The Prehistory of the Solar System
Image courtesy of STScl and NASA
Presolar Grain Research
Presolar grains condensed in the expanding envelopes of dying stars like AGB stars, Wolf-Rayet stars, novae, supernovae and other mass-loss stars and have survived solar system formation (see also Figure 1). They were captured into primitive meteorites which didn't undergo any significant metamorphism and so were preserved. Known grain populations include micron-sized SiC, corundum, graphite, spinel and silicates and nanometre sized diamonds ('nano-diamonds'). Each was identified because of order-of-magnitude or more variation in the isotope ratios of their major and trace elements. They represent a new window on late-stage stellar evolution and molecular cloud processes as significant as any of the bands of the electromagnetic spectrum. The association between their exotic isotope ratios and the chemical conditions necessary for their formation are now used to test models of nucleosynthesis [1].
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| Figure 1: A simplified sketch of the stellar material cycle. Elements heavier than H and He are produced in stars which eject big proportions of the newly synthetised material into space at the end of their life. This material is incorporated into new stars and planets. |
Micron sized presolar grains of SiC, corundum and graphite constitute at most a few ppm of even primitive meteorites. They are conventionally separated from their host meteorites by sequential dissolution of silicates and organic materials with strong acids and oxidising agents [2,3] (see Figure 2 for an exmple). Thousands of isotopic measurements have been acquired from these grains over the last 20 years to identify their origin. However, this severe acid treatment seems to alter element ratios and possibly even the isotopic ratios in the outer surface of the grains [4]. Furthermore, any non-refractory coating on the grains acquired in the interstellar medium is completely lost. We have developed a new method of separating SiC grains by density and size separation [5] which retains any non-refractory coating on the grains and leaves the grain unaltered ( Figure 3). These grains also preserve coatings that they may have gained in interstellar space. We have undertaken a study of these grains along with SiC grains separated by the conventional acid treatment.
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| Figure 2: SEM-image of a presolar SiC-grain extracted from the Murchison meteorite by the use of harsh acids. |
Measurements of lithium and boron abundance in the grains showed higher values just below the surfaces of the grains, dropping to low levels in the cores of the grains [6]. this pattern was interpreted as being due to implantation of fast atoms into the grains from the intersellar medium.
These samples thus provide the first evidence of samples from the interstellar medium recognised for study on Earth.
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| Figure 3: SEM-image of a presolar SiC-grain extracted by our own, new method avoiding the use of any acids. |
References:
[1] Bernatowicz T and Zinner E (Eds) 'Astrophysical consequences of the laboratory study of presolar materials' AIP Proceedings 402, AIP, Woodbury, New York, 1997;
[2] Amari S. et al. (1994) GCA, 58, 459-470
[3] Lewis R. S. et al. (1987) Nature, 326, 160-162
[4] Henkel et al.,
[5] Tizard, J., Henkel, T. and Lyon, I. (2005) 'The Gentle separation of SiC grains from meteorites' Meteor. Plan. Sci. 40(3): 335-342
[6] Lyon, I.C., Tizard, J.M. and Henkel, T. (2007) Evidence for lithium and boron from star-forming regions implanted in presolar SiC grains. Meteor.Plan.Sci 42(3): 373-385
[7] Tizard, J., Henkel, T. and Lyon, I. (2008) Elemental abundances in presolar SiC: a comparison of grains separated by acid residue and gentle separation techniques. Meteor.Plan.Sci. (submitted)