Speaker
Description
Presolar grains are microscopic dust grains that formed in the gas outflows or explosions of ancient stars that died before the formation of the solar system. The majority (~90% in number) of presolar SiC grains, including Types mainstream (MS), Y, and Z, came from low-mass C-rich asymptotic giant branch (AGB) stars, which is supported by the ubiquitous presence of SiC dust observed in the circumstellar envelope of AGB stars and the signatures of the slow neutron-capture process (s-process) preserved in these grains.
We report NanoSIMS Si and Mg–Al isotopic data (and C, N, and Ti isotopic data, when available) for 62 new MS and Y grains from the CM2 Murchison meteorite. 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.
In addition, we report 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. The Y and Z grains show Sr and Ba isotopic compositions similar to MS grains, which challenges their proposed low-metallicity AGB stellar origins especially given that 88Sr/87Sr is expected to increase linearly with decreasing initial stellar metallicity based on AGB stellar nucleosynthesis models and stellar observations. We find that the Si, Sr, and Ba isotopic compositions of our Y and Z grains can be consistently explained if the amount of 13C in the 13C pocket is reduced by a factor of 4.0–7.8 in Torino AGB models for a 0.3 Z8 AGB star 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 (1) the indistinguishable Mo isotopic compositions of MS, Y, and Z grains and (2) the lack of SiC grains in the presolar SiC grain inventory from low metallicity AGB stars.
| Session | Dust and presolar grains |
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