Titanium isotopic compositions of rare presolar SiC grain types from the Murchison meteorite

摘要

We report the Ti isotopic compositions of 8 mainstream, 22 Y, 9 Z, and 26 AB presolar SiC grains from two SiC-rich residues of the Murchison CM2 meteorite together with Si, C and some Mg-Al isotopic data for the same grains. Mainstream, Y and Z grains are believed to originate in asymptotic giant branch (AGB) stars of varying metallicities, but the stellar sources of AB grains are poorly understood. We find that the Ti-46,Ti-47,Ti-49/Ti-48 ratios are correlated with Si-29/Si-28 for all of the grain types, indicating that these ratios are mainly dominated by Galactic chemical evolution (GCE). The mainstream, Y and Z grains all show enrichments in Ti-50 from neutron capture nucleosynthesis. However, AGB models predict smaller excesses in Ti-50 (and Ti-49) than are observed in these grains. For Z grains and especially for Y grains, the enhancement of Ti-50 is greater than the enhancement in Si-30, indicating that the C-13 neutron source produced a greater total fluence of neutrons than the Ne-22 source in the low metallicity parent AGB stars. The Z grains plot below the mainstream correlation lines at more Ti-48-and Si-28-rich compositions in plots of Ti-46,Ti-47,Ti-49/Ti-48 vs. Si-29/Si-28. On the other hand, the Y grains plot close to the mainstream correlation line. This could imply that the Ti isotopes evolved non-linearly at metallicities below similar to 1/3 solar. The AB grains in this study have Ti isotopic compositions that fall along correlation lines defined by the mainstream grains, suggesting origins in close to solar metallicity stars. However, these grains fall below the mainstream correlation lines in plots of Ti-46,Ti-49,Ti-50/Ti-48 vs. Si-29/Si-28 and do not show enhancements in Ti-50, indicating that their parent stars did not undergo significant s-process nucleosynthesis. These data support origins of AB grains in J-type C stars rather than born-again AGB stars that undergo s-process nucleosynthesis. AB grains that do not have Ti-50 excesses may pro