On the ALMA observability of nascent massive multiple systems formed by
  gravitational instability
  Massive young stellar object (MYSOs) form during the collapse of high-mass
pre-stellar cores, where infalling molecular material is accreted through a
centrifugally-balanced accretion disc that is subject to efficient
gravitational instabilities. In the resulting fragmented accretion disc of the
MYSO, gaseous clumps and low-mass stellar companions can form, which will
influence the future evolution of massive protostars in the Hertzsprung-Russell
diagram. We perform dust continuum radiative transfer calculations and compute
synthetic images of disc structures modelled by the
gravito-radiation-hydrodynamics simulation of a forming MYSO, in order to
investigate the Atacama Large Millimeter/submillimeter Array (ALMA)
observability of circumstellar gaseous clumps and forming multiple systems.
Both spiral arms and gaseous clumps located at ~a few 100 au from the protostar
can be resolved by interferometric ALMA Cycle 7 C43-8 and C43-10 observations
at band 6 (1.2 mm), using a maximal 0.015" beam angular resolution and at least
10-30 min exposure time for sources at distances of 1-2 kpc. Our study shows
that substructures are observable regardless of their viewing geometry or can
be inferred in the case of an edge-viewed disc. The observation probability of
the clumps increases with the gradually increasing efficiency of gravitational
instability at work as the disc evolves. As a consequence, large discs around
MYSOs close to the zero-age-main-sequence line exhibit more substructures than
at the end of the gravitational collapse. Our results motivate further
observational campaigns devoted to the close surroundings of the massive
protostars S255IR-NIRS3 and NGC 6334I-MM1, whose recent outbursts are a
probable signature of disc fragmentation and accretion variability.
