Disk as Seen with ALMA

The formation of planets occurs within protoplanetary disks surrounding young stars, resulting in perturbation of the gas and dust surface densities. Here we report the first evidence of spatially resolved gas surface density (Sigma(g)) perturbation toward the AS 209 protoplanetary disk from the optically thin (CO)-O-18 (J = 2-1) emission. The observations were carried out at 1.3 mm with Atacama Large Millimeter/submillimeter Array at a spatial resolution of about 0." 3. x 0." 2 (corresponding to similar to 38 x 25 au). The (CO)-O-18 emission shows a compact (<= 60 au), centrally peaked emission and an outer ring peaking at 140 au, consistent with that observed in the continuum emission, and its azimuthally averaged radial intensity profile presents a deficit that is spatially coincident with the previously reported dust map. This deficit can only be reproduced with our physico-thermochemical disk model by lowering Sigma(gas) by nearly an order of magnitude in the dust gaps. Another salient result is that, contrary to (CO)-O-18, the DCO+ (J = 3-2) emission peaks between the two dust gaps. We infer that the best scenario to explain our observations ((CO)-O-18 deficit and DCO+ enhancement) is a gas perturbation due to one or more forming planets, which is commensurate with previous continuum observations of the source along with hydrodynamical simulations. Our findings confirm that the previously observed dust gaps are very likely due to perturbation of the gas surface density that is induced by a planet of at least 0.2M(J) in formation. Finally, our observations also show the potential of using CO isotopologues to probe the presence of one or more Saturn-mass planets.