Mg and Ti isotopes in presolar Al_2O_3

Abstract

Previously, we reported O isotopic compositions and initial ^(26)Al/^(27) Al ratios for two presolar Al_2O_3 grains, Orgueil B and Bishunpur B39 [1-3]. In Orgueil B, ^(17)O/^(16)O is about twice the solar value but ^(18)O/^(16)O is normal. In B39, ^(17)O/^(16)O is ~7x higher than solar and ^(18)O/^(16)O is 0.6x the solar value. Spectroscopic observations of red giant stars show similar O compositions [4], which result when material that has experienced partial H burning is mixed into the stellar envelope by the first dredge-up [3,5]. The ^(17)O/^(16)O ratios indicate that Orgueil B originated around a star of ~1.5 M_☉, while B39 formed around either a ~2 Me or 4-7 M_☉ [3,5]. Both grains formed with ^(26)Al/^(27)Al ratios of ~10^(-3). Aluminum-26 is produced in the H shell after core H burning has ceased and first dredge-up has occurred. It is spread through the envelope by the third dredge-up, which occurs in low- and intermediate-mass stars as a series of mixing events driven by thermal pulses in the He shell. In intermediate-mass stars (3-8 M_☉), ^(26)Al can also be brought to the surface by the second dredge-up, which occurs at the end of core He burning.

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