Presolar SiC grains of type Y: Origin from low-metallicity asymptotic giant branch stars

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 ¹²C/¹³C > 100 (solar = 89) and ¹⁴N/¹⁵N > 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 ¹²C/¹³C ratio in the ion microprobe. We report C, N, and Si isotopic ratios of all 27 grains, inferred initial ²⁶Al/ ²⁷Al ratios of 18, and Ti isotopic ratios of 20 grains. Whereas ¹⁴N/¹⁵N and ²⁶Al/²⁷Al 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 ³⁰Si/²⁸Si excesses up to 183‰ relative to solar. The average ²⁹Si/²⁸Si ratio of Y grains is by 59‰ smaller than that of mainstream grains. Ti isotopic ratios relative to ⁴⁸Ti are some-what similar to those of mainstream grains, but extend to more extreme anomalous compositions. One grain has ⁴⁶Ti/⁴⁸Ti, ⁴⁹Ti/⁴⁸Ti, and ⁵⁰Ti/⁴⁸Ti 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 ¹²C 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 ¹²C/¹³C, ³⁰Si/ ²⁸Si, and ⁴⁹Ti/⁴⁸Ti and ⁵⁰Ti/⁴⁸Ti 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 (²⁹Si and ³⁰Si) relative to ²⁸Si prior to solar system formation was lower than has been generally assumed, implying larger contributions of ²⁸Si from Type Ia supernovae compared to those from Type II supernovae. The Si isotopic ratios of Galactic cosmic rays also suggest such an evolution.