10 - The primary solar system objects and related processes

Summary

Solar nebula: initial composition and early development

Initial composition

Initially gas and dust were the only constituents of the cold molecular interstellar cloud, the raw material for the solar nebula. Spectroscopic observations of dense interstellar clouds along with modelling indeed suggest that the difference between the local dust/gas ratio and that in other regions of the Galaxy can be attributed entirely to a difference in metallicity (Vuong et al., 2003). According to the solar metallicity (Table 3.2), the bulk-mass dust/gas ratio in the presolar cloud was ∼ 1/100. The gas included highly volatile elements, first of all H₂, He, N₂, CH₄, CO, the noble gases etc. The dust grains, which were mineral or amorphous condensates, varied in composition from ices (H₂O, CO, CO₂, NH₃, C₂H₆) to refractory grains (e.g. Al₂O₃, SiC, graphite and diamond). The mean size of the dust grains was generally ∼ 10⁻⁴ cm but could vary from ∼ 10⁻⁸ cm up to 1 cm (Elmegreen, 1981).

The dust was chemically and isotopically heterogeneous (Section 3.3), implying a number of presolar sources of the raw material. More than 1000 stars might have contributed to the presolar cloud (Flam, 1991).

Early T Tauri stage: high-temperature processing

There are three approaches allowing PT-t parameters (i.e. pressure and temperature conditions varying in the course of time t), which govern the early evolution of matter of the solar nebula, to be estimated.