Evidence of a link between circumstellar and interstellar aromatic dust
Centre d'Étude Spatiale des Rayonnements, Université de Toulouse et CNRS, Observatoire Midi-Pyrénées, 9 Av. du Colonel Roche, 31028 Toulouse Cedex 04, France e-mail: email@example.com
2 N. Copernicus Astronomical Center, Rabianska 8, 87-100 Toruń, Poland
3 Nicolaus Copernicus University, Gagarina 11, 87-100 Toruń, Poland
Accepted: 4 August 2008
Context. 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.
Aims. 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.
Methods. The mid-IR (6–14 μm) emission spectra of PNe and compact 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.
Results. 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 PAHx are proposed as the carriers of a characteristic band at 7.90 μm. This band can be used as part of diagnostics that identify PNe in nearby galaxies and is also observed in galactic compact regions.
Conclusions. 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.
Key words: astrochemistry / stars: carbon / ISM: planetray nebulae: general / infrared: ISM / methods: data analysis
This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.
© ESO, 2008