Signatures of MRI-Driven Turbulence in Protoplanetary Disks: Predictions for ALMA Observations
Simon et al
Spatially resolved observations of molecular line emission provide unique constraints on protoplanetary disk turbulence. Using local non-ideal MHD simulations and radiative transfer calculations, we assess the ability of ALMA observations to robustly detect and characterize disk turbulence. We specifically predict the outcome of the magnetorotational instability (MRI) in the disk around HD 163296, a promising observational target. We find that the MRI can support the observed level of accretion if the outer disk surface is ionized by far-UV photons and threaded by a weak net vertical magnetic field. We identify two classes of MRI solution - dynamo solutions in which the surface magnetic field reverses periodically, and non-dynamo solutions in which much of the Maxwell stress is steady and large scale. In both classes the small-scale turbulence increases in strength with height above the mid-plane, and can be represented as a microturbulent component. Using vertical profiles of the turbulent velocity from simulations at different radii, we use radiative transfer calculations to quantify the observational signatures. We show that the peak to line center flux ratio is a robust diagnostic of turbulence that is only mildly degenerate with uncertainties in disk temperature. For the CO(3-2) line variations in the predicted peak-to-trough ratio between our most and least turbulent models are ~15%. We develop predictions for other molecular lines and for channel maps whose morphology allows for independent constraints on turbulence.