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

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 mainstream (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 of 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 low-mass asymptotic giant branch (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, which are classical AGB stars.

Slow Neutron-Capture Process: Low-Mass Asymptotic Giant Branch Stars and Presolar Silicon Carbide Grains

“…Thus, the stellar origins of Y and Z grains remain ambiguous. Finally, the wide range of 14 N/ 15 N ratios observed in types MS, Y, and Z grains result mainly from contamination with terrestrial and asteroidal materials [51]. The intrinsic N isotopic compositions of AGB dust grains are characterized by large 14 N excesses compared to the solar composition [51].…”

“…The intrinsic N-isotopic compositions of AGB dust grains are characterized by large 14 N excesses compared to the solar composition [50]. It remains a question whether "uncontaminated" MS, Y, and Z grains have different 14 N/ 15 N ratios [51]. Presolar SiC grains from AGB stars include mainstream (MS, ~85-90%), Y (~1-3%), and Z (~1-3%) grains, which are classified based on C and Si isotope ratios (Figure 2).…”

The Achievements of the RockStar Group (Perugia) on Astrophysical Modelling and Pallasite Geochemistry

“…The isotopic compositions of presolar grains are determined in laboratories all over the world, e.g., [13][14][15][16][17]. A convenient source for accessing the data for a wide range of isotopic compositions and grain types is the "Presolar Grain Database" (PGD) maintained by the Department of Physics of Washington University in St. Louis [18,19].…”

“…It is a collection of spreadsheets containing the isotope data on presolar grains (SiC, Graphite, Oxides, and Silicates; [18,19]). As an example, the PGD for Mainstream SiC contains ~20,000 isotopic ratio measurements for Si and C, mainly determined by Nanoscale Secondary Ion Mass Spectrometry (NanoSims, e.g., [17]), and a few identifications of trace heavy elements, analyzed by Resonance Ionization Mass Spectrometry (RIMS, e.g., [20,21]). The majority of those grains (>90%), known as mainstream (MS) SiC grains, have been shown to provide crucial knowledge about the nucleosynthesis of the progenitor stars with higher accuracy ( 10%) than direct stellar observations (typically about a factor of two).…”

Underground Measurements of Nuclear Reaction Cross-Sections Relevant to AGB Stars

“…Currently the success in reproducing 12 C/ 13 C ratios appears to indicate that AGB stars experience extra mixing episodes [46]. Furthermore, discrepancies were found in the recent analysis of the presolar SiC grains [47], where it is evidenced that the discrepancy could be solved by extra mixing caused by the magnetic buoyancy [48]. Hence, further insight into complex AGB mixing events can be obtained by constraining the isotopic ratio predictions.…”