Observational Evidence Linking Interstellar UV Absorption to PAH
  Molecules
  The 2175 \AA\ UV extinction feature was discovered in the mid-1960s, yet its
physical origin remains poorly understood. One suggestion is absorption by
Polycyclic Aromatic Hydrocarbons (PAH) molecules, which is supported by
theoretical molecular structure computations and by laboratory experiments.
PAHs are positively detected by their 3.3, 6.2, 7.7 8.6, 11.3 & 12.7 $\mu$m IR
emission bands, which are specified by their modes of vibration. A definitive
empirical link between the 2175 \AA\ UV extinction, and the PAH IR emission
bands, however, is still missing. We present a new sample of hot stars that
have both 2175 \AA\ absorption and PAH IR emission. We find significant shifts
of the central wavelength of the UV absorption feature, up to 2350 \AA, but
predominantly in stars that also have IR PAH emission. These UV shifts depend
on stellar temperature in a fashion that is similar to the shifts of the 6.2
and 7.7$\mu$m PAH IR bands, namely the features are increasingly more
red-shifted as the stellar temperature decreases, but only below $\sim 15$ kK.
Above 15 kK both UV and IR features retain their nominal values. Moreover, we
find a suggestive correlation between the UV and IR shifts. We hypothesize that
these similar dependences of both the UV and IR features on stellar temperature
hint to a common origin of the two in PAH molecules and may establish the
missing link between the UV and IR observations. We further suggest that the
shifts depend on molecular size, and that the critical temperature of $\sim 15$
kK above which no shifts are observed is related to the onset of UV driven
hot-star winds and their associated shocks.
