EXTINCT 93Zr IN SINGLE PRESOLAR SiC GRAINS FROM LOW MASS ASYMPTOTIC GIANT BRANCH STARS AND CONDENSATION FROM Zr-DEPLETED GAS

Synchrotron X-ray fluorescence was used in this study for the first time to measure trace element abundances in single presolar grains. The abundances of Zr and Nb were determined in SiC grains of the KJF size-separate. These grains are most likely from C-rich asymptotic giant branch stars (mainstream grains). Comparison of the data with s-process calculations suggests that the relatively short-lived isotope Zr-93 (t(1/2) = 1.5 x 10(6) yr) condensed into the grains. The Nb/Zr ratios of the majority of the grains are higher than the s-process and CI chondrite ratios. This is probably due to grains condensing from stellar gas that was depleted in Zr, potentially because of earlier condensation of ZrC, but not depleted in Nb. However, grain contamination with solar system Nb is possible as well. Upper limits on the initial Zr-93/Zr ratios in the grains agree with the ratios observed in late-type S stars.