The ALMA view of W33A: A spiral filament feeding the candidate disc in MM1-Main

We targeted the massive star-forming region W33A using the Atacama large sub/millimeter array in bands 6 (230 GHz) and 7 (345 GHz) to search for a sub-1000 au disc around the central O-type massive young stellar object W33A MM1-Main. Our data achieve a resolution of ~0.2 arcsec (~500 au) and resolve the central core, MM1, into multiple components and reveal complex and filamentary structures. There is strong molecular line emission covering the entire MM1 region. The kinematic signatures are inconsistent with only Keplerian rotation although we propose that the shift in the emission line centroids within ~1000 au of MM1-Main could hint at an underlying compact disc with Keplerian rotation. We cannot however rule out the possibility of an unresolved binary or multiple system. A putative smaller disc could be fed by the large-scale spiral 'feeding filament' we detect in both gas and dust emission. We also discuss the nature of the now-resolved continuum sources.