Evidence for Non-zero Turbulence in the Protoplanetary disc around IM Lup

TL;DR

This study provides direct evidence of significant non-zero turbulence in the protoplanetary disc around IM Lup using ALMA observations, impacting understanding of planet formation processes.

Contribution

First direct measurement of turbulence levels in the IM Lup disc, demonstrating the presence of non-zero turbulence and evaluating potential turbulence sources.

Findings

Turbulence level of 0.18-0.30 c_s detected

Gravito-turbulence unlikely as the source

Magneto-rotational or hydrodynamic instabilities possible

Abstract

The amount of turbulence in protoplanetary discs around young stars is critical for determining the efficiency, timeline, and outcomes of planet formation. It is also difficult to measure. Observations are still limited, but direct measurements of the non-thermal, turbulent gas motion are possible with the Atacama Large Millimeter/submillimeter Array (ALMA). Using CO(2-1)/$^{13}$CO(2-1)/C$^{18}$O(2-1) ALMA observations of the disc around IM Lup at ~0.4" (~60 au) resolution we find evidence of significant turbulence, at the level of $\delta v_{\rm turb}=(0.18-0.30)$c$_s$. This result is robust against systematic uncertainties (e.g., amplitude flux calibration, midplane gas temperature, disc self-gravity). We find that gravito-turbulence as the source of the gas motion is unlikely based on the lack of an imprint on the rotation curve from a massive disc, while magneto-rotational instabilities and hydrodynamic instabilities are still possible, depending on the unknown magnetic field strength and the cooling timescale in the outer disc.