EDP Sciences
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Volume 429, Number 1, January I 2005
Page(s) 193 - 204
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20041247

A&A 429, 193-204 (2005)
DOI: 10.1051/0004-6361:20041247

Spectroscopy of polycyclic aromatic hydrocarbons and very small grains in photodissociation regions

M. Rapacioli1, C. Joblin1 and P. Boissel2

1  Centre d'Étude Spatiale des Rayonnements, CNRS-Université Paul Sabatier - Observatoire Midi-Pyrénées, 9 avenue du Colonel Roche, 31028 Toulouse, France
    e-mail: mathias.rapacioli@cesr.fr
2  Laboratoire de Chimie Physique, CNRS-Université Paris Sud, Bât. 350, 91405 Orsay Cedex, France

(Received 6 May 2004 / Accepted 5 August 2004)

We have coupled a singular value decomposition method with a Monte Carlo search algorithm to analyse the mid-infrared ISOCAM spectral maps of photodissociation regions (PDRs) in NGC 7023 and $\rho$ Oph-SR3.Three different spectra and their associated spatial distribution were extracted from this analysis. It is shown that they can be associated with polycyclic aromatic hydrocarbons (PAHs) in their cationic and neutral forms and a third population of carbonaceous very small grains (VSGs). The method allows for the first time (i) to separate the contribution of neutral PAHs to the interstellar emission spectrum from that of cationic PAHs; (ii) to show that the 7.8  $\mu$m component of the "7.7  $\mu$m" broad feature is carried by VSGs, whereas the 7.6  $\mu$m component is due to PAHs; and (iii) to give evidence that free-flying PAHs are produced in PDRs by photoevaporation of VSGs. It is proposed that these carbonaceous VSGs are indeed PAH clusters. We derived a minimal size of 400 carbon atoms per cluster and estimated a UV absorption power of the order of 10 -24 W C -1 for their dissociation.

Key words: ISM: lines and bands -- ISM: clouds -- ISM: dust, extinction -- methods: data analysis -- astrochemistry

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