Azimuthal Metallicity Structure in the Milky Way Disk
  Elemental abundance patterns in the Galactic disk constrain theories of the
formation and evolution of the Milky Way. HII region abundances are the result
of billions of years of chemical evolution. We made radio recombination line
and continuum measurements of 21 HII regions located between Galactic azimuth
Az = 90-130 degree, a previously unexplored region. We derive the plasma
electron temperatures using the line-to-continuum ratios and use them as
proxies for the nebular [O/H] abundances, because in thermal equilibrium the
abundance of the coolants (O, N, and other heavy elements) in the ionized gas
sets the electron temperature, with high abundances producing low temperatures.
Combining these data with our previous work produces a sample of 90 HII regions
with high quality electron temperature determinations. We derive kinematic
distances in a self-consistent way for the entire sample. The radial gradient
in [O/H] is -0.082 +/- 0.014 dex/kpc for Az = 90-130 degree, about a factor of
two higher than the average value between Az = 0-60 degree. Monte Carlo
simulations show that the azimuthal structure we reported for Az = 0-60 degree
is not significant because kinematic distance uncertainties can be as high as
50% in this region. Nonetheless, the flatter radial gradients between Az = 0-60
degree compared with Az = 90-130 degree, are significant within the
uncertainty. We suggest that this may be due to radial mixing from the Galactic
Bar whose major axis is aligned toward Az ~30 degree.
