FEEDBACK from the NGC7538 HII region
  Context: How do expanding HII regions interact with their environmental
cloud? This is one of the central questions driving the SOFIA legacy program
FEEDBACK. Here, we present a case study toward the prototypical H{\sc ii}
region NGC7538. Methods: With SOFIA we mapped an area of ~210'^2 around NGC7538
in the [CII] line at 1.9THz. Complementary observed atomic carbon [CI] and
high-J CO(8-7) data as well as archival NIR/FIR, cm continuum, CO(3-2) and HI
data are folded into the analysis. Results: While the overall [CII] morphology
follows the general ionized gas, the channel maps show multiple bubble-like
structures with sizes on the order of ~80-100" (~1.0-1.28pc). While at least
one of them may be an individual feedback bubble driven by the main exciting
sources of the region, the other bubble-morphologies may also be due to the
intrinsically porous structure of the HII region. An analysis of the expansion
velocities around 10km s^{-1} indicates that thermal expansion is not
sufficient but that wind-driving from the central O-stars is required. The most
blue-shifted [CII] component has barely any molecular or atomic counterparts.
At the interface to the molecular cloud, we find a typical photon-dominated
region (PDR) with a bar-shape. Ionized, atomic and molecular carbon show a
layered structure in this PDR. The carbon in the PDR is dominated by its
ionized form with atomic and molecular masses of ~0.45+-0.1M_{\odot} and
~1.2+-0.1M_{\odot}, respectively, compared to the ionized carbon in the range
of 3.6-9.7M_{\odot}. Conclusions: The NGC7538 HII region exhibits a diverse set
of sub-structures that interact with each other as well as with the adjacent
cloud. Compared to other recent [CII] observations of HII regions (e.g., Orion
Veil, RCW120, RCW49), bubble-shape morphologies revealed in [CII] emission,
indicative of expanding shells, are recurring structures of PDRs.
