Interpreting observations of edge-on gravitationally unstable accretion flows

TL;DR

This study analyzes edge-on gravitationally unstable accretion flows in a star-forming region, using observations and modeling to reveal spiral structures and instability signatures that influence star formation.

Contribution

It introduces a detailed analysis of gravitational instability in accretion flows around OB star-forming regions, combining observational PV diagrams with modeling to interpret gas morphology.

Findings

PV diagrams show asymmetry indicating complex gas motions.

Accretion flow within 0.3 pc exhibits infall velocities of 1-2 km/s.

Flow appears Toomre unstable with spiral structures.

Abstract

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.