New Multielement Isotopic Compositions of Presolar SiC Grains: Implications for Their Stellar Origins

TL;DR

This study provides new isotopic data for presolar SiC grains from the Murchison meteorite, revealing contamination effects and offering insights into their stellar origins, especially related to carbon star processes.

Contribution

It presents the first comprehensive isotopic analysis of presolar SiC grains with improved contamination control, refining stellar origin models and nucleosynthesis processes.

Findings

Contamination mainly affects surface isotopic signals.

New data align with carbon star observations.

Constraints on stellar processes like cool bottom processing.

Abstract

We report NanoSIMS Si and Mg-Al isotopic data (and C, N, and Ti isotopic data when available) for 85 submicron- to micron-sized presolar SiC grains from the CM2 Murchison meteorite, including 60 MS, 8 AB1, 8 X, 7 AB2, and 2 Y grains. The MS and Y grain data demonstrate that (1) C and N contamination mainly appears as surface contamination, and sufficient presputtering is needed to expose a clean grain surface for obtaining intrinsic C and N signals, and (2) Mg and Al contamination appears as adjacent grains and rims, and high-resolution imaging and the choice of small regions of interest during data reduction together are effective in suppressing the contamination. Our results strongly indicate that previous studies on presolar SiC grains could have sampled differing degrees of contamination for C, N, Mg, and Al. Compared to the literature data, our new MS and Y grains are in better agreement with carbon star observations for both the C and N isotopic ratios. By comparing our new, tighter distributions of 12C/13C, 14N/15N, and initial 26Al/27Al ratios for MS and Y grains with FRUITY AGB stellar models, we provide more stringent constraints on the occurrence of cool bottom processing and the production of 26Al in N-type carbon stars, classical asymptotic giant branch stars.