Evidence for Non-zero Turbulence in the Protoplanetary Disc Around IM Lup

The amount of turbulence in protoplanetary discs around young stars iscritical for determining the efficiency, timeline, and outcomes of planetformation. It is also difficult to measure. Observations are still limited, butdirect measurements of the non-thermal, turbulent gas motion are possible withthe Atacama Large Millimeter/submillimeter Array (ALMA). UsingCO(2-1)/^13CO(2-1)/C^18O(2-1) ALMA observations of the disc around IMLup at  0.4" ( 60 au) resolution we find evidence of significant turbulence, atthe level of δ v_ turb=(0.18-0.30)c_s. This result is robustagainst systematic uncertainties (e.g., amplitude flux calibration, midplanegas temperature, disc self-gravity). We find that gravito-turbulence as thesource of the gas motion is unlikely based on the lack of an imprint on therotation curve from a massive disc, while magneto-rotational instabilities andhydrodynamic instabilities are still possible, depending on the unknownmagnetic field strength and the cooling timescale in the outer disc.