Presolar Silicon Carbide Grains of Types Y and Z: Their Molybdenum 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 Sr-88/Sr-87 and more variable Ba-138/Ba-136 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(circle dot) <= Z < 1.00 Z(circle dot), in which the C-13 neutron exposure for the slow neutron-capture process is greatly reduced with respect to that required by MS grains for a 1.0 Z(circle dot) 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.