Sagittarius B1 -- A Patchwork of H II Regions and PhotoDissociation Regions

TL;DR

Sgr B1 is a complex, patchwork of multiple H II regions and PDRs with widely dispersed ionizing stars, lacking a central cluster, and characterized by diverse geometries and physical conditions.

Contribution

This study provides new insights into the structure and physical conditions of Sgr B1 using SOFIA FIFI-LS observations and modeling, revealing its fragmented nature and the absence of a central star cluster.

Findings

Sgr B1 consists of multiple smaller H II regions and PDRs with varied orientations.

The ionizing stars are widely dispersed, not forming a central cluster.

Pressure equilibrium is unlikely without additional turbulent or magnetic pressure contributions.

Abstract

Sgr B1 is a luminous H II region in the Galactic Center immediately next to the massive star-forming giant molecular cloud Sgr B2 and apparently connected to it from their similar radial velocities. In 2018 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 must be widely spread throughout the region. Here we present SOFIA FIFI-LS observations of the [O I] 146 and [C II] 158 micron lines formed in the surrounding photodissociation regions (PDRs). We find that these lines correlate neither with each other nor with the [O III] lines although together they correlate better with the 70 micron Herschel PACS images from Hi-GAL. We infer from this 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. We used the PDR Toolbox to estimate densities and the far-ultraviolet intensities exciting the PDRs. Using models computed with Cloudy, we demonstrate possible appearances of edge-on PDRs and show that the density difference between the PDR densities and the electron densities estimated from the [O III] line ratios is incompatible with pressure equilibrium unless there is a substantial pressure contribution from either turbulence or magnetic field or both. We likewise conclude that the hot stars exciting Sgr B1 are widely spaced throughout the region at substantial distances from the gas with no evidence of current massive star formation.