Structure of the W3A Low-density Foreground Region

• Published in: The Astrophysical journal Vol. 952; no. 2; pp. 102 - 120
• Main Authors: Goldsmith, Paul. F., Langer, William D., Seo, Youngmin, Pineda, Jorge, Stutzki, Jürgen, Guevara, Christian, Aladro, Rebeca, Justen, Matthias
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.08.2023; IOP Publishing
• Subjects: Absorption; Astrophysics; Emission; Fine structure; Gas absorption; Giant molecular clouds; Low density materials; Modelling; Photodissociation; Photodissociation regions; Proton density (concentration); Star & galaxy formation; Star formation; Stars; Submillimeter astronomy; Tracers
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/acd842

We present analysis of [O i ] 63 μ m and CO J = 5 − 4 and 8 − 7 multiposition data in the W3A region and use it to develop a model for the extended low-density foreground gas that produces absorption features in the [O i ] and J = 5 − 4 CO lines. We employ the extinction to the exciting stars of the background H ii region to constrain the total column density of the foreground gas. We have used the Meudon photodissociation region code to model the physical conditions and chemistry in the region employing a two-component model with a high-density layer near the H ii region responsible for the fine-structure line emission and an extended low-density foreground layer. The best-fitting total proton density, constrained largely by the CO lines, is n (H) = 250 cm −3 in the foreground gas and 5 × 10 5 cm −3 in the material near the H ii region. The absorption is distributed over the region mapped in W3A and is not restricted to the foreground of either the embedded exciting stars of the H ii region or the protostar W3 IRS5. The low-density material associated with regions of massive-star formation, based on an earlier study by Goldsmith et al., is quite common, and we now see that it is extended over a significant portion of W3A. It thus should be included in modeling of fine-structure line emission, including interpreting low-velocity-resolution observations made with incoherent spectrometer systems, in order to use these lines as accurate tracers of massive-star formation.