2016ApJ...830...32W

The gas and dust are spatially segregated in protoplanetary disks due to the vertical settling and radial drift of large grains. A fuller accounting of the mass content and distribution in disks therefore requires spectral line observations. We extend the modeling approach presented in Williams & Best to show that gas surface density profiles can be measured from high fidelity ¹³CO integrated intensity images. We demonstrate the methodology by fitting ALMA observations of the HD 163296 disk to determine a gas mass, M_gas=0.048 M_☉, and accretion disk characteristic size R_c=213 au and gradient γ=0.39. The same parameters match the C¹⁸O 2-1 image and indicate an abundance ratio [¹²CO]/[C¹⁸O] of 700 independent of radius. To test how well this methodology can be applied to future line surveys of smaller, lower mass T Tauri disks, we create a large ¹³CO 2-1 image library and fit simulated data. For disks with gas masses 3-10 M_Jup at 150 pc, ALMA observations with a resolution of 0.''2-0.''3 and integration times of ∼20 minutes allow reliable estimates of R_c to within about 10 au and γ to within about 0.2. Economic gas imaging surveys are therefore feasible and offer the opportunity to open up a new dimension for studying disk structure and its evolution toward planet formation.