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

TL;DR

This study models Carbon Recombination Line emission from the PDR around the UCHII region W48A, demonstrating how line ratios can reveal PDR density and the region's physical conditions.

Contribution

It provides a detailed model linking CRL emission to PDR density and radiation, enabling better interpretation of observations around UCHII regions.

Findings

Inner CII layer significantly contributes to CRL emission.

CRL line ratios can constrain PDR density.

High-density PDRs produce bright high-frequency CRL emission.

Abstract

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.