Tracing interactions between HII regions and their natal molecular clouds
Feedback from young, high mass stars causes violent disturbances in the surrounding gas of their parent clouds. These processes are critically important to the evolution of the interstellar medium, yet are poorly constrained. In this 10-month thesis we aim to study the kinematics of gas internal and external to HII regions to provide much needed physical constraints on these environments and dynamical interactions. To achieve this we use VLA observations of three Galactic HII regions, G049.205-0.343, G034.256+0.145 and G024.471+0.492, in multiple hydrogen radio recombination lines (RRL), broad-band continuum emission, and the four ground-state 18-cm OH lines. In particular, we draw on recent work which suggests that the peculiar OH spectral feature known as the OH satellite-line ‘flip’ may trace a shock front from HII region thermal expansion moving through the surrounding molecular cloud.
We design and apply our own Gaussian decomposition code to map the complex OH emission and absorption across our targets, finding that the satellite-line ‘flip’ occurs in 2/3 sources, G049.205-0.343 and G034.256+0.145. Drawing on auxiliary CO data, we use CO line ratios as crude tracers of the molecular gas conditions, and investigate the broader parent clouds of the sources in the context of the ‘flip’ model, but the results are inconclusive.
We then derive basic physical properties of the ionised and molecular gas and construct a simple HII region model to further evaluate whether the ‘flip’ model can explain the observed spectra at each HII region. We find that the estimated molecular hydrogen densities and OH column densities of the sources are consistent with the physical conditions expected to produce the observed OH spectra, lending support to the ‘flip’ model.
Finally, from investigation of the morphological and spectral characteristics of the ionised and molecular gas, we find that G049.205-0.343 and G034.256+0.145 are well described as ‘champagne flow’ HII regions. We also suggest that the OH from G024.471+0.492, without the ‘flip’, traces separate molecular gas structures, not shocked and unshocked gas. In future, excitation modelling of the observed spectra will help to constrain the physical parameters of the gas and to better constrain the environments of the HII regions.
