Modelling Carbon Radio Recombination Line observation towards the Ultra-Compact HII region W48A

We model Carbon Recombination Line (CRL) emission from the Photo Dissociation Region (PDR) surrounding the Ultra-Compact (UC) HII region W48A. Our modelling shows that the inner regions ($A_V \sim 1$) of the CII layer in the PDR contribute significantly to the CRL emission. The dependence of line ratios of CRL emission with the density of the PDR and the far ultra-violet (FUV) radiation incident on the region is explored over a large range of these parameters that are typical for the environments of UCHII regions. We find that by observing a suitable set of CRLs it is possible to constrain the density of the PDR. If the neutral density in the PDR is high ($\gtrsim 10^7$ \cmthree) CRL emission is bright at high frequencies ($\gtrsim 20$ GHz), and absorption lines from such regions can be detected at low frequencies ($\lesssim 10$ GHz). Modelling CRL observations towards W48A shows that the UCHII region is embedded in a molecular cloud of density of about $4 \times$ 10$^7$ \cmthree.