Deuterated polycyclic aromatic hydrocarbons: Revisited
  The amount of deuterium locked up in polycyclic aromatic hydrocarbons (PAHs)
has to date been an uncertain value. We present a near-infrared (NIR)
spectroscopic survey of HII regions in the Milky Way, Large Magellanic Cloud
(LMC), and Small Magellanic Cloud (SMC) obtained with AKARI, which aims to
search for features indicative of deuterated PAHs (PAD or Dn-PAH) to better
constrain the D/H ratio of PAHs. Fifty-three HII regions were observed in the
NIR (2.5-5 {\mu}m), using the Infrared Camera (IRC) on board the AKARI
satellite. Through comparison of the observed spectra with a theoretical model
of deuterated PAH vibrational modes, the aromatic and (a)symmetric aliphatic
C-D stretch modes were identified. We see emission features between 4.4-4.8
{\mu}m, which could be unambiguously attributed to deuterated PAHs in only six
of the observed sources, all of which are located in the Milky Way. In all
cases, the aromatic C-D stretching feature is weaker than the aliphatic C-D
stretching feature, and, in the case of M17b, this feature is not observed at
all. Based on the weak or absent PAD features in most of the observed spectra,
it is suggested that the mechanism for PAH deuteration in the ISM is uncommon.
