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Microscopic presolar grains extracted from primitive meteorites haveextremely anomalous isotopic compositions revealing the stellar origin of thesegrains. The composition of presolar spinel grain OC2 is different from that ofall other presolar spinel grains. Large excesses of the heavy Mg isotopes arepresent and thus an origin from an intermediate-mass (IM) asymptotic giantbranch (AGB) star was previously proposed for this grain. We discuss theisotopic compositions of presolar spinel grain OC2 and compare them totheoretical predictions. We show that the isotopic composition of O, Mg and Alin OC2 could be the signature of an AGB star of IM and metallicity close tosolar experiencing hot bottom burning, or of an AGB star of low mass (LM) andlow metallicity suffering very efficient cool bottom processing. Largemeasurement uncertainty in the Fe isotopic composition prevents us fromdiscriminating which model better represents the parent star of OC2. However,the Cr isotopic composition of the grain favors an origin in an IM-AGB star ofmetallicity close to solar. Our IM-AGB models produce a self-consistentsolution to match the composition of OC2 within the uncertainties related toreaction rates. Within this solution we predict that the 16O(p,g)17F and the17O(p,a)14N reaction rates should be close to their lower and upper limits,respectively. By finding more grains like OC2 and by precisely measuring theirFe and Cr isotopic compositions, it may be possible in the future to deriveconstraints on massive AGB models from the study of presolar grains.Comment: 10 pages, 8 figures, accepted for publication on Astronomy &  Astrophysic

On the asymptotic giant branch star origin of peculiar spinel grain OC2