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 (∼1.5–3 M ⊙ ) rather than intermediate-mass (>3–∼9 M ⊙ ) AGB stars. Because of the low efficiency of 22 Ne( α , n ) 25 Mg at the low T MAX values attained in low-mass AGB stars, model calculations cannot explain the large 30 Si excesses of Z grains as arising from neutron capture, so these excesses remain a puzzle at the moment.