Gas density perturbations induced by forming planet(s) 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 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).
