ISOTOPICALLY ANOMALOUS TI IN PRESOLAR SIC FROM THE MURCHISON METEORITE

The Ti-isotopic compositions of 14 large (8-23-mu-m) interstellar SiC grains from the Murchison meteorite have been measured. Three SiC grains with normal isotopic compositions of C, N, and Si also have normal Ti but the other 11 grains with anomalous C-, N-, and Si-isotopic compositions also have anomalous Ti. The anomalous Ti qualitatively bears the signature of neutron-capture nucleosynthesis with excesses in the minor isotopes Ti-46, Ti-47, Ti-49, and Ti-50, relative to the major isotope Ti-48. The anomalies range in magnitude up to 15% for Ti-46 and Ti-50 with smaller anomalies at Ti-47 and Ti-49 relative to Ti-48 when normalized to the solar abundances. The Si- and Ti-isotopic compositions are correlated in the sense that the grains with the largest Ti-50 excesses have the largest Si-29 excesses. While the anomalies are qualitatively consistent with the patterns predicted by n-capture nucleosynthesis, the correlations between anomalies in different isotopes differ substantially from predictions for the He-burning shell of AGB stars and core He-burning in massive stars. However, uncertainties in n-capture cross sections are large and require refinement before a full evaluation of the predictions for nucleosynthesis in proposed stellar sources can be made.