INTERSTELLAR SIC WITH UNUSUAL ISOTOPIC COMPOSITIONS - GRAINS FROM A SUPERNOVA

Five single SiC grains, 2-9-mu-m in size, from the Murchison carbonaceous meteorite have been analyzed by ion microprobe mass spectrometry for their C, N, Si, Mg-Al, Ca, and Ti isotopic compositions. While most interstellar SiC grains from primitive meteorites are characterized by C-13, N-14, Si-29, Si-30, Ti-46, Ti-47, Ti-49, and Ti-50 excesses relative to solar isotopic ratios, and Al-26/Al-27 ratios of up to approximately 10(-2), carrying the signatures of H- and He-burning, these five grains (=grains X) have large excesses in C-12 (up to 28 times solar) and N-15 (up to 22 times solar), depletion in Si-29 and Si-30 (up to 59%), and Al-26/Al-27 ratios between 0.1 and 0.6. They furthermore have Ti-49 excesses (up to 95%), and one grain has a large Ca-44 excess (300%). While the Ca and Ti anomalies point toward explosive nucleosynthesis in supernovae and the in situ decay of the radioactive precursors Ti-44 and V-49 in SiC grains formed in supernova ejecta, there is no simple formation scenario that can give a consistent explanation for the isotopic compositions of these grains.