Aluminum-, Calcium- and Titanium-rich Oxide Stardust in Ordinary Chondrite Meteorites

We report O-, Al-Mg-, K-, Ca-, and Ti-isotopic data for a total of 96 presolar oxide grains found in residues of several unequilibrated ordinary chondrite meteorites. Identified grain types include Al2O3, MgAl2O4, hibonite (CaAl12O19), and Ti oxide. This work greatly increases the presolar hibonite database, and is the first report of presolar Ti oxide. O-isotopic compositions of the grains span previously observed ranges and indicate an origin in red giant and asymptotic giant branch (AGB) stars of low mass (<2.5M ) for most grains. Cool bottom processing in the parent AGB stars is required to explain isotopic compositions of many grains. Potassium-41 enrichments in hibonite grains are attributable to in situ decay of now-extinct Ca. Inferred initial Ca/Ca ratios are in good agreement with model predictions for low-mass AGB star envelopes, provided that ionization suppresses Ca decay. Stable Mg and Ca isotopic ratios of most of the hibonite grains reflect primarily the initial compositions of the parent stars and are generally consistent with expectations for Galactic chemical evolution, but require some local interstellar chemical inhomogeneity. Very high O/O or Mg/Mg ratios suggest an origin for some grains in binary star systemswhere mass transfer from an evolved companion has altered the parent star compositions. A supernova origin for the hitherto enigmatic O-rich Group 4 grains is strongly supported by multielement isotopic data for two grains. The Group 4 data are consistent with an origin in a single supernova in which variable amounts of material from the deep O-rich interior mixed with a unique end-member mixture of the outer layers. The Ti oxide grains primarily formed in low-mass AGB stars. They are smaller and rarer than presolar Al2O3, reflecting the lower abundance of Ti than Al in AGB envelopes. Subject headinggs: dust, extinction — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: AGB and post-AGB — supernovae: general