Presolar SiC Grains of Type Y: Origin from Low‐Metallicity Asymptotic Giant Branch Stars

Abstract

We report isotopic data for 27 presolar SiC grains of the rare subtype Y in an acid-resistant residue of the Murchison (CM2) meteorite. Presolar SiC grains of type Y constitute only ≃1% of Murchison SiC grains larger than ≃2 μm and are defined as having 12 C/ 13 C > 100 (solar = 89) and 14 N/ 15 N > 272 (solar). In a Si 3-isotope plot, their Si isotopic compositions plot to the right of the correlation line defined by the majority of presolar SiC grains (the mainstream population), whose isotopic compositions indicate an origin in C-rich asymptotic giant branch (AGB) stars of near-solar metallicity. Because of their low abundance, the new Y grains were identified by automatic isotopic imaging of the 12 C/ 13 C ratio in the ion microprobe. We report C, N, and Si isotopic ratios of all 27 grains, inferred initial 26 Al/ 27 Al ratios of 18, and Ti isotopic ratios of 20 grains. Whereas 14 N/ 15 N and 26 Al/ 27 Al ratios exhibit the same range as mainstream grains, the C, Si, and Ti isotopic ratios are distinct. Carbon-12/carbon-13 ratios range up to 295 and 30 Si/ 28 Si excesses up to 183‰ relative to solar. The average 29 Si/ 28 Si ratio of Y grains is by 59‰ smaller than that of mainstream grains. Ti isotopic ratios relative to 48 Ti are some-what similar to those of mainstream grains, but extend to more extreme anomalous compositions. One grain has 46 Ti/ 48 Ti, 49 Ti/ 48 Ti, and 50 Ti/ 48 Ti excesses of 183‰, 365‰, and 990‰, respectively, relative to solar. These features exhibited by Y grains point to an origin in AGB stars of somewhat lower than solar metallicity. In the envelope of such stars the proportion of 12 C and s-processed material dredged up from deep zones that experienced partial He burning and was mixed with original material is higher than in stars of solar metallicity. Their envelopes are therefore expected to have larger 12 C/ 13 C, 30 Si/ 28 Si, and 49 Ti/ 48 Ti and 50 Ti/ 48 Ti ratios than mainstream grains. We compare the C, Si, and Ti isotopic compositions of Y grains with the results of theoretical models of AGB stars with 1.5, 3, and 5 M ⊙ and Z = 0.006, 0.01, and 0.02. While solar-metallicity (Z = 0.02) AGB models cannot account for the Y grain data, the models with Z = 0.01 can reproduce the measured isotopic compositions reasonably well. A range of stellar masses (from 1.5 M ⊙ possibly up to 5 M ⊙ ) is indicated by the grain data. The present study together with additional data on SiC grains of type Z furthermore indicate that the rate of change of the ratios of the secondary Si isotopes ( 29 Si and 30 Si) relative to 28 Si prior to solar system formation was lower than has been generally assumed, implying larger contributions of 28 Si from Type Ia supernovae compared to those from Type II supernovae. The Si isotopic ratios of Galactic cosmic rays also suggest such an evolution.