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We present high-resolution (30 mas or 130 au at 4.2 kpc) Atacama LargeMillimeter/submillimeter Array observations at 1.2 mm of the disc around theforming O-type star AFGL 4176 mm1. The disc (AFGL 4176 mm1-main) has a radiusof ~1000 au and contains significant structure, most notably a spiral arm onits redshifted side. We fitted the observed spiral with logarithmic andArchimedean spiral models. We find that both models can describe its structure,but the Archimedean spiral with a varying pitch angle fits its morphologymarginally better. As well as signatures of rotation across the disc, weobserve gas arcs in CH$_3$CN that connect to other millimetre continuum sourcesin the field, supporting the picture of interactions within a small clusteraround AFGL 4176 mm1-main. Using local thermodynamic equilibrium modelling ofthe CH$_3$CN K-ladder, we determine the temperature and velocity field acrossthe disc, and thus produce a map of the Toomre stability parameter. Our resultsindicate that the outer disc is gravitationally unstable and has alreadyfragmented or is likely to fragment in the future, possibly producing furthercompanions. These observations provide evidence that disc fragmentation is onepossible pathway towards explaining the high fraction of multiple systemsaround high-mass stars.

Spiral arms and instability within the AFGL 4176 mm1 disc