  It has been shown that the diversity of the aromatic emission features can be
rationalized into different classes of objects, in which differences between
circumstellar and interstellar matter are emphasised. We probe the links
between the mid-IR emitters observed in planetary nebulae (PNe) and their
counterparts in the interstellar medium in order to probe a scenario in which
the latter have been formed in the circumstellar environment of evolved stars.
The mid-IR (6-14 um) emission spectra of PNe and compact HII regions were
analysed on the basis of previous work on photodissociation regions (PDRs).
Galactic, Large Magellanic Cloud (LMC), and Small Magellanic Cloud (SMC)
objects were considered in our sample.We show that the mid-IR emission of PNe
can be decomposed as the sum of six components. Some components made of
polycyclic aromatic hydrocarbon (PAH) and very small grain (VSG) populations
are similar to those observed in PDRs. Others are fitted in an evolutionary
scenario involving the destruction of the aliphatic component observed in the
post-AGB stage, as well as strong processing of PAHs in the extreme conditions
of PNe that leads to a population of very large ionized PAHs. This species
called PAH^x are proposed as the carriers of a characteristic band at 7.90 um.
This band can be used as part of diagnostics that identify PNe in nearby
galaxies and is also observed in galactic compact HII regions. These results
support the formation of the aromatic very small dust particles in the
envelopes of evolved stars, in the Milky Way, as well as in the LMC and SMC,
and their subsequent survival in the interstellar medium.
