9. Concluding remarks
In this paper we have considered the various dust components present in a protoplanetary accretion disk. We have seen that the initial mixture of grains entering the disk from the parent molecular cloud, as given for instance by the P94 model, does not correspond to the mixture of solids which one encounters in a chemical equilibrium state for a Solar System element mixture. As the disk material slowly spirals inwards during the viscous evolution of the accretion disk the material gradually becomes hotter. This activates previously kinetically forbidden local rearrangement processes within the grain lattice and solid diffusion in the grain material. As a result of such processes a considerable change of the structural properties and of the chemical composition of the dust component in the protostellar accretion disk can be expected to occur in the warm inner parts of the accretion disk. The annealing of the initially amorphous lattice structure of dust grains and chemical fractionation processes strongly modify the extinction properties of the dust component of the disk material which in turn strongly modifies the radial temperature structure of the accretion disk. A realistic modelling of the structure and evolution of protoplanetary accretion disks, thus, requires a detailed consideration of the chemical transmutation of the dust material during the accretion process. The present paper is a first attempt to figure out what type of processes may be important in this connection and what are the consequences of such processes for the disk structure.
The calculations with respect to the possible condensates in a chemical equilibrium state to some extent repeats earlier calculations on condensation sequences for the Solar Nebula (e.g. Grossman 1972, Lattimer et al. 1978, Saxena & Eriksson 1986, Sharp & Huebner 1990). Though it seems now clear that the type of processes envisaged by the early calculations - condensation of solids from the gas phase in a contracting cooling nebula - is not the process really operating in a protoplanetary disk the predictions for the appearance and disappearance of certain minerals at certain temperatures does not depend critically on the assumptions with respect to the structure of the object. While most of the earlier calculations are performed for only one or a few fixed values of the pressure, the method used in this paper of constructing curves for a given degree of condensation has the advantage of showing clearly the region of existence of the various dust materials in a wide region of the pressure and temperature plane. The results therefore give a broader insight into which dust material may be important under varying pressure-temperature conditions in the protoplanetary accretion disk.
A major shortcoming of the type of equilibrium calculations based on the methods of chemical thermodynamics as have been performed in the present paper and in all earlier paper on condensation sequences is that in a real accretion disk the evolution of the chemical system into the chemical equilibrium state may be hindered either by activation energy barriers or by the slowness of the solid state diffusion processes required if certain atomic species have to be transported from the gas phase into the interior of a grain or have to be driven out of the grain during the conversion from one condensate to another one at the border of stability between the two compounds. A more realistic treatment of the chemical transmutation of the dust components in a protoplanetary disk requires a much more detailed treatment of the transport processes within the grains as it was possible to perform in the present paper. Additionally a kinetical treatment of the chemical surface reactions during growth or destruction of the different dust components is required but presently this is not possible since the details of the individual reaction steps are not known. Further progress in the understanding of the structure and evolution of the inner parts of a protoplanetary system requires further efforts with respect to clarifying which are the basic processes responsible for the chemical evolution of the dust component.
In our calculations we did not account for radial mixing due to turbulent motions which inevitably occurs during the viscous evolution of the disk (e.g. Morfill et al. 1985, Stevenson & Lunine 1988). This process intermingles material from different zones of the disk or accumulates material in the vaporization-condensation zone of solids and the rather simple radial distribution of different dust materials obtained in the present calculation would become more complicated instead. Since mixing occurs on timescales comparable to the evolutionary timescale of the disk such effects can only be treated within the frame of time dependent calculations of the disk evolution.
© European Southern Observatory (ESO) 1998
Online publication: March 30, 1998