More and more observations of the interstellar medium infrared emission support the existence of aromatic carbonaceous molecules like PAHs, originally proposed by Léger & Puget (1984) and by Allamandola et al.(1985). The observations of the Galactic plane performed with the AROME balloon-borne experiment, allowed the first detection of the emission features at (Giard et al. 1988, 1989 and 1994), and at (Ristorcelli et al. 1994), at large scale in the diffuse interstellar medium. Giard et al.(1994) deduce from their measurements and from the dust model of Désert et al. (1990), that PAHs should include about 8% of the cosmic carbon and be homogeneously mixed into the general Interstellar Medium (ISM). They infer an average production rate of about for the Galaxy and address the question of the formation of PAHs. The processes involved as well as the sites of production have given rise to many discussions (Allamandola et al. 1985, Omont 1986). In particular, Allamandola et al.(1985) and Jura et al.(1987) proposed that C-rich red giants could be a significant source of PAHs, since they eject an important mass of gas into the interstellar medium. Direct observations of the emission bands attributed to PAHs are difficult in giant star envelopes because of the lack of UV photons required for the excitation mechanism. However, these stars are the precursors of planetary nebulae whose spectra present the IR bands, when they are carbon rich. It is also the case for Post-AGB objects (Buss et al. 1990, Lequeux et al. 1990). Then, one can ask at which stellar evolution stage should the PAH formation start and according to which process.
Chemical models have been developed to study PAH formation in stellar envelopes involving classical nucleation from the precursor acetylene molecule (Frenklach & Feigelson 1989, Cherchneff et al. 1992). However, the chemical growth is restricted to a narrow temperature range , and the resulting production rate is very low. In this scheme, the blocking step is the production of the first aromatic ring. In this paper, we consider the possibility of a different formation process, based on the catalytic cyclotrimerization of acetylene with iron, originally proposed by Chaudret et al. (1991) and Serra et al. (1992) in their study of organometallic chemistry in the ISM. Such a mechanism could take place in circumstellar media but also inside molecular clouds allowing in-situ formation of aromatic molecules. The aim of this paper is to investigate the efficiency of this chemical process in the typical AGB stellar envelope of the well-observed star IRC+10216.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998