A Three-dimensional View of Turbulence: Constraints on Turbulent Motions in the HD 163296 Protoplanetary Disk Using DCO⁺

Gas kinematics are an important part of the planet formation process. Turbulence influences planetesimal growth and migration from the scale of submicron dust grains through gas-giant planets. Radio observations of resolved molecular line emission can directly measure this non-thermal motion and, taking advantage of the layered chemical structure of disks, different molecular lines can be combined to map the turbulence throughout the vertical extent of a protoplanetary disk. Here we present ALMA observations of three molecules (DCO + (3-2), C 18 O(2-1) and CO(2-1)) from the disk around HD 163296. We are able to place stringent upper limits ( v turb < 0.06 c s , <0.05 c s , and <0.04 c s for CO(2-1), C 18 O(2-1), and DCO + (3-2) respectively), corresponding to α ≲ 3 × 10 −3 , similar to our prior limit derived from CO(3-2). This indicates that there is little turbulence throughout the vertical extent of the disk, contrary to theoretical predictions based on the magnetorotational instability and gravitoturbulence. In modeling the DCO + emission, we also find that it is confined to three concentric rings at 65.7 ± 0.9 au, , and 259 ± 1 au, indicative of a complex chemical environment.