5 Concluding remarks

This paper considers some important chemical reactions of the products of carbon dust oxidation in the warm inner zone ( $r~{\hskip 1pt}{\raise 1pt \hbox{$<$ }}{\hskip- 7.5pt}{\lower 3pt \hbox{$\sim$ }}{\hskip 2pt}\ 1$ AU) of a protoplanetary accretion disc and the mixing of these products into the outer disc regions by turbulent diffusion. It is shown that radial mixing results in a rather high abundance of CH₄ and C₂H₂ in the outer disc regions beyond 10 AU where water ice is stable and where most likely the nuclei of the present day long period comets in our Solar System have been formed 4.6 Gyr ago. These gas phase species then are expected to exist in significant amounts in the ice mixture of cometary nuclei. This is in accord with the observed high abundance of CH₄and C₂H₂ in cometary ices.

The model calculation presented in this paper shows that at least some fraction of the observed CH₄ and C₂H₂ ices should result from the oxidation of the interstellar carbon dust grains and from mixing the burning products into the formation zone of comets. The model cannot explain, however, the observed high abundance of C₂H₆ and of CH₃OH. These molecules can be formed by gas phase reactions from the combustion products of carbon black only in very small quantities. The present model shares this property with models for the chemistry in molecular clouds, if only gas phase reactions are considered. Calculations of models for the chemistry in molecular clouds considering grain surface reactions with H atoms (Aikawa & Herbst 1999,1999; Willacy & Langer 2000) and laboratory investigations of H atom reactions with organic ices at very low temperature (Hiraoka et al. 1998) have shown, however, that C₂H₆, CH₃OH, and H₂CO are formed in large quantities by surface reactions of C₂H₂ and of CO with H atoms.

Since the outer regions of the accretion disc and the disc surface in less far extended regions of the disc are very cold, it seems well possible that the C₂H₂ formed in the inner disc and mixed outwards and precipitated as part of the ice mixture on grain surfaces later is partially processed on the surfaces into C₂H₆ by free H atoms. The abundant species CH₃OH and H₂CO cannot be formed by carbon oxidation. They probably are products of a different mechanism not related to carbon dust oxidation. This, at the same time suggests, that also some of the CH₄ and C₂H₂ present in the cold disc zones might result from the same processes that formed CH₃OH and H₂CO because of the similarities in molecular abundances in comet Hale-Bopp and in hot molecular cores (cf. Bockelée-Morvan et al. 2000). The present model calculation neglects all such processes since the primary process, turbulent mixing into the outer disc region, presently is calculated only for stationary disc models, which is not realistic for the transport into the outermost disc regions. Such calculations are postponed until time dependent models for disc evolution and mixing are available.

Despite the strong simplifications of the model calculation, it shows that at least some fraction and perhaps even most part, but not all, of the hydrocarbons observed to exist in cometary nuclei might originate (i) from the destruction of the interstellar carbon dust in protoplanetary accretion discs and (ii) from extended radial mixing processes operating in such discs.

Acknowledgements

This work is part of a project of the special research project SFB359 "Reactive Flows, Diffusion and Transport'' which is supported by the Deutsche Forschungsgemeinschaft (DFG).