Isotopic Compositions of Ruthenium Predicted from the NuGrid Project

TL;DR

This study uses NuGrid stellar models to predict ruthenium isotopic compositions in presolar grains, confirming low-mass stars as the likely source and accounting for decay contributions, thus enhancing understanding of stellar nucleosynthesis.

Contribution

It provides the first detailed isotopic predictions for Ru in presolar SiC grains from NuGrid models, linking stellar parameters to observed isotopic signatures.

Findings

Low-mass stars with low metallicity explain Ru isotopic compositions.

SiC grains do not form in massive star winds.

In-situ decay of ${^{99}}$Tc contributes to ${^{99}}$Ru abundance.

Abstract

The isotopic compositions of ruthenium (Ru) are measured from presolar silicon carbide (SiC) grains. In a popular scenario, the presolar SiC grains formed in the outskirt of an asymptotic giant branch (AGB) star, left the star as a stellar wind, and joined the presolar molecular cloud from which the solar system formed. The Ru isotopes formed inside the star, moved to the stellar surface during the AGB phase, and were locked into the SiC grains. Following this scenario, we analyze the NuGrid data which provide the abundances of the Ru isotopes in the stellar wind for a set of stars in a wide range of initial masses and metallicities. We apply the C>O (carbon abundance larger than the oxygen abundance) condition which is commonly adopted for the condition of the SiC formation in the stellar wind. The NuGrid data confirm that SiC grains do not form in the winds of massive stars. The isotopic compositions of Ru in the winds of low-mass stars can explain measurements. We find that lower-mass stars ($1.65~M_\odot$ and $2~M_\odot$) with low metallicity (Z=0.0001) can explain most of the measured isotopic compositions of Ru. We confirm that the abundance of ${^{99}}$Ru inside the presolar grain includes the contribution from the in-situ decay of ${^{99}}$Tc. We also verify our conclusion by comparing the isotopic compositions of Ru integrated over all the pulses with those calculated at individual pulses.