Circumstellar Hibonite and Corundum and Nucleosynthesis in Asymptotic Giant Branch Stars

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Abstract
We report the discovery of two hibonite grains (CaAl12O19) whose isotopic compositions show that they formed in the winds of red giant and asymptotic giant branch (AGB) stars. While hibonite is the second major phase (after corundum, Al2O3) expected to condense from stellar ejecta with C/O < 1, it has not previously been found. One circumstellar hibonite grain is highly enriched in 17O and slightly depleted in 18O relative to the solar composition and has large excesses in 26Mg and 41K, decay products of 26Al and 41Ca. The inferred initial values (26Al/27Al)0 ≈ 5 × 10-3 and (41Ca/40Ca)0 ≈ 1.5 × 10-4 of this grain are consistent with models of nucleosynthesis in an AGB star. The other hibonite is enriched in 17O, strongly depleted in 18O, shows no evidence of 41Ca and formed with (26Al/27Al)0 ≈ 2 × 10-2. The low 18O/16O and very high (26Al/27Al)0 may indicate substantial proton exposure during cool bottom processing in a low-mass parent star. The low upper limit on 41Ca/40Ca (≤ 3.2 × 10-5) implies that little or no He-shell material had been dredged into the envelope when this grain formed. We also report isotopic compositions for 12 new circumstellar corundum grains. The compositions of 11 of these grains are consistent with current models for red giant and AGB stars. One corundum grain has extremely high 17O/16O and near-solar 18O/16O and may have formed in a star that was initially enriched in 17O and 18O.