SOFIA FIFI-LS Observations of Sgr B1: Ionization Structure and Sources
  of Excitation
  The current paradigm of Galactic Center (GC) gas motions and star formation
envisions sequential star formation in streams of gas as they pass near the
supermassive black hole, Sgr A*. This is based on the relative positions of
dense molecular clouds, the very young star-forming region Sgr B2, the much
older region Sgr C, and the several Myr old Arches and Quintuplet Clusters.
Because Sgr B1 is found with Sgr B2 in a common envelope of molecular gas and
far-infrared emission, the two sources are thought to be physically related,
even though there are indicators of a significantly greater age for Sgr B1. To
clarify the status of Sgr B1, we have mapped it with the FIFI-LS spectrometer
on SOFIA in the far-infrared lines of [O III] 52 and 88 micron. From the ratios
of these lines and lines measured with the Spitzer Infrared Spectrograph, we
find that there are at least eight separate sub-regions that must contain the
stars that excite the gas. We infer spectral energy distributions (SEDs) of the
ionizing sources from models and find they are in agreement only with SEDs of
late O stars augmented at the highest frequencies with interstellar X-rays from
fast shocks. We suggest that although the gas, from its velocity structure,
must be part of the very young Sgr B2 complex, the stars that are ionizing the
gas were not formed there but are the remnants of a previous generation of star
formation in the GC.
