Interpreting observations of edge-on gravitationally unstable accretion flows

The case of G10.6-0.4

European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
e-mail: baobabyoo@gmail.com

Received: 25 August 2016
Accepted: 21 September 2016

Abstract

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.