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

TL;DR

This study analyzes molybdenum isotopic compositions in presolar SiC grains of types Y and Z, revealing their stellar origins from low-mass AGB stars and highlighting unresolved issues regarding silicon isotopic anomalies.

Contribution

It provides the first detailed Mo isotopic analysis of Y and Z grains, constrains stellar temperatures during grain formation, and suggests their origins from low-mass AGB stars.

Findings

Y and Z grains have similar Mo isotopic compositions to mainstream grains.

Mo isotopic data constrains AGB stellar temperatures during thermal pulses.

Low-mass AGB stars are likely sources of these presolar grains.

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({\alpha},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.