Molybdenum Isotopes in Presolar Silicon Carbide Grains: Details of s-process Nucleosynthesis in Parent Stars and Implications for r- and p-processes

1,2Thomas Stephan,1,2,6Reto Trappitsch,3Peter Hoppe,1,2,4Andrew M. Davis,1,2,4,5Michael J. Pellin,1,2Olivia S. Pardo
The Astrophysical Journal 877,101 Link to Article [https://doi.org/10.3847/1538-4357/ab1c60]
1Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637, USA
2Chicago Center for Cosmochemistry, Chicago, IL, USA
3Max Planck Institute for Chemistry, 55128 Mainz, Germany
4The Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
5Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
6Present address: Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.

We have analyzed molybdenum isotopes, together with strontium and barium isotopes, in 18 presolar silicon carbide grains using the Chicago Instrument for Laser Ionization (CHILI), a resonance ionization mass spectrometer. All observed isotope ratios can be explained by mixtures of pure s-process matter with isotopically solar material. Grain residues were subsequently analyzed for carbon, nitrogen, silicon, and sulfur isotopes, as well as a subset for 26Al–26Mg systematics using the NanoSIMS. These analyses showed that all but one grain are mainstream grains, most probably coming from low-mass asymptotic giant branch (AGB) stars. One grain is of the AB type, for which the origin is still a matter of debate. The high precision of molybdenum isotope measurements with CHILI provides the best estimate to date for s-process molybdenum made in low-mass AGB stars. The average molybdenum isotopic abundances produced by the s-process found in the analyzed mainstream SiC grains are 0% 92Mo, 0.73% 94Mo, 13.30% 95Mo, 36.34% 96Mo, 9.78% 97Mo, 39.42% 98Mo, and 0.43% 100Mo. Solar molybdenum can be explained as a combination of 45.9% s-process, 30.6% r-process, and 23.5% p-process contributions. Furthermore, the observed variability in the individual grain data provides insights into the variability of conditions (neutron density, temperature, and timescale) during s-process nucleosynthesis in the grains’ parent stars, as they have subtle effects on specific molybdenum isotope ratios. Finally, the results suggest that the ratio between p– and r-process molybdenum in presolar SiC from many different types of parent stars is Mo p /Mo r  = 0.767, the value inferred for the solar system and consistent with what has been found in bulk samples and leachates of primitive meteorites.

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