Bipolar HII Regions produced by Colliding Clouds

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CONTEXT. When two interstellar clouds collide at high speed, they produce a shock-compressed layer, which becomes gravitationally unstable and fragments, producing a cluster of stars. If the cluster includes massive stars, their radiation excites an HII region (a bubble of hot ionised gas like the Orion Nebula), then they blow off powerful winds, and finally they explode as supernovae. The fact that the cluster is located in a layer of residual interstellar gas (i.e. a distribution of dense gas that is much thinner in one dimension than the other two) means that the the HII region quickly breaks out of the layer in the two directions perpendicular to the midplane of the layer, producing a bipolar HII region. At the same time, a second generation of stars may form at the edge of the HII region where the ionisation front pushes against the dense gas in the layer.PROJECT. This project will use numerical simulations to establish (i) the range of collision parameters that lead to the formation of shock compressed layers and massive star clusters; (ii) the dynamics and morphology of the resulting star clusters and of the HII regions they excite; (iii) the efficiency of star formation and the relation between first and second generation star formation; (iv) the external appearance of the layer and the HII region, and its dependence on viewing angle; and (v) the extent to which the layer is dispersed by the action of the HII region. These issues (i through v) are critical to understanding nearby star-formation regions and the evolution of disc galaxies like the Milky Way.SKILLS. The student will become expert in interstellar gas dynamics and the associated chemical and radiative processes, triggered star formation, and numerical hydrodynamics.