Speaker
Description
We have measured carbon, nitrogen, zirconium, molybdenum, and barium isotopes in two large presolar graphite grains found in situ in sections of the CM2 carbonaceous chondrites Murchison and Maribo. Carbon and nitrogen were analyzed using nanometer-scale secondary ion mass spectrometry (NanoSIMS), and heavy elements were measured by resonance ionization mass spectrometry (RIMS) with the Chicago Instrument for Laser ionization (CHILI) [1].
The grain from Maribo showed nearly pure s-process molybdenum in a subgrain, probably a refractory carbide, also containing highly enriched s-process zirconium. The Murchison grain is enriched in s-process molybdenum and zirconium as well. Barium concentrations in both grains are low, but isotopic ratios consistent with the s-process were also observed. Both grains show significant enrichments in $^{13}$C, while nitrogen isotope ratios are close to the terrestrial value.
These findings are consistent with condensation in the outflows of low-mass, low-metallicity asymptotic giant branch (AGB) stars. However, the observation of nearly pure s-process molybdenum in the Maribo grain is inconsistent with current AGB star models [3–5]. Similar enrichments in s-process ruthenium, also outside the range of such models, have been observed in a graphite grain extracted from Murchison [2]. All these AGB star models assume homogenization of s-process material in the convective stellar envelope after each third dredge-up (TDU) episode, and prior to grain condensation. One explanation for our findings is that these grains formed in isotopically inhomogeneous regions, high-density clumps, arising during TDU events, as has been suggested before [6]. Another option could be a very late thermal pulse from a post-AGB star [7].
Furthermore, neutron capture at $^{95}$Zr, an important s-process branch point, was only marginally activated for our grains, further challenging the stellar models and theoretically derived $^{95}$Zr neutron-capture cross sections.
Improved stellar nucleosynthesis models should take these new observations from presolar grains into account. Published isotope data of presolar silicon carbide and graphite grains are compiled in the Presolar Grain Database [8, 9] (latest versions available at https://zenodo.org/doi/10.5281/zenodo.8187219 and
https://zenodo.org/doi/10.5281/zenodo.11188115).
[1] Stephan T. et al. (2016) Int. J. Mass Spectrom. 407, 1–15. [2] Stephan T. et al. (2026) Eur. Phys. J. A 62, 47. [3] Cristallo S. et al. (2011) Astrophys. J. Suppl. Ser. 197, 17. [4] Cristallo S. et al. (2015) Astrophys. J. Suppl. Ser. 219, 40. [5] Szányi B. et al. (2025) Astron. Astrophys. 697, A48. [6] Croat T. K. et al. (2005) Astrophys. J. 631, 976–987. [7] Jadhav M. et al. (2013) Astrophys. J. Lett. 777, L27. [8] Stephan T. et al. (2024) Astrophys. J. Suppl. Ser. 270, 27. [9] Stephan T. et al. (2024) Meteorit. Planet. Sci. 59, A399 (#6388).
| Career stage | Pre-tenure/untenured mid-to-late-career researcher |
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