2021ApJ...916....6G

We investigate the origin of self-absorption in [O I] 63 µm line emission, which is very clearly seen in approximately half of the 12 Galactic giant molecular cloud (GMC)/H II regions observed. For this study, we observed velocity-resolved spectra of photon-dominated region (PDR) and H II region tracers, the [O I] 63 µm, [N II] 205 µm, and CO J = 5-4 and 8-7 lines, with the upGREAT instrument in the 4GREAT configuration on the NASA/DLR Stratospheric Observatory For Infrared Astronomy (SOFIA). To probe the origin of the [O I] absorption and line shape and what they tell us about the physical conditions, we focus on the W3 region, for which we obtained data for eight positions along a line near the H II region W3 A. We derive the foreground column density of low-excitation atomic oxygen to be in the range 2-7 x 10¹⁸ cm^–2. At the position of strongest [O I] emission and greatest absorbing column density, 24% of the oxygen in the PDR is in the form of low-excitation atomic oxygen. We employ the Meudon PDR code to study the chemical and thermal structure of the PDR and to understand the large column density of neutral oxygen throughout the PDR. The reduction in the integrated intensity of the [O I] 63 µm emission is a factor of ≃2-4 in directions with strong [O I] emission. The results from our sample, if general, would significantly impact the use of the [O I] 63 µm line as a tracer of massive star formation and could play a significant role in explaining the "63 µm [O I] deficit" seen in very luminous extragalactic sources.