# Gas density perturbations induced by forming planet(s) in the AS 209 protoplanetary disk as seen with ALMA

## TL;DR

This study provides the first spatially resolved evidence of gas surface density perturbations caused by forming planet(s) in the AS 209 protoplanetary disk, using ALMA observations of C$^{18}$O and DCO$^{+}$ emissions.

## Contribution

It demonstrates that gas surface density gaps are caused by planet formation, confirmed through ALMA observations and physico-thermochemical modeling, revealing the presence of at least a 0.2 Jupiter-mass planet.

## Key findings

- Gas surface density deficits coincide with dust gaps.
- DCO$^{+}$ emission peaks between dust gaps, indicating gas perturbation.
- Gas perturbation likely caused by a forming planet of at least 0.2 Jupiter masses.

## Abstract

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 towards the AS~209 protoplanetary disk from the optically thin C$^{18}$O ($J=2-1$) emission. The observations were carried out at 1.3~mm with ALMA at a spatial resolution of about 0.3$\arcsec$ $\times$ 0.2$\arcsec$ (corresponding to $\sim$ 38 $\times$ 25 au). The C$^{18}$O emission shows a compact ($\le$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 C$^{18}$O, the DCO$^{+}$ ($J=3-2$) emission peaks between the two dust gaps. We infer that the best scenario to explain our observations (C$^{18}$O deficit and DCO$^{+}$ enhancement) is a gas perturbation due to forming-planet(s), that 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 0.2~M$\rm_{Jupiter}$ in formation. Finally, our observations also show the potential of using CO isotopologues to probe the presence of saturn mass planet(s).