Understanding how planets form and how they become habitable is at the forefront of astronomical research and is of great public and scientific interest. When stars form they are surrounded by an orbiting disk of gas and dust which, are the building materials of planets (see schematic in Fig1). Approximately 20 different molecules have been detected in protoplanetary disks to date. By studying these disks with Atacama Large Millimetre Array (ALMA) we can map the spatial distribution of gas and dust with unprecedented detail. There now is ever-growing evidence for the presence of planets in disks (e.g. the rings in HL Tau and HD 163296).  I focus on detecting simple molecules in these planet-forming (and planet-hosting) disks and using them as diagnostic tools to understand the physical conditions associated with planet formation.


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Fig1. Cartoon highlighting some of the features (and observables) in planet-forming disks.


Detection of Sulphur Monoxide in the HD100546 protoplanetary disk

Sulphur-bearing volatiles are observed to be significantly depleted in interstellar and circumstellar regions. This missing sulphur is postulated to be mostly locked up in refractory form. With ALMA we have detected sulphur monoxide (SO), a known shock tracer, in the HD 100546 protoplanetary disk.  Two rotational transitions: J = 77-66 (301.286 GHz) and J = 78-67 (304.078 GHz) are detected in their respective integrated intensity maps. The stacking of these transitions results in a clear 5σ detection in the stacked line profile. The stacked integrated intensity map is shown in Fig2 and the stacked SO line profile in Fig3.  The kinematics and spatial distribution of the SO emission are not consistent with that expected from a purely Keplerian disk. One hypothesis is that a possible inner disk warp (seen in CO emission) directly exposes the north-east side of the disk to heating by the central star, creating locally the conditions to launch a disk wind.  An alternative explanation is that the SO is tracing an accretion shock from a circumplanetary disk associated with the proposed protoplanet embedded in the disk at approximately 50 au.

Fig2. Integrated intensity map of the stacked SO emission.
Fig3. Line profiles of the CO J=3-2 and the stacked SO emission.