C, N, and O Isotopic Heterogeneities in Low-density Supernova Graphite Grains from Orgueil

TL;DR

This study uses NanoSIMS isotope imaging to analyze low-density supernova graphite grains from Orgueil, revealing isotopic heterogeneities and hotspots indicating complex stellar formation environments.

Contribution

It provides detailed spatial isotopic maps of supernova graphite grains, highlighting internal heterogeneities and correlations that suggest formation in dynamic stellar environments.

Findings

Hotspots of $^{18}O$ and $^{15}N$ are spatially correlated and enriched.

Grains show surface heterogeneities with extreme isotope excesses.

C isotopic compositions exhibit radial gradients and anomalous pockets.

Abstract

We report on the results of NanoSIMS isotope imaging of low-density supernova graphite grains from the Orgueil meteorite. 70 nm thick microtomed sections of three supernova graphite grains were deposited on Si wafers and isotopically imaged in the NanoSIMS. These sections contain hotspots of excesses in $^{18}O$ and $^{15}N$ which are spatially well correlated, and are likely carried by internal TiC subgrains. These hotspots are considerably more enriched in $^{18}O$ and $^{15}N$ than the host graphite grain. Correlations between $^{18}O$ and $^{15}N$ excesses indicate that the grains incorporated material from the He/C supernova zone. Isotope images of the surfaces of some grains show heterogeneities in their N and O isotope compositions, with extreme excesses in $^{15}N$ and $^{18}O$. In the microtome sections, we also observe two types of heterogeneities in the grains' C isotopic compositions: smooth, radial gradients in $^{12}C/^{13}C$, with this ratio trending toward solar with increasing radius; and highly anomalous pockets up to 2 {\mu}m in size with $^{12}C/^{13}C$ > solar that are located near the centers of the grain sections. Partial isotopic equilibration does not likely explain the C isotopic heterogeneities. These grains and their constituent parts probably formed in a stellar environment with changing isotopic composition.