Evolution of prolate molecular clouds at H ii boundaries – II. Formation of BRCs of asymmetrical morphology

• Published in: Monthly notices of the Royal Astronomical Society Vol. 450; no. 1; pp. 1017 - 1031
• Main Authors: Kinnear, T. M., Miao, J., White, G. J., Sugitani, K., Goodwin, S.
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 11.06.2015
• Subjects: Astronomy; Asymmetry; Boundaries; Clouds; Density; Evolution; Fluid mechanics; Flux; Initial conditions; Ionizing radiation; Morphology; Simulation; Star & galaxy formation; stars: formation; regions; ISM: evolution; radiative transfer; ISM: kinematics and dynamics; H; hydrodynamics
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stv637

A systematic investigation on the evolution of a prolate cloud at an H ii boundary is conducted using smoothed particle hydrodynamics in order to understand the mechanism for a variety of irregular morphological structures found at the boundaries of various H ii regions. The prolate molecular clouds in this investigation are set with their semimajor axes at inclinations between 0° and 90° to a plane-parallel ionizing radiation flux. A set of four parameters, the number density n, the ratio of major to minor axis γ, the inclination angle φ and the incident flux F EUV, are used to define the initial state of the simulated clouds. The dependence of the evolution of a prolate cloud under radiation-driven implosion (RDI) on each of the four parameters is investigated. It is found that (i) in addition to the well-studied standard type A, B or C bright-rimmed clouds (BRCs), many other types such as asymmetrical BRCs, filamentary structures and irregular horse-head structures could also be developed at H ii boundaries with only simple initial conditions; (ii) the final morphological structures are very sensitive to the four initial parameters, especially to the initial density and the inclination; (iii) the previously defined ionizing radiation penetration depth can still be used as a good indicator of the final morphology. Based on the simulation results, the formation time-scales and masses of the early RDI-triggered star formation from clouds of different initial conditions are also estimated. Finally a unified mechanism for the various morphological structures found in many different H ii boundaries is suggested.