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 nH (which gives electron density and temperature), are varied, and the sets of parameters (G0 and nH) 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.

The ionization fraction in OMC-2 and OMC-3

“…The lines observed from this cascade are known as carbon radio recombination lines (CRRLs, Gordon & Sorochenko 2009), and along with the far-infrared line of C + at 158 µm, [CII], they are important tracers of PDRs. Since the intensities of [CII] and CRRLs have different dependencies on the gas temperature and density, their combined use is a powerful probe of the gas properties in a PDR (e.g., Natta et al 1994;Sorochenko & Tsivilev 2010;Tsivilev 2014;Salgado et al 2017b;Cuadrado et al 2019;Salas et al 2019;Siddiqui et al 2020), such as the envelopes of molecular clouds.…”