Structure of the W3A Low Density Foreground Region

We present analysis of [O i] 63 µm and CO J = 5–4 and 8–7 multi–position data in the W3A region 8 and use it to develop a model for the extended low–density foreground gas that produces absorption 9 features in the [O i] and J = 5–4 CO lines. We employ the extinction to the exciting stars of the 10 background H ii region to constrain the total column density of the foreground gas. We have used 11 the Meudon PDR code to model the physical conditions and chemistry in the region employing a 12 two–component model with high density layer near the H ii region responsible for the fine structure 13 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 5×105 cm−3 14 in the 15 material near the H ii region. The absorption is distributed over the region mapped in W3A, and is not 16 restricted to the foreground of either the embedded exciting stars of the H ii region or the protostar 17 W3 IRS5. The low–density material associated with regions of massive star formation, based on an 18 earlier study by Goldsmith et al. (2021), is quite common, and we now see that it is extended over 19 a significant portion of W3A. It thus should be included in modeling of fine structure line emission, 20 including interpreting low–velocity resolution observations made with incoherent spectrometer systems, 21 in order to use these lines as accurate tracers of massive star formation.