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It has been shown that the diversity of the aromatic emission features can berationalized into different classes of objects, in which differences betweencircumstellar and interstellar matter are emphasised. We probe the linksbetween the mid-IR emitters observed in planetary nebulae (PNe) and theircounterparts in the interstellar medium in order to probe a scenario in whichthe 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 wereanalysed 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 PNecan be decomposed as the sum of six components. Some components made ofpolycyclic aromatic hydrocarbon (PAH) and very small grain (VSG) populationsare similar to those observed in PDRs. Others are fitted in an evolutionaryscenario involving the destruction of the aliphatic component observed in thepost-AGB stage, as well as strong processing of PAHs in the extreme conditionsof PNe that leads to a population of very large ionized PAHs. This speciescalled 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 nearbygalaxies and is also observed in galactic compact HII regions. These resultssupport the formation of the aromatic very small dust particles in theenvelopes of evolved stars, in the Milky Way, as well as in the LMC and SMC,and their subsequent survival in the interstellar medium.

Carriers of the mid-IR emission bands in PNe reanalysed. Evidence of a link between circumstellar and interstellar aromatic dust