Presolar Silicon Carbide Grains of Types Y and Z: Their Molybdenum Isotopic Compositions and Stellar Origins

1,2,3Nan Liu,4,5Thomas Stephan,6,7Sergio Cristallo,8Roberto Gallino,4,5Patrick Boehnke,3Larry R. Nittler,3Conel M. O’D. Alexander,4,5,9Andrew M. Davis,4,5,10Reto Trappitsch,4,5,11Michael J. Pellin,12,13Iris Dillmann
The Astrophysical Journal 881, 28 Link to Article []
1Laboratory for Space Sciences and Physics Department, Washington University in St. Louis, St. Louis, MO 63130, USA
2McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA
3Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA
4Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
5Chicago Center for Cosmochemistry, Chicago, IL, USA
6INAF-Osservatorio Astronomico d’Abruzzo, Teramo 64100, Italy
7INFN-Sezione di Perugia, Perugia 06123, Italy
8Dipartimento di Fisica, Università di Torino, Torino 10125, Italy
9The Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
10Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
11Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
12TRIUMF, 4004 Westbrook Mall, Vancouver, British Columbia V6T 2A3, Canada
13Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia V8P 5C2, Canada

We report Mo isotopic compositions of 37 presolar SiC grains of types Y (19) and Z (18), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. Direct comparison of the Y and Z grain data with data for mainstream grains from AGB stars of close-to-solar metallicity demonstrates that the three types of grains have indistinguishable Mo isotopic compositions. We show that the Mo isotope data can be used to constrain the maximum stellar temperatures (T MAX) during thermal pulses in AGB stars. Comparison of FRUITY Torino AGB nucleosynthesis model calculations with the grain data for Mo isotopes points to an origin from low-mass (~1.5–3 M ) rather than intermediate-mass (>3–~9 M ) AGB stars. Because of the low efficiency of 22Ne(α, n)25Mg at the low T MAX values attained in low-mass AGB stars, model calculations cannot explain the large 30Si excesses of Z grains as arising from neutron capture, so these excesses remain a puzzle at the moment.


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