Ultra-Compact HII Regions

In this thesis I have studied hydrodynamical models of cometary HII regions and distributions of UCHII regions in simulated surveys, for comparison with CORNISH. I present the numerical method used to model the evolution of cometary HII regions produced by ZAMS stars of O and B spectral types, which are driving strong winds and are born off-centre from spherically symmetric cores with power-law (alpha = 2) density slopes. A model parameter grid was produced that spans stellar mass, age and core density. Exploring this parameter space I investigated limb-brightening, a feature commonly seen in cometary HII regions. It was found that all of the models produce this feature. The models have a cavity, bounded by a contact discontinuity separating hot shocked wind and ionised ambient gas, that is similar in size to the surrounding HII region. Due to early pressure confinement, shocks outside of the contact discontinuity were not seen, but the cavities were found to continue to grow. The cavity size in each model plateaus as the expansion of the HII region stagnates, which could be due to the Kelvin-Helmholtz instabilities at the interface mixing in cooler gas. SEDs of the models are similar to those from identical stars evolving in uniform density fields. The turn-over frequency is lower in the power-law models due to a higher proportion of low density gas covered by the HII regions. Following from this I have simulated CORNISH surveys for stars, varying the local density at the location of stellar birth. I have shown that the models used can reproduce the observed size and flux distributions in the CORNISH survey. Higher density environments generally lead to better fits to the observed size and flux distributions. A good match between the overall number of UCHII regions in the simulated surveys for a SFR = 1.5 solar masses per year can be achieved if it is considered that stars are born in a distribution of local densities rather than a single density.