ALMA MAPS program IV: CO gas distribution in Protoplaentary Disks

CO is one of the most abundant and readily observable molecules in protoplanetary disks. It is a key tracer species of the gas mass and a primary benchmark of abundances of other molecules in disks. Here we present the radial distribution of CO gas mass in five protoplanetary disks at 10-20 au resolution, using the MAPS ALMA large program observations of C13O and C18O (2-1)/(1-0), and C17O (1-0) lines. We retrieve CO gas distributions with three independent methods: (1) fitting spatially resolved CO line images with 2-dimensional temperature structures of gas and dust from thermo-chemical models, (2) fitting line images with empirical gas temperature measured from optically-thick CO lines, (3) fitting ratios among C17O hyperfine lines. Results from all three methods generally show good agreement with each other. We find that CO gas gaps only has a weak correlation with the dust gap locations. The CO gas gaps has 30-70% of the mass depression compared to nearby regions, a much smaller contrast compared to these in dust mass distribution or in models of giant planet-disk interaction. Assuming a gas-to-dust mass ratio of 100, all five disks have a CO gas abundance of 10-100 times lower than the ISM CO ratio of 10^-4. MWC 480 and HD 163296 show a steep increase of CO gas column density inside their CO snowlines compared to that of thermo-chemical models, suggesting their CO abundance inside the CO snowline is significantly higher than the warm molecular layer in the outer disk region. Four of the largest disks show a similar CO gas distribution between 150-350 au, which can be well characterized by a power-law of Σ_CO ∝ R^-2.4. This slowly decreasing tail is strongly consistent with viscous disks, confirming a long-standing theory of disk evolution.