2019A&A...627L...6M

We present the highest angular resolution (∼20x15mas-44x33au) 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 16km probe scales <50au 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 ν₂=1 (E_u=3461.9K). The mass of the central source found by modelling the Keplerian rotation is consistent with 45±10M_☉. The H30α (231.9GHz) radio-recombination line and the SiO (5-4) molecular line were detected at up to the ∼10σ level. The estimated disc mass is 0.6-2.6M_☉ 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 >150K; this does not preclude that the substructures formed earlier through disc fragmentation.