Structure of the W3A Low-density Foreground Region

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.