MEASURING PROTOPLANETARY DISK GAS SURFACE DENSITY PROFILES WITH ALMA

Abstract

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 13 CO 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 C 18 O 2–1 image and indicate an abundance ratio [ 12 CO]/[C 18 O] 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 13 CO 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.