Chasing discs around O-type (proto)stars - ALMA evidence for an SiO disc
  and disc wind from G17.64+0.16
  We present high angular resolution 0.2 arcsec continuum and molecular
emission line Atacama Large Millimeter/sub-millimeter Array (ALMA) observations
of G17.64+0.16 in Band 6 (220GHz) taken as part of a campaign in search of
circumstellar discs around (proto)-O-stars. At a resolution of 400au the main
continuum core is essentially unresolved and isolated from other strong and
compact emission peaks. At a resolution of 400au the main continuum core is
essentially unresolved and isolated from other strong and compact emission
peaks. We detect SiO (5-4) emission that is marginally resolved and elongated
in a direction perpendicular to the large-scale outflow seen in the 13CO (2-1)
line using the main ALMA array in conjunction with the Atacama Compact Array
(ACA). Morphologically, the SiO appears to represent a disc-like structure.
Using parametric models we show that the position-velocity profile of the SiO
is consistent with the Keplerian rotation of a disc around an object between
10-30Mo in mass, only if there is also radial expansion from a separate
structure. The radial motion component can be interpreted as a disc wind from
the disc surface. Models with a central stellar object mass between 20 and 30Mo
are the most consistent with the stellar luminosity (100000 Lo) and indicative
of an O-type star. The H30a millimetre recombination line (231.9GHz) is also
detected, but spatially unresolved, and is indicative of a very compact, hot,
ionised region co-spatial with the dust continuum core. Accounting for all
observables, we suggest that G17.64 is consistent with a O-type young stellar
object in the final stages of protostellar assembly, driving a wind, but that
has not yet developed into a compact HII region. The existance and detection of
the disc in G17.64 is likely related to its isolated and possibly more evolved
nature, traits which may underpin discs in similar sources.
