Spitzer observations of extragalactic H II regions - III. NGC 6822 and
  the hot star, H II region connection
  Using the short-high module of the Infrared Spectrograph on the Spitzer Space
Telescope, we have measured the [S IV] 10.51, [Ne II] 12.81, [Ne III] 15.56,
and [S III] 18.71-micron emission lines in nine H II regions in the dwarf
irregular galaxy NGC 6822. These lines arise from the dominant ionization
states of the elements neon (Ne$^{++}$, Ne$^+$) and sulphur (S$^{3+}$,
S$^{++}$), thereby allowing an analysis of the neon to sulphur abundance ratio
as well as the ionic abundance ratios Ne$^+$/Ne$^{++}$ and S$^{3+}$/S$^{++}$.
By extending our studies of H II regions in M83 and M33 to the lower
metallicity NGC 6822, we increase the reliability of the estimated Ne/S ratio.
We find that the Ne/S ratio appears to be fairly universal, with not much
variation about the ratio found for NGC 6822: the median (average) Ne/S ratio
equals 11.6 (12.2$\pm$0.8). This value is in contrast to Asplund et al.'s
currently best estimated value for the Sun: Ne/S = 6.5. In addition, we
continue to test the predicted ionizing spectral energy distributions (SEDs)
from various stellar atmosphere models by comparing model nebulae computed with
these SEDs as inputs to our observational data, changing just the stellar
atmosphere model abundances. Here we employ a new grid of SEDs computed with
different metallicities: Solar, 0.4 Solar, and 0.1 Solar. As expected, these
changes to the SED show similar trends to those seen upon changing just the
nebular gas metallicities in our plasma simulations: lower metallicity results
in higher ionization. This trend agrees with the observations.
