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

Abstract

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

arXiv commentThe Astrophysical Journal (Accepted)

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• Cosmic nucleosynthesis: a multi-messenger challenge
• On the mass and metallicity distribution of parent AGB stars of presolar SiC
• New Multielement Isotopic Compositions of Presolar SiC Grains: Implications for Their Stellar Origins
• Presolar O- and C-anomalous grains in unequilibrated ordinary chondrite matrices
• Slow Neutron-Capture Process: Low-mass AGB stars and presolar silicon carbide grains
• Presolar Silicon Carbide Grains of Types Y and Z: Their Strontium and Barium Isotopic Compositions and Stellar Origins
• Presolar grains
• Representation of s-process abundances for comparison to data from bulk meteorites
• Impact of $\text{T}$- and $\rho$-dependent decay rates and new (n,$\gamma$) cross sections on the $\text{s}$ process in low-mass AGB stars