摘要：Context. 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 ~10%).
Aims. 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) Hii
region, address the gravitational stability of the accreting gas, and outline the
observed potential signatures of gravitational instability.
Methods. The dense gas accretion flow around the central UC Hii
region in G10.6-0.4 is geometrically flattened, and is in an approximately edge-on
projection. 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.
Results. We find 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. We propose that the morphology of dense
gas structures 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 is likely,
however, to be in an approximately face-on projection.
Conclusions. 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 localized gas condensations. The proposed approaches for
data analyses may be applied to the observations of Class 0/I low-mass protostars, in
diagnosis of disk gravitational instability.
关键词：stars: formation;ISM: jets and outflows;ISM: structure
