Science Highlights - Spiral Arms in the Protoplanetary Disc HD100453 Detected with ALMA

The protoplanetary disc orbiting HD100453 is very well known thanks to its two symmetric spiral arms seen in scattered light high-resolution images. Such spirals, possibly launched by an external stellar companion, did not have sub-mm counterparts detected so far. In their 2020 paper, Rosotti and collaborators present new high-resolution (∼30 milliarcseconds) ALMA observations of the source, showing that the scattered light spirals have in fact clear sub-mm counterparts both in the continuum and gas CO(3-2) emission. They show that the spirals are actual structures in the disc surface density and not only in the surface layers of the disc. Since one of the two spiral arms detected in CO emission points to the location of the known companion, they also confirm that the companion is responsible for launching the spirals. The pitch angle of the sub-millimeter continuum spirals (∼6 degrees) is lower than the one in scattered light (∼16 degrees). The authors use hydrodynamical simulations of binary-disc interaction to show that the difference in pitch angle can be explained only if the midplane is colder than the upper layers of the disc, as expected for the case of externally irradiated discs.