An In-Depth View of the Mid-Infrared Properties of Point Sources and the
  Diffuse ISM in the SMC Giant HII Region, N66
  (abridged)The focus of this work is to study mid-infrared point sources and
the diffuse interstellar medium (ISM) in the low-metallicity (~0.2 solar) giant
HII region N66 using the Spitzer Space Telescope's Infrared Spectrograph. We
study 14 targeted infrared point sources as well as spectra of the diffuse ISM
that is representative of both the photodissociation regions (PDRs) and the HII
regions. Among the point source spectra, we spectroscopically confirm that the
brightest mid-infrared point source is a massive embedded young stellar object,
we detect silicates in emission associated with two young stellar clusters, and
we observe spectral features of a known B[e] supergiant that are more commonly
associated with Herbig Be stars. In the diffuse ISM, we provide additional
evidence that the very small grain population is being photodestroyed in the
hard radiation field. The 11.3 um PAH complex emission exhibits an unexplained
centroid shift in both the point source and ISM spectra that should be
investigated at higher signal-to-noise and resolution. Unlike studies of other
regions, the 6.2 um and 7.7 um band fluxes are decoupled; the data points cover
a large range of I7.7/I11.3 PAH ratio values within a narrow band of I6.2/I11.3
ratio values. Furthermore, there is a spread in PAH ionization, being more
neutral in the dense PDR where the radiation field is relatively soft, but
ionized in the diffuse ISM/PDR. By contrast, the PAH size distribution appears
to be independent of local ionization state. Important to unresolved studies of
extragalactic low-metallicity star-forming regions, we find that emission from
the infrared-bright point sources accounts for only 20-35% of the PAH emission
from the entire region. These results make a comparative dataset to other
star-forming regions with similarly hard and strong radiation fields.
