Molybdenum isotopic composition of individual presolar silicon carbide grains from the Murchison meteorite

摘要

We report the isotopic composition of molybdenum in twenty-three presolar SiC grains from the Murchison meteorite which have been measured by resonant ionization mass spectrometry (RIMS). Relative to terrestrial abundance (and normalized to s-process-only ~(96)Mo), the majority of the analyzed grains show strong depletions in the p-process isotopes ~(92)Mo and ~(94)Mo and the r-process isotope ~(100)Mo. Sixteen of these grains have δ-values <-6005 for these three isotopes. The observed isotopic patterns of Mo from mainstream SiC grains clearly reveal the signature of s-process nucleosynthesis. Three-isotope plots of all grain data (δ~iMo vs. δ~(92)Mo) show strong linear correlations with characteristic slopes. This finding suggests mixing of solar-like material and pure s-process material in the parent stars. Comparison with evolutionary calculations of nucleosynthesis and mixing in red giants suggests that low-mass thermally-pulsed symptotic giant branch (TP-AGB) stars are the most likely site for the observed s-process nucleosynthesis.