Observational studies on the Near-Infrared Unidentified Emission Bands
  in Galactic HII regions
  Using a large collection of near-infrared spectra (2.5-5.4 um) of Galactic
HII regions and HII region-like objects, we perform a systematic investigation
of the astronomical polycyclic aromatic hydrocarbon (PAH) features. 36 objects
were observed by the use of the infrared camera onboard the AKARI satellite as
a part of a directer's time program. In addition to the well-known 3.3-3.6 um
features, most spectra show a relatively-weak emission feature at 5.22 um with
sufficient signal-to-noise ratios, which we identify as the PAH 5.25 um band
previously reported. By careful analysis, we find good correlations between the
5.25 um band and both the aromatic hydrocarbon feature at 3.3 um and the
aliphatic ones at around 3.4-3.6 um. The present results give us convincing
evidence that the astronomical 5.25 um band is associated with C-H vibrations
as suggested by previous studies and show its potential to probe the PAH size
distribution. The analysis also shows that the aliphatic to aromatic ratio of
I(3.4-3.6)/I(3.3) decreases against the ratio of the 3.7 um continuum intensity
to the 3.3 um band, I(3.7 cont)/I(3.3), which is an indicator of the ionization
fraction of PAHs. The mid-infrared color of I(9)/I(18) also declines steeply
against the ratio of the hydrogen recombination line Bralpha at 4.05 um to the
3.3 um band, I(Bralpha)/I(3.3). These facts indicate possible dust processing
inside or at the boundary of ionized gas.
