Isotopic signatures and systematics of heavy elements in stardust silicon carbide grains

Abstract

Isotopic and elemental abundance signatures of heavy elements determined in presolar stardust grains have provided important constraints on nucleosynthetic processes in individual stars. The present study reports on new analytical protocols developed to measure the isotopic compositions of heavy elements in large (> 4 urn) individual presolar stardust SiC grains by Sensitive High-Resolution Ion Microprobe (SHRIMP). Isotopic analyses of Ba, Eu, W, Pb, U and Th in stardust SiC grains extracted from the Murchison meteorite are presented. Most of the studied grains show Si-, C-, and N-isotopic compositions in the range displayed by the socalled "mainstream grains" and, therefore, likely condensed in the outflows oflow mass carbonrich asymptotic giant branch (AGB) stars with close-to-solar metallicity. A detailed investigation by SHRIMP of the mass regIOn around Ba and Eu using a suite of carbide and silicate reference materials revealed several significant molecular interferences. Importantly, the interferences observed in carbide matrices, including stardust SiC grains, were not identified in silicate matrices. We found that employing an energy offset to select secondary ions from the high range of energies, in combination with high-mass resolution, was sufficient to suppress the unwanted interferences. These findings emphasize the importance of matrixmatching standards to obtain accurate ion microprobe isotopic data. The Ba-isotopic compositions determined in large stardust SiC grains employing such an approach were found to be mostly normal, which could suggest that the conditions for growth of large SiC grains are more favourable in the early C-rich stages of AGB evolution before much dredge-up episodes have happened. One of the main goals of the present study was to measure the isotopic compositions of heavy elements affected by branching points, such as Eu and W, in individual mainstream SiC grains and investigate their potential as proxies for temperature and neutron density during s-process nucleosynthesis. The Eu-isotopic compositions determined in large stardust SiC grains were found to be in agreement with observations in carbon-enhanced metal-poor stars enriched in 5process elements, despite differences in metallicity. Unfortunately, given the low Eu abundances in the SiC grains, the 151Eu isotope fractions [fr(151Eu) = 151Eu/(151Eu+153Eu)] derived from our measurements show large uncertainties and, in most cases, cannot be distinguished from the solar value. On the other hand, thefi(15IEu) value derived for a SiC-enriched bulk sample was very well constrained, matching those of carbon-enhanced metal-poor stars, but approximately 12% higher than current s-process predictions. The W-isotopic compositions determined in large stardust SiC grains were found to be anomalous in comparison with solar system materials. They are also in disagreement with current s-process predictions, unless the 183W and 185W neutron capture cross sections are increased. The results of this study provide an alternative explanation for the problem of the overproduction of s-process 1860s, since the higher observed 186W/ 184W ratios could eventually lead to lower 1860s as a consequence of the activation of the branching point at 185W during thermal pulse phases, when the neutron density is sufficiently high to modify the 186W abundance. Also investigated was utility of the U-Th-Pb system to obtain insights into the timescale of stardust grains and their parent stars. The Ph-isotopic compositions determined in large stardust SiC grains were found to be highly anomalous with respect to both modern terrestrial and primordial lead, showing enrichment in the non-radiogenic s-only 204Pb. It is speculated that the Pb-isotopic compositions of stardust grains may result from mixing of three components: common Pb, s-process Pb, and radiogenic Pb. The application of conventional radiometric dating using the U-Th-Pb decay system in stardust SiC grains was found not to be viable, given the large observed Pb isotopic anomalies. A rough correlation exists between Th and U concentrations and 829Si/ 28Si, which likely reflects the average Galactic Chemical Evolution trends of these elements. It is suggested that 238U/232Th ratios may be used as a relative chronometer of star formation, provided that unfractionated Th and U were incorporated into SiC grains. If assumed that the 238U/232Th ratios are initially unfractionated, then the results indicate that 29,30Si/28Si ratios decrease with time in the interstellar medium for close-to-solar metallicities. These results, therefore, would confirm that the placement of the Solar System Si isotopes near the bottom of the mainstream line is indeed what one would expect for its age and current location. Show more