Optical and infrared emission of H II complexes as a clue to the PAH
  life cycle
  We present an analysis of optical spectroscopy and infrared aperture
photometry of more than 100 H II complexes in nine galaxies. Spectra obtained
with the 6-m telescope of SAO RAS are used along with archival data from
Spitzer and several ground-based telescopes to infer a strength of polycyclic
aromatic hydrocarbon (PAH) emission, age, properties of the UV radiation field,
and metallicity of studied H II complexes. Physical properties (age, radiation
field parameters, metallicity) are related to the $F_{8}/F_{24}$ ratio used as
a proxy for the PAH abundance in order to reveal factors that may influence the
PAH evolution in H II complexes. The well-known correlation between the
$F_{8}/F_{24}$ ratio and metallicity is confirmed in the studied complexes. The
infrared flux ratio also correlates with the [O III]$\lambda
5007/\mathrm{H\beta}$ ratio which is often considered as an indicator of the
radiation field hardness, but this correlation seems to be a mere reflection of
a correlation between [O III]$\lambda 5007/\mathrm{H\beta}$ and metallicity. In
separate metallicity bins, the $F_{8}/F_{24}$ ratio is found to correlate with
an age of an H II complex, which is estimated from the equivalent width of
$\mathrm{H}\beta$ line. The correlation is positive for low metallicity
complexes and negative for high metallicity complexes. Analysing various
mechanisms of PAH formation and destruction in the context of found
correlations, we suggest that PAH abundance is likely altered by the UV
radiation within H II complexes, but this is not necessarily due to their
destruction. If PAHs can also form in H II complexes due to some processes like
aromatisation, photodestruction, shattering and sputtering of very small
grains, the net $F_{8}/F_{24}$ ratio is determined by a balance between all
these processes that can be different at different metallicities.
