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

We present the highest angular resolution (similar to 20 x 15 mas-44 x 33 au) Atacama Large Millimeter/sub-millimeter Array (ALMA) observations that are currently possible of the proto-O-star G17.64+0.16 in Band 6. The Cycle 5 observations with baselines out to 16 km probe scales <50 au and reveal the rotating disc around G17.64+0.16, a massive forming O-type star. The disc has a ring-like enhancement in the dust emission that is especially visible as arc structures to the north and south. The Keplerian kinematics are most prominently seen in the vibrationally excited water line, H2O 5(5,0)-6(4,3) nu(2) = 1 (E-u = 3461.9 K). The mass of the central source found by modelling the Keplerian rotation is consistent with 45 +/- 10 M-circle dot. The H30 alpha (231.9 GHz) radio-recombination line and the SiO (5-4) molecular line were detected at up to the similar to 10 sigma level. The estimated disc mass is 0.6-2.6 M-circle dot under the optically thin assumption. Analysis of the Toomre Q parameter in the optically thin regime indicates that the disc stability is highly dependent on temperature. The disc currently appears stable for temperatures >150 K; this does not preclude that the substructures formed earlier through disc fragmentation.