AI 中文总结
该研究通过数值实验探究云-云碰撞是否为双极电离氢区及多数电离氢区的成因，明确了OB星与电离氢区的形成机制及观测判别指标。
AI 中文摘要
我们通过数值实验探究两种可能性：(i) 双极电离氢区（H II Regions）是云-云碰撞（Cloud-Cloud Collisions，CCCs）的结果；(ii) 若考虑此类碰撞的混沌特性、双极阶段的短持续时间及不同观测视角，所有电离氢区中很大比例可能是云-云碰撞的产物。为减少参数空间，我们的实验仅考虑两个质量为500倍太阳质量（$500$ M$_\bigodot$）的云之间的对撞，设置三种不同湍流水平和两种不同碰撞速度；碰撞速度定义了“碰撞轴”。在所有实验中，OB星仅在垂直于碰撞轴的激波压缩层形成并碎裂产生枢纽丝系统后才凝聚，OB星在枢纽中形成。OB星的电离辐射激发电离氢区，该区域倾向于在接近碰撞轴的方向更快膨胀，在垂直于碰撞轴的方向膨胀更慢，因为该方向会遇到激波压缩层的致密气体。因此，若从与碰撞轴足够大的角度观测，电离氢区在演化过程的短时间内可能呈现双极形态；从较小角度观测时，其腰部呈现亮边缘，与传统近似球形的电离氢区类似。在此情况下，还有其他指标——基于弥散自由-自由发射的范围、复合射电谱线的速度弥散、中红外波长的尘埃发射——可指示云-云碰撞的产物，并确立云-云碰撞是大质量恒星形成的主要触发因素。
英文摘要
We use numerical experiments to explore two possibilities: (i) that Bipolar H II Regions are the result of Cloud-Cloud Collisions (CCCs), and (ii) that -- when allowance is made for the chaotic nature of such collisions, the short duration of the bipolar phase, and different viewing angles -- a large proportion of all H II Regions might be the aftermath of CCCs. To reduce the parameter space, our experiments only consider head-on collisions between two $500$ M$_\odot$ clouds, with three different levels of turbulence, and two different collision velocities; the collision velocities define the `collision axis'. In all experiments, OB stars only condense out after a Shock-Compressed Layer has formed (perpendicular to the collision axis), and fragmented to produce a Hub Filament System, with the OB stars forming in the Hub. Ionising radiation from the OB stars excites an H II Region, which tends to expand more rapidly in directions close to the collision axis, and more slowly in directions orthogonal to the collision axis, where it encounters the dense gas of the Shock-Compressed Layer. Consequently, the H II Region may appear bipolar, for a short period during its evolution, if observed at sufficiently large angle to the collision axis. Viewed from smaller angles, the waist appears as a Bright-Rim, similar to conventional approximately spherical H II Regions. Under this circumstance, there are other metrics -- based on the extent of diffuse freefree emission, the velocity dispersion of Radio Recombination Lines, dust emission at mid-infrared wavelengths -- that might indicate the aftermath of a CCC, and establish CCCs as a dominant trigger for high-mass star formation.
Comments18 pages, 13 figures, 1 table, submitted to MNRAS
