2021ApJS..257...15B

Constraining the distribution of gas and dust in the inner 20 au of protoplanetary disks is difficult. At the same time, this region is thought to be responsible for most planet formation, especially around the water ice line at 3-10 au. Under the assumption that the gas is in a Keplerian disk, we use the exquisite sensitivity of the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA large program to construct radial surface brightness profiles with a ∼3 au effective resolution for the CO isotopologue J = 2-1 lines using the line velocity profile. IM Lup reveals a central depression in ¹³CO and C¹⁸O that is ascribed to a pileup of ∼500 M_⊕ of dust in the inner 20 au, leading to a gas-to-dust ratio of around <10. This pileup is consistent with an efficient drift of grains (>=100 M_⊕ Myr^–1) and a local gas-to-dust ratio that suggests that the streaming instability could be active. The CO isotopologue emission in the GM Aur disk is consistent with a small (∼15 au), strongly depleted gas cavity within the ∼40 au dust cavity. The radial surface brightness profiles for both the AS 209 and HD 163296 disks show a local minimum and maximum in the C¹⁸O emission at the location of a known dust ring (∼14 au) and gap (∼10 au), respectively. This indicates that the dust ring has a low gas-to-dust ratio (>10) and that the dust gap is gas-rich enough to have optically thick C¹⁸O. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.