Solar system formation environment

Hester, J. J. and Desch, S. J. (2005) Understanding our origins Star formation in HII region environments. In Chondrites and the Protoplanetary Disk, ASP Conference Series, 341, eds. Krot, A. N., Scott, E. R. D. and Reipurth, A. San Francisco Astronomical Society of the Pacific, pp. 107-130. A clear and up-to-date review of astronomical observations that constrain models for solar system formation. 

Stars form in dense cores within giant molecular clouds (see Fig. 1.4, Alves et al. 2001). About 1 % of their mass is in dust grains, produced in the final phases of stellar evolution. Molecular clouds are complex entities with extreme density variations, whose nature and scales are defined by turbulence. These transient environments provide dynamic reservoirs that thoroughly mix dust grains of diverse origins and composition before the violent star-formation process passes them on to young stars and planets. Remnants of this primitive dust from the Solar System formation exist as presolar grains in primitive chondritic meteorites and IDPs. 

Based on the probabilities, we might infer that the Sun originated in such a setting of cluster star formation. Cosmochemistry provides ways to test this idea. As discussed in Chapter 9, the short-lived radionuclides in the early solar system carry a record of nucleosynthesis in the galaxy and in the immediate environment where the solar system formed. 

The depletion in FeO may be understood in at least two ways. First, the crystalline grains may be equilibrium condensates from a hot solar nebular composition gas with iron sequestered to metals or sulfides (see e.g. Chapter 4). In this case the condensed grains either had to condense slowly to form crystal domains, or had been reheated and thermally annealed at a later epoch. The second, alternative explanation is that ferromagnesian amorphous silicate grains were thermally annealed in a reducing environment, e.g. in the presence of carbon. Heating such precursors leads to the formation of metallic spheroids embedded between the forsterite crystals, as the initial FeO component is reduced (see e.g. Fig. 8.3 and Connolly et al. 1994 Jones Danielson 1997 Leroux et al. 2003 Davoisne et al. 2006). Because carbon is ubiquitously present in primitive Solar System materials, this pathway offers a natural explanation to the observed FeO-poor silicate crystals. It is yet to be determined whether low-temperature crystallization processes, discussed in Section 8.1.1, would also lead to FeO depletion. 

Guided by early compilations of the cosmic abundances as reflected in solar system material (e.g., Suess and Urey, 1956), Burbidge cr a/. (1957) and Cameron (1957) identified the nuclear processes by which element formation occurs in stellar and supernova environments (i) hydrogen burning, which powers stars for —90% of their lifetimes (ii) helium burning, which is responsible for the production of and the two most abundant elements heavier than helium (iii) the a-process, which we now understand as a combination of... 

Comets are primitive but complex bodies that potentially contain solid materials from a wide sampling of the cold regions of the solar nebula disk. Comet formation appears to be a common consequence of star formation and studies of solar system comets provide important links between solar system studies and a broad range of astronomical investigations. Although the comets are undoubtedly the best preserved materials from the solar nebula, they have potentially been influenced and processed by a significant number of nebular and parent-body processes. The properties and mysteries of comets contain important clues to numerous materials, environments, and processes that occurred both in the cold regions of the solar nebula and in the environments that preceded it. 

Processes that occurred during the formation of the solar system, e.g. in the accretionary disk environment. 

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