Radio Recombination Line Modelling of the Cometary Ultra-Compact HII Region DR21

This thesis presents the inaugural hydrodynamic modelling of cometary ultra-compact (UC) HII regions, with a direct comparison to observational kinematic data. The hydrodynamic models were applied to the cometary UCHII region DR21, offering the most precise comparison to date of the velocity structure of such regions. In these models, the star is offset from the cloud core, incorporating stellar winds and parameters specific to DR21. Through logical deduction, the density power law index value, α, was determined to be 1.0, the distance from the star to the cloud core, Rs, was 0.07 pc, and the initial number density at the star location, no, was 1.28 × 105 cm−3. The thermal dust continuum emission from DR21 at 450μm was modelled to find the best-fit value for the density power law index. By overlaying and comparing the radio and dust continuum emissions of both the observed DR21 and the models, the distance between the star and the cloud core, Rs, was further validated. The cometary DR21 was determined to be 5 kyr old by comparing the model size to observations. The spectral energy distribution (SED) of the model and its turnover frequency fit the required density. The RRL radiative transfer code, developed by Steggles, was tested, modified, and compared to DR21 UCHII region observations. After identifying and correcting bugs in the code, the emission measure values obtained were consistent with literature test cases. Tests for expanding and contracting ionised gas under LTE and non-LTE conditions aligned well with previous literature results. The modified RRL radiative code modelled the radio recombination lines of the cometary UCHII region DR21, focusing on its velocity structure. Moment maps, position velocity diagrams, and line profiles for both the observed DR21 and the models were examined within the context of DR21’s cometary morphology. A comparison of the velocity structure for the observed DR21 and models projected at 30◦, 45◦ and 60◦ showed reasonable agreement in the main body of the UCHII region, albeit with a ≈ 5 kms−1. This highlights the necessity for accurate stellar motion representation and support for the hybrid picture.