Signatures of MRI-Driven Turbulence in Protoplanetary Disks: Predictions for ALMA Observations

TL;DR

This study combines simulations and radiative transfer calculations to predict how ALMA observations can detect MRI-driven turbulence in protoplanetary disks, focusing on the HD 163296 system.

Contribution

It provides a novel method to diagnose MRI-driven turbulence using molecular line emission, linking simulation-based turbulence models to observable signatures.

Findings

Peak line flux to line center flux ratio is a robust turbulence diagnostic.

Predicted turbulence-induced line profile variations are around 15%.

Lower optical depth tracers probe regions with lower velocities.

Abstract

Spatially resolved observations of molecular line emission have the potential to yield unique constraints on the nature of turbulence within protoplanetary disks. Using a combination of local non-ideal magnetohydrodynamic simulations and radiative transfer calculations, tailored to properties of the disk around HD 163296, we assess the ability of ALMA to detect turbulence driven by the magnetorotational instability (MRI). Our local simulations show that the MRI produces small-scale turbulent velocity fluctuations that increase in strength with height above the mid-plane. For a set of simulations at different disk radii, we fit a Maxell-Boltzmann distribution to the turbulent velocity and construct a turbulent broadening parameter as a function of radius and height. We input this broadening into radiative transfer calculations to quantify observational signatures of MRI-driven disk turbulence. We find that the ratio of the peak line flux to the flux at line center is a robust diagnostic of turbulence that is only mildly degenerate with systematic uncertainties in disk temperature. For the CO(3-2) line, which we expect to probe the most magnetically active slice of the disk column, variations in the predicted peak-to-trough ratio between our most and least turbulent models span a range of approximately 15%. Additional independent constraints can be derived from the morphology of spatially resolved line profiles, and we estimate the resolution required to detect turbulence on different spatial scales. We discuss the role of lower optical depth molecular tracers, which trace regions closer to the disk mid-plane where velocities in MRI-driven models are systematically lower.