Orion Bar as a window to the evolution of PAHs
  We investigate the mid-infrared (IR) emission in the Orion Bar
photodissociation region, using archival photometric and spectroscopic
observations from UKIRT, Spitzer, ISO, and SOFIA telescopes. Specifically, we
consider flux densities of the emission bands at 3.3, 3.4, 3.6, 6.6, 7.7,
11.2~$\mu$m in several locations and a spectrum from 3 to 45~$\mu$m in one
location. We study the behaviour of band flux ratios, which are sensitive to
external conditions, as revealed by their variations with the distance from an
ionizing source. Assuming that the mid-IR emission arises mostly from
polycyclic aromatic hydrocarbons (PAHs), and that a weak emission feature at
3.4~$\mu$m is related to PAHs with extra hydrogen atoms (H-PAHs), we trace
variations of the ratios using a model for PAH evolution. Namely, we estimate
how populations of PAHs of different sizes, hydrogenation and ionization states
change across the Orion Bar over a time interval approximately equal to its
lifetime. The obtained ensembles of PAHs are further used to calculate the
corresponding synthetic spectra and band flux densities. The model
satisfactorily describes the main features of the ratios $I_{3.6}/I_{11.2}$,
$I_{7.7}/I_{11.2}$, $I_{7.7}/I_{3.6}$ and $I_{3.3}/I_{3.4}$. We conclude that
the best coincidence between modelling and observations is achieved if C loss
of PAHs is limited by the number of carbon atoms $N_{\rm C}=60$, and the band
at 3.4~$\mu$m may indeed be attributed to H-PAHs. We confirm that large cations
dominate at the surface of the PDR but small neutral PAHs and anions are
abundant deeper in the molecular cloud.
