Substructures in the Keplerian disc around the O-type (proto)star
  G17.64+0.16
  We present the highest angular resolution (20x15mas - 44x33au) Atacama Large
Millimeter/sub-millimeter Array (ALMA) observations currently possible of the
proto-O-star G17.64+0.16 in Band 6. The Cycle 5 observations with baselines out
to 16km probes 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, especially visible as arc structures to the north and south. The
Keplerian kinematics are most prominently seen in the vibrationally excited
water line, H2O (Eu=3461.9K). The mass of the central source found by modelling
the Keplerian rotation is consistent with 45+/-10Mo. The H30alpha (231.9GHz)
radio-recombination line and the SiO (5-4) molecular line were detected at up
to the 10 sigma$ level. The estimated disc mass is 0.6-2.6Mo 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.
