Structure of the W3A Low Density Foreground Region

Abstract

We present analysis of OI 63 micron and CO $J$ = 5-4 and 8-7 multi-position 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 OI and $J$ = 5-4 CO lines. We employ the extinction to the exciting stars of the background HII region to constrain the total column density of the foreground gas. We have used the Meudon PDR code to model the physical conditions and chemistry in the region employing a two-component model with high density layer near the HII 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$\times$10${}^5$ cm${}^{-3}$ in the material near the HII 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 HII 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. (2021), 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.

References in corpus (7)

• Water maser motions in W3(OH) and a determination of its distance
• [CII] absorption and emission in the diffuse interstellar medium across the Galactic Plane
• The star-forming content of the W3 giant molecular cloud
• Herschel Observations of the W3 GMC: Clues to the Formation of Clusters of High-Mass Stars
• Self-absorption in [CII], ${}^{12}$CO, and HI in RCW120. Building up a geometrical and physical model of the region
• Carbon radio recombination lines from gigahertz to megahertz frequencies towards Orion A
• HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules -- I. Survey description and a first look toward W3(OH), W3 IRS5 and NGC 7538 IRS1