Presolar silicon carbide grains of types Y and Z: their strontium and barium isotopic compositions and stellar origins.

We report the Sr and Ba isotopic compositions of 18 presolar SiC grains of types Y (11) and Z (7), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. We find that the Y and Z grains show higher ⁸⁸Sr/⁸⁷Sr and more variable ¹³⁸Ba/¹³⁶Ba ratios than mainstream (MS) grains. According to FRANEC Torino AGB models, the Si, Sr, and Ba isotopic compositions of our Y and Z grains can be consistently explained if the grains came from low-mass AGB stars with 0.15 Z_⊙ ≤ Z < 1.00 Z_⊙, in which the ¹³C neutron exposure for the slow neutron-capture process is greatly reduced with respect to that required by MS grains for a 1.0 Z_⊙ AGB star. This scenario is in line with the previous finding based on Ti isotopes, but it fails to explain the indistinguishable Mo isotopic compositions of MS, Y, and Z grains. [ABSTRACT FROM AUTHOR]