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

¹ Sterrenkundig Instituut, University of Utrecht, Postbus 80000, 3508 TA Utrecht, The Netherlands e-mail: M.Lugaro@phys.uu.nl
² Origins Institute, Department of Physics & Astronomy, McMaster University, Hamilton ON, Canada
³ Carnegie Institution of Washington, Department of Terrestrial Magnetism Washington DC 20015, USA
⁴ Max-Planck-Institute for Chemistry, PO Box 3060, 55020 Mainz, Germany
⁵ Department of Physics and Astronomy, 176 Phillips Hall, University of North Carolina, Chapel Hill, NC 27599-3255, USA
⁶ Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800, Australia

Received: 7 June 2006
Accepted: 7 October 2006

Abstract

Context.Microscopic presolar grains extracted from primitive meteorites have extremely anomalous isotopic compositions revealing the stellar origin of these grains. Multiple elements in single presolar grains can be analysed with sensitive mass spectrometers, providing precise sets of isotopic compositions to be matched by theoretical models of stellar evolution and nucleosynthesis.

Aims.The composition of presolar spinel grain OC2 is different from that of all other presolar spinel grains. In particular, large excesses of the heavy Mg isotopes are present and thus an origin from an intermediate-mass (IM) asymptotic giant branch (AGB) star was previously proposed for this grain. We discuss the O, Mg, Al, Cr and Fe isotopic compositions of presolar spinel grain OC2 and compare them to theoretical predictions.

Methods.We use detailed models of the evolution and nucleosynthesis of AGB stars of different masses and metallicities to compare to the composition of grain OC2. We analyse the uncertainties related to nuclear reaction rates and also discuss stellar model uncertainties.

Results.We show that the isotopic composition of O, Mg and Al in OC2 could be the signature of an AGB star of IM and metallicity close to solar experiencing hot bottom burning, or of an AGB star of low mass (LM) and low metallicity (0.004) suffering very efficient cool bottom processing. Large measurement uncertainty in the Fe isotopic composition prevents us from discriminating 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 of metallicity close to solar.

Conclusions.Our IM-AGB models produce a self-consistent solution to match the composition of OC2 within the uncertainties related to reaction rates. Within this solution we predict that the ¹⁶O(F and the ¹⁷O(N reaction rates should be close to their lower and upper limits, respectively. By finding more grains like OC2 and by precisely measuring their Fe and Cr isotopic compositions, it may be possible in the future to derive constraints on massive AGB models from the study of presolar grains.