Bipolar HII regions Produced by Cloud-Cloud Collisions
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.