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Astron. Astrophys. 317, 962-967 (1997)

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4. Discussion and conclusion

Our modelling leads to high abundances of [FORMULA] in the intermediate envelope of IRC+10216, but only if the dissociation energy of the organometallic intermediates [FORMULA] and [FORMULA]) is higher than 1.2 eV. This is the main constraint on the organometallic catalysis for the cyclotrimerization of acetylene in this circumstellar medium. We have seen that [FORMULA] bonding energy was expected to be found between 0.8 and 1.5 eV. The dissociation energy of organometallic intermediates can be assessed to be equal to their bonding energy if they are small complexes (i.e. less than about 10 atoms). However, if the metallic cores M were aggregates, we have seen in Sect. 2 that, according to unimolecular theory, the intermediate complexes could have a sufficient number of vibrational modes to radiate their internal energy instead of breaking the organometallic bond. As a consequence, large organometallic complexes could have a dissociation energy higher than 1.2 eV even if their bonding energy is smaller.
For a given initial metal abundance, whatever the value of the dissociation energy higher than 1.2 eV, the final steady-state abundance of [FORMULA] does not vary significantly. In the case [FORMULA], the first coordination leading to [FORMULA] becomes much more efficient then the photodestruction, depleting rapidly the gas phase of free [FORMULA] molecules, and then slowing down the following step of the cyclotrimerization. We have also studied the effect of the initial depletion of M. The final abundance of [FORMULA] is about the same, [FORMULA], for [FORMULA] to 0.1, with El=1.2eV. This is due to the catalytic property of M. However, if [FORMULA], the production of [FORMULA] falls by more than one order of magnitude. The most favourable cases for the production of [FORMULA] have been obtained for a dissociation energy of the intermediate complexes around 1.2 eV and for a metallic depletion [FORMULA] included between 1 and 0.1. In the case [FORMULA], the abundance of free acetylene decreases up to a factor 60 from its initial value in the internal envelope ([FORMULA]). The acetylene abundance in the envelope has been measured by means of two independent methods: i) directly deduced from the measurement of its absorption band at [FORMULA] (Ridgway et al. 1976, Keady et al. 1993): it corresponds to [FORMULA] (with a factor 2 of accuracy), ii) indirectly deduced from the emission at 87GHz of its photodissociation product in the external envelope, [FORMULA] (Huggins et al. 1984, Truong-Bach et al. 1987): it gives an abundance at least one order of magnitude lower ([FORMULA]). Truong-Bach et al. (1987) have shown that an accretion process of acetylene onto the grains could not be sufficient to explain this depletion and they suggest the existence of an unknown very efficient chemical process in the expanding envelope. The catalytic process that we propose in the intermediate envelope, could contribute significantly to this depletion.
More laboratory and theoretical data on the bonding energy and on the cross sections of the involved organometallic species, are needed to improve the modelling and to give more accurate results. In addition, many other reactions involving oligomerization of unsaturated hydrocarbons, should be taken into account to produce larger PAHs. Similar organometallic processes could also be efficient to form very small grains of metal-aromatic aggregates as already discussed by Marty et al. (1994). Our simplified modelling shows that organometallic catalysis could contribute efficiently to the formation of aromatic molecules in C-rich circumstellar media and should be included in general chemical modellings. Moreover, such processes could take place inside molecular clouds. As discussed by Giard et al.(1994), in-situ formation of PAHs is necessary to account for the large amount of these molecules observed and to provide an homogeneous mixing in the ISM. Interstellar acetylene has bected for the first time by Lacy et al. (1989) in the spectra of three infrared sources embedded in molecular clouds. The inferred abundances ([FORMULA] to [FORMULA] of CO) could lead to a significant production of aromatic molecules through the catalytic process proposed in this paper. Moreover, this chemical process is thought to be more efficient in dense interstellar clouds than in circumstellar envelopes because produced molecules are protected from UV radiation by a large dust extinction.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998