Spatial variations of PAH properties in M17SW revealed by Spitzer/IRS
  spectral mapping
  We present $Spitzer$/IRS mid-infrared spectral maps of the Galactic
star-forming region M17 as well as IRSF/SIRIUS Br$\gamma$ and Nobeyama
45-m/FOREST $^{13}$CO ($J$=1--0) maps. The spectra show prominent features due
to polycyclic aromatic hydrocarbons (PAHs) at wavelengths of 6.2, 7.7, 8.6,
11.3, 12.0, 12.7, 13.5, and 14.2 $\mu$m. We find that the PAH emission features
are bright in the region between the HII region traced by Br$\gamma$ and the
molecular cloud traced by $^{13}$CO, supporting that the PAH emission
originates mostly from photo-dissociation regions. Based on the
spatially-resolved $Spitzer$/IRS maps, we examine spatial variations of the PAH
properties in detail. As a result, we find that the interband ratio of PAH 7.7
$\mu$m/PAH 11.3 $\mu$m varies locally near M17SW, but rather independently of
the distance from the OB stars in M17, suggesting that the degree of PAH
ionization is mainly controlled by local conditions rather than the global UV
environments determined by the OB stars in M17. We also find that the interband
ratios of the PAH 12.0 $\mu$m, 12.7 $\mu$m, 13.5 $\mu$m, and 14.2 $\mu$m
features to the PAH 11.3 $\mu$m feature are high near the M17 center, which
suggests structural changes of PAHs through processing due to intense UV
radiation, producing abundant edgy irregular PAHs near the M17 center.
