Integrated approach to the PDR/H II complex by the application of carbon radio recombination lines in the millimetre range and fine-structure lines of C II and O I

Abstract

We have performed self-consistent calculations to estimate the physical parameters of photodissociation regions (PDRs) associated with objects, namely, NGC 2024, Orion A and W3, using far-infrared continuum emission, fine-structure lines of C II and O I, and radio recombination lines of carbon. Typically, PDRs separate H II regions from the molecular cloud; therefore, necessary corrections for the contribution to C II line emission due to the H II region are made. For that purpose, using observational data, theoretical calculations are performed to obtain the best fit for the said observations. Three parameters, angular size, θ (in arcminutes), far-ultraviolet radiation field G₀, and hydrogen density n_H (which gives electron density and temperature), are varied, and the sets of parameters (G₀ and n_H) obtained for the NGC 2024, Orion A and W3 PDRs are (7.6 × 10⁴ and 1.2 × 10⁵ cm^-3), (2.8 × 10⁵ and 2.3 × 10⁵ cm^-3) and (3.7 × 10⁵ and 1.9 × 10⁵ cm^-3), respectively. The relationship between line and continuum emissions from PDRs associated with H II regions leads us to conclude that exciting stars for the NGC 2024, Orion A and W3 H II regions are O8-O9V, O6-O7V and O5-O6V, respectively.