Bipolar HII Regions produced by Colliding Clouds

上下文。当两个星际云高速碰撞时，它们会产生一个激波压缩层，这个层在引力上变得不稳定并破碎，从而产生一个星团。如果星团中包含大质量恒星，它们的辐射会激发一个HII区域（一个像猎户座星云一样的热电离气体气泡），然后它们会吹出强风，最后它们会爆炸成超新星。星团位于残留星际气体层（即密度较大的气体分布，在一个维度上比另外两个维度薄得多）的事实意味着，HII区域在垂直于该层中间层的两个方向上迅速冲出该层，产生双极HII区域。与此同时，第二代恒星可能会在HII区域的边缘形成，在那里电离锋会推动致密气体层。该项目将使用数值模拟来确定(i)导致激波压缩层和大质量星团形成的碰撞参数范围；（ii）由此产生的星团及其激发的HII区域的动力学和形态学；（iii）恒星形成的效率以及第一代和第二代恒星形成的关系；（iv）层和HII区域的外观及其与视角的依赖关系；(v)受HII区域作用的地层分散程度。这些问题（i到v）对于理解附近的恒星形成区域和像银河系这样的盘状星系的演化至关重要。该学生将成为星际气体动力学和相关的化学和辐射过程、触发恒星形成和数值流体动力学方面的专家。

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.