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Abstract

Molecular line observations are powerful tracers of the physical and chemical conditions across the different evolutionary stages of star, disk, and planet formation. The high angular resolution and unprecedented sensitivity of the Atacama Large Millimeter Array (ALMA) enables the current drive to detect small-scale gas structures in protoplanetary disks that can be attributed directly to forming planets. We report high angular resolution ALMA Band 7 observations of sulphur monoxide (SO) in the nearby planet-hosting disk around the Herbig star HD 100546. SO is rarely detected in evolved protoplanetary disks, but in other environments, it is most often used as a tracer of shocks. The SO emission from the HD 100546 disk primarily originates from gas within the ≈20 au millimeter-dust cavity and shows a clear azimuthal brightness asymmetry of a factor of 2. In addition, the difference in the line profile shape is significant when these new Cycle 7 data are compared to Cycle 0 data of the same SO transitions. We discuss the different physical and chemical mechanisms that might cause this asymmetry and time variability, including disk winds, disk warps, and a shock triggered by a (forming) planet. We propose that SO is enhanced in the cavity by the presence of a giant planet. The SO asymmetry complements evidence for hot circumplanetary material around giant planet HD 100546 c that is traced via CO ro-vibrational emission. This work sets the stage for further observational and modelling efforts to detect and understand the chemical imprint of a forming planet on its parent disk.