Trace, Isotopic Analysis of Stardust

Scientific Achievement

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The isotopic distributions of Mo, Sr, Zr, Ba, Si, C, and N have been determined in individual micron sized SiC and Graphite grains culled from acid dissolutions of the Murchison meteorite for the first time. These grains, dubbed presolar grains, condensed in stellar outflows, traveled through interstellar space, were trapped in the gravitational well of our forming solar system, and finally agglomerated in the Murchison meteorite at the time of solar system formation. Truly stardust, these grains contain within them a geminate record of the nucleosynthetic processes of the original star around which it condensed. This analysis is only possible with the SARISA (Surface Analysis by Resonant Ionization of Sputtered Atoms) instrument developed under the DOE BES Materials Chemistry Program at Argonne National Laboratory. This instrument leads the world in the combination high sensitivity with high discrimination. As such it is well matched to overcome the difficulty in grain measurement -- the need to measure < 1-ppm concentrations on very few atoms. SARISA, for instance, is able to measure with high Signal to Noise the 96Zr isotope even though there are only a few thousand 96Zr atoms present in a grain made up of over a billion Si and C atoms. It accomplishes this while removing isobaric (same mass) interferences from 96Mo and Si3C.

Significance

These studies contribute greatly to our understanding of the nucleosynthetic origin of the elements in our solar system and to our understanding of how our sun and stars in general work. Isotopic analysis of grains whose Si and C isotopic ratios clearly identify their originating star as an AGB star such as our sun demonstrate for the first time the range of stars and nucleosynthesis conditions that contributed to our solar system. Moreover these studies validate many of the theoretical models for low mass AGB stars and point the way for modification of these models. Isotopic analysis of grains whose origin is most likely a supernova demonstrate a new nucleosynthetic mechanism, a neutron burst mechanism, which must be considered in the production of the elements. In this mechanism, an intense neutron burst of limited duration is produced perhaps by C/H+ reactions in the collapsing He shell of the supernovae. While the extent of the contribution of such material to universe remains unclear, the signature of this event has now been detected in the isotopic record of four elements Ba, Mo, Zr, and Sr.

Performers

M. J. Pellin, M. Savina, W. F. Calaway, Argonne National Laboratory