Heavy Element Abundances in Presolar Silicon Carbide Grains from Low-Metallicity AGB Stars

Abstract

AbstractPrimitive meteorites contain small amounts of presolar minerals that formed in the winds of evolved stars or in the ejecta of stellar explosions. Silicon carbide is the best studied presolar mineral. Based on its isotopic compositions it was divided into distinct populations that have different origins: Most abundant are the mainstream grains which are believed to come from 1.5–3 M⊙ AGB stars of roughly solar metallicity. The rare Y and Z grains are likely to come from 1.5–3 M⊙ AGB stars as well, but with subsolar metallicities (0.3–0.5 times solar). Here we report on C and Si isotope and trace element (Zr, Ba) studies of individual, submicrometer-sized SiC grains. The most striking results are: (1) Zr and Ba concentrations are higher in Y and Z grains than in mainstream grains, with enrichments relative to Si and solar of up to 70 times (Zr) and 170 times (Ba), respectively; (2) For the Y and Z grains there is a positive correlation between Ba concentrations and amount of s-process Si. This correlation is well explained by predictions for 2–3 M⊙ AGB stars with metallicities of 0.3–0.5 times solar. This confirms low-metallicity stars as most likely stellar sources for the Y and Z grains.