Presolar Silicon Carbide Grains of Types Y and Z: Their Molybdenum Isotopic Compositions and Stellar Origins

We report Mo isotopic compositions of 37 presolar SiC grains of types Y (19) and Z (18), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. Direct comparison of the Y and Z grain data with data for mainstream grains from AGB stars of close-to-solar metallicity demonstrates that the three types of grains have indistinguishable Mo isotopic compositions. We show that the Mo isotope data can be used to constrain the maximum stellar temperatures (T-MAX) during thermal pulses in AGB stars. Comparison of FRUITY Torino AGB nucleosynthesis model calculations with the grain data for Mo isotopes points to an origin from low-mass (similar to 1.5-3 M-circle dot) rather than intermediate-mass (> 3-similar to 9 M-circle dot) AGB stars. Because of the low efficiency of Ne-22(alpha, n)Mg-25 at the low T-MAX values attained in low-mass AGB stars, model calculations cannot explain the large Si-30 excesses of Z grains as arising from neutron capture, so these excesses remain a puzzle at the moment.