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Astron. Astrophys. 325, 1264-1279 (1997)

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6. Summary

In this work we have investigated the gas phase chemistry in a protoplanetary accretion disk including the dust destruction processes.

The usual time independent accretion disk's model has been used (Sect. 2). We calculate the chemistry in a gas parcel by solving a system of more than 100 ordinary differential equations corresponding to the different chemical species and the radii of the dust particles. The calculation is performed in the comoving frame. The chemical network is composed of about 600 chemical reactions (Sect. 3). The coupling between temperature and opacity is calculated by solving a supplementary algebraic equation (Sect. 2.1).

We have used a more realistic approximation for the gas opacity as in Paper I, where it was assumed to be constant. The mass extinction coefficients due to molecules and ions H [FORMULA] have been calculated from analytical approximations to tabular values (Sect. 2.1).

Our dust model is composed of carbon (with the thermodynamical properties of graphite), olivine (Mg2 SiO4) and troilite (FeS). We assume that these grains are embedded in two distinct layers of water and CO ice until the vapourisation temperature for CO ([FORMULA] 25 K) and water ([FORMULA] 150 K) are reached (Sect. 4).

We have considered the influence of carbon dust erosion by free O atoms and OH molecules (oxidation). This last process becomes efficient at about 2 AU. The oxidation by OH molecules modifies dramatically the hydrocarbons chemistry in the actual vicinity of the Earth by leading to the formation of huge amounts of methane. Some other hydrocarbon compounds are expected in this region to be formed but we must probably consider other types of surface reactions on grains to model their formation. The oxidation of carbon by free O atoms and the carbon vapourisation turn out to be inefficient. Methane disappears promptly because of oxidation reactions as soon as the olivine vapourisation becomes efficient (Sect. 5).

The treatment of troilite vapourisation (at [FORMULA] 700 K) leads to huge densities of H2 S in the region of the terrestrial planets (between 2 and 0.8 AU). As in the case of methane, the oxygen bearing species liberated by the olivine grains oxidate H2 S, which is rapidly transformed into SO2 (Sect. 5).

The olivine dissociation ([FORMULA] 1 650 K) produces SiO molecules, which lead to some other silicon bearing compounds like SiOOH or SiO2, but the silicon chemistry has no noticeable consequences on the chemistry of the other elements (Sect. 5).

Close to the young central star ([FORMULA] 6 000 K), only free H, C, N, O, Mg, Fe, Si and S atoms are present.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998