Evolution of Prolate Molecular Clouds at Hii Boundaries: II. Formation of BRCs of asymmetrical morphology

TL;DR

This study uses SPH simulations to explore how prolate molecular clouds evolve under ionizing radiation, revealing diverse morphologies including asymmetrical BRCs, and highlights the influence of initial conditions on final structures.

Contribution

It demonstrates that a variety of complex BRC morphologies can develop from simple initial conditions and identifies key parameters influencing these structures.

Findings

Asymmetrical BRCs and irregular structures can form at Hii boundaries.

Final morphology is highly sensitive to initial density and inclination.

Ionizing radiation penetration depth predicts final cloud shape.

Abstract

A systematic investigation on the evolution of a prolate cloud at an Hii boundary is conducted using Smoothed Particle Hydrodynamics (SPH) in order to understand the mechanism for a variety of irregular morphological structures found at the boundaries of various Hii regions. The prolate molecular clouds in this investigation are set with their semi-major axes at inclinations between 0 and 90 degrees to a plane parallel ionizing radiation flux. A set of 4 parameters, the number density n, the ratio of major to minor axis gamma, the inclination angle phi 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 Hii boundaries with only simple initial conditions; ii) the final morphological structures are very sensitive to the 4 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 efficiency of the RDI triggered star formation from clouds of different initial conditions is also estimated. Finally a unified mechanism for the various morphological structures found in many different Hii boundaries is suggested.