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Astron. Astrophys. 332, 1099-1122 (1998)

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2. Initial composition of the dust mixture

The question of which types of dust formed from the most abundant dust forming elements, i.e., formed from the elements C, N, O, Mg, Si, S and Fe, can be expected to exist in molecular cloud cores and in the cool outer parts of a protoplanetary accretion disk has thoroughly been discussed by Pollak et al. (1994). They arrived at the conclusion that in the outer parts of the disk there exists a multicomponent mixture of several kinds of dust species, which is dominated most likely by the following few species:

  • [FORMULA] (Olivine)
  • [FORMULA] (Orthopyroxene)
  • [FORMULA] (Quartz)
  • Fe (metallic iron)
  • FeS (Troilite)
  • CHON-Material (Kerogen).

Table 1 shows the best estimate of Pollak et al. (1994) for the fraction of the key elements Mg, Fe, Si, and S bound into these dust materials: The elements Mg, Fe, and Si are assumed to be completely condensed into this dust mixture while for S some fraction remains in the gas phas. The carbon is assumed to form a complex organic material containing considerable portions of H, N, and O besides the carbon. The numbers in Table 1 result from a critical discussion and evaluation of the observational material and probably represent the most reliable model for the composition of the dust material in molecular cloud cores and in protostellar disks which can presently be derived. We base our considerations in this paper with respect to the Mg-Fe-Si dust components on precisely this model. We shall call this the P94-mixture.


[TABLE]

Table 1. Abundant dust species in the protoplanetary accretion disk, the fraction of the abundant elements condensed in the dust species, the stoichiometric coefficient x for the silicates, and their resp. melting temperatures


The P94 model for the dust composition is quite different from models for the interstellar dust composition, as for instance the widely used Mathis-Rumpl-Norsieck (1977) model or the model proposed by Li & Greenberg (1997). Especially the assumption of an iron and a troilite component is unusual. It is motivated by results of thermochemical equilibrium calculations (e.g. Grossman 1972, Lattimer et al. 1978, Prinn & Fegley 1989) and the existence of such grains in primitive meteorites. The recent detection of iron and troilite inclusions in glassy silicate grains of presumably interstellar origin (Bradley 1994, cf. also the discussion in Martin 1995, Goodman & Whittet 1995) lends some support to the assumptions of the P94 model. Also the models for the composition of interstellar dust discussed by Mathis (1996) shows for instance iron grains to be a possible component of the interstellar dust mixture.

Throughout the protostellar accretion disk, these dust components are in the solid state (cf. Table 1 and Fig. 3) because the upper limit of stability under protoplanetary disk conditions of all these dust components turns out to be well below their respective melting points.

The dust formed from the group of elements O, Mg, Fe, Si, and C is the main source of opacity in the protoplanetary accretion disk and, by this, determines the structure and time evolution of that part of the accretion disk where the disk is cool enough for these dust components to exist. From studies of primitive meteorites it is known that additional minor dust components formed from such elements existed in the protoplanetary disk (e.g. Sears & Dodd 1988), but they cannot be of substantial importance, neither for the disk structure and evolution nor for the chemistry of the abundant elements. Such minor dust components formed from the abundant elements O, Mg, Fe, Si, and C are neglected in the P94 dust model and so we will do.

The dominating dust components are not the most stable dust species existing at high temperatures. Compounds of the less abundant elements Al and Ca, for instance, can survive even if heated up to much higher temperatures than the Mg-Fe-Si-compounds. Dust species formed from these elements, especially corundum, then become the dominant opacity sources in part of the protoplanetary disk close to the protostar where the disk is too hot for the silicates to exist. From analysis of abundances ratios for oxygen and aluminium isotopes in meteoritic material it is known that corundum (Al2 O3) and spinel (MgAl2 O4) grains of definitely circumstellar origin have been present in the protoplanetary disk material (e.g. Hutcheon et al. 1994, Huss et al. 1994, Nittler et al. 1994). Thus, at least part of the total aluminium content of the infalling material from the parent molecular cloud entered the protoplanetary disk as a separate dust component. The aluminium dust components are considered in some detail in Sect.  5.

Compounds of the less abundant elements Ti and Zr may exist at temperatures above the stability limit of the silicates and at even much higher temperatures than the aluminium-calcium compounds. Their abundance is too low as that they can become important sources of dust opacity. We do not consider such dust species in this paper, though they may be important as seed nuclei for dust condensation.

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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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