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Astron. Astrophys. 359, 799-810 (2000)

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4. Conclusion

We have developed a multicomponent model of carbon cluster growth using reaction rate constants, deduced from combustion chemistry, geometrical parameters and dissociation energies as input data. The three forms of carbon, sp, sp2 and sp3 are simultaneously considered. In the framework of this mean field model a number of very efficient radicalar reactions are taken into account together with some other reactions involving an activation barrier. The calculations show that the distribution of carbon clusters in carbon-rich stellar atmospheres depends on the relative energetics of the clusters and on the kinetics of their growth. When the carbon to hydrogen abundance ratio is low, only small (non-condensable) molecules are formed in agreement with laboratory experiments on carbon vapour deposition. On the other hand, when this ratio is high ([FORMULA]), medium-sized species are produced of various types: carbon chains [FORMULA] or [FORMULA], carbon monocycles, [FORMULA], and PAHs accompanied by their corresponding dehydrogenated congeners. At [FORMULA]K, carbon chains or monocycles are clearly dominant over dehydrogenated (regular and relatively small) PAHs. In contrast, at [FORMULA]-[FORMULA]K, big (regular and irregular) PAHs are formed at much larger rate and compete with cumulenic structures.

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© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000