A Galactic Plane Defined by the Milky Way HII Region Distribution
  We develop a framework for a new definition of the Galactic midplane,
allowing for tilt (rotation about Galactic azimuth 90deg.), and roll (rotation
about Galactic azimuth 0deg.) of the midplane with respect to the current
definition. Derivation of the tilt and roll angles also determines the solar
height above the midplane. Here we use nebulae from the WISE Catalog of
Galactic HII Regions to define the Galactic high-mass star formation (HMSF)
midplane. We analyze various subsamples of the WISE catalog and find that all
have Galactic latitude scale heights near 0.30deg. and z-distribution scale
heights near 30pc. The vertical distribution for small (presumably young) HII
regions is narrower than that of larger (presumably old) HII regions (~25pc
versus ~40pc), implying that the larger regions have migrated further from
their birth sites. For all HII region subsamples and for a variety of fitting
methodologies, we find that the HMSF midplane is not significantly tilted or
rolled with respect to the currently-defined midplane, and therefore the Sun is
near to the HMSF midplane. These results are consistent with other studies of
HMSF, but are inconsistent with many stellar studies, perhaps due to
asymmetries in the stellar distribution near the Sun. Our results are sensitive
to latitude restrictions, and also to the completeness of the sample,
indicating that similar analyses cannot be done accurately with less complete
samples. The midplane framework we develop can be used for any future sample of
Galactic objects to redefine the midplane.
