Interpreting observations of edge-on gravitationally unstable accretion
  flows
  Gravitational collapse of molecular cloud or cloud core/clump may lead to the
formation of geometrically flattened, rotating accretion flow surrounding the
new born star or star cluster. Gravitational instability may occur in such
accretion flow when the gas to stellar mass ratio is high (e.g. over
$\sim$10\%). This paper takes the OB cluster-forming region G10.6-0.4 as an
example. We introduce the enclosed gas mass around its central ultra compact
(UC) H\textsc{ii} region, addresses the gravitational stability of the
accreting gas, and outline the observed potential signatures of gravitational
instability. The position-velocity (PV) diagrams of various molecular gas
tracers on G10.6-0.4 consistently show asymmetry in the spatial and the
velocity domain. We deduce the morphology of the dense gas accretion flow by
modeling velocity distribution of the azimuthally asymmetric gas structures,
and by directly de-projecting the PV diagrams. We found that within the 0.3 pc
radius, an infall velocity of 1-2 km\,s$^{-1}$ may be required to explain the
observed PV diagrams. In addition, the velocity distribution traced in the PV
diagrams can be interpreted by spiral arm-like structures, which may be
connected with exterior infalling gas filaments. The morphology of dense gas
structures we propose appears very similar to the spatially resolved gas
structures around the OB cluster-forming region G33.92+0.11 with similar gas
mass and size, which however is likely to be approximately in a face-on
projection. The dense gas accretion flow around G10.6-0.4 appears to be Toomre
unstable, which is consistent with the existence of large-scale spiral arm-like
structures, and the formation of localize gas condensations.
