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

TL;DR

This study analyzes the Sr and Ba isotopic compositions of presolar SiC grains of types Y and Z, linking their stellar origins to low-metallicity AGB stars and providing insights into nucleosynthesis processes.

Contribution

It provides the first detailed isotopic analysis of Y and Z presolar SiC grains and links their compositions to specific low-metallicity AGB stellar sources.

Findings

Y and Z grains show higher 88Sr/87Sr ratios than mainstream grains.

The isotopic compositions suggest origins in low-mass AGB stars with reduced 13C neutron exposure.

The Mo isotopic compositions remain indistinguishable among grain types.

Abstract

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 88Sr/87Sr and more variable 138Ba/136Ba 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 Zsun <= Z < 1.00 Zsun, in which the 13C neutron exposure for the slow neutron-capture process is greatly reduced with respect to that required by MS grains for a 1.0 Z8 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.