Surveying the Giant HII Regions of the Milky Way with SOFIA: IV. Sgr D,
  W42, and a Reassessment of the Giant HII Region Census
  This is the fourth paper exploring the infrared properties of giant HII
regions with the FORCAST instrument on the Stratospheric Observatory For
Infrared Astronomy (SOFIA). Our survey utilizes the census of 56 Milky Way
giant HII regions identified by Conti & Crowther (2004), and in this paper we
present the 20 and 37 micron imaging data we have obtained from SOFIA for
sources Sgr D and W42. Based upon the SOFIA data and other multi-wavelength
data, we derive and discuss the detailed physical properties of the individual
compact sources and sub-regions as well as the large scale properties of Sgr D
and W42. However, improved measurements have revealed much closer distances to
both regions than previously believed, and consequently both sources are not
powerful enough to be considered giant HII regions any longer. Motivated by
this, we revisit the census of giant HII regions, performing a search through
the last two decades of literature to update each source with the most recent
and/or most accurate distance measurements. Based on these new distance
estimates, we determine that 14 sources in total (25%) are at sufficiently
reliable and closer distances that they are not powerful enough to be
considered giant HII regions. We briefly discuss the observational and physical
characteristics specific to Sgr D and W42 and show that they have properties
distinct from the giant HII regions previously studied as a part of this
survey.
