SOFIA Elucidates the Structure of the Galactic Center H II Region/PDR Sgr B1

Sgr B1 is the luminous H II region in the Galactic Center just south of the massive giant molecular cloud Sgr B2. The two sources are thought to be connected due to their similar radial velocities, even though Sgr B2, which currently has the highest star formation rate in the Galaxy, appears to be much younger. Earlier, we showed from SOFIA FIFI-LS observations of the [O III] 52 and 88 micron lines that there is no central exciting star cluster and that the ionizing stars, which are evolved with ages of order several million years, must be widely spread throughout the region (Simpson et al. 2018). Here we present SOFIA FIFI-LS observations of the [C II] 158 and [O I] 146 micron lines formed in the Sgr B1 photo-dissociation regions (PDRs). We find that the intensities of these lines correlate better with the 70 micron Herschel PACS images from Hi-GAL then they do with the [O III] line intensities. From the relative distributions of the different line morphologies, we infer that Sgr B1 consists of a number of smaller H II regions plus their associated PDRs, some seen face-on and the others seen more or less edge-on. Using the PDR Toolbox (Pound & Wolfire 2008; Kaufman et al. 2006), we estimated the gas densities and the far-ultraviolet intensities exciting the PDRs. We also computed models with Cloudy that demonstrate possible appearances of edge-on PDRs. The results show that the density difference between the PDR densities and the electron densities estimated from the [O III] 52/88 micron line ratios is incompatible with pressure equilibrium unless there is a substantial pressure contribution from either turbulence or magnetic field or both. We also discuss possible feedback from the massive O supergiant and Wolf-Rayet stars that have been identified in the Sgr B1 region and show that these could be some of the evolved stars exciting the gas and heating the dust. From the low densities of the PDR (as well as the H II region) and the lack of significant connection to the cold dust and molecular gas of the GC, we suggest that the ionizing stars of Sgr B1 represent a currently dispersing OB association.