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We present the highest angular resolution (20x15mas - 44x33au) Atacama LargeMillimeter/sub-millimeter Array (ALMA) observations currently possible of theproto-O-star G17.64+0.16 in Band 6. The Cycle 5 observations with baselines outto 16km probes scales <50au and reveal the rotating disc around G17.64+0.16, amassive forming O-type star. The disc has a ring-like enhancement in the dustemission, especially visible as arc structures to the north and south. TheKeplerian kinematics are most prominently seen in the vibrationally excitedwater line, H2O (Eu=3461.9K). The mass of the central source found by modellingthe Keplerian rotation is consistent with 45+/-10Mo. The H30alpha (231.9GHz)radio-recombination line and the SiO (5-4) molecular line were detected at upto the 10 sigma$ level. The estimated disc mass is 0.6-2.6Mo under theoptically thin assumption. Analysis of the Toomre Q parameter, in the opticallythin regime, indicates that the disc stability is highly dependent ontemperature. The disc currently appears stable for temperatures >150K, thisdoes not preclude that the substructures formed earlier through discfragmentation.

Substructures in the Keplerian disc around the O-type (proto)star G17.64+0.16