J-type Carbon Stars: A Dominant Source of 14N-rich Presolar SiC Grains of Type AB

Nan Liu1, Thomas Stephan2,3, Patrick Boehnke2,3, Larry R. Nittler1, Conel M. O’D. Alexander1, Jianhua Wang1, Andrew M. Davis2,3,4, Reto Trappitsch2,3,5, and Michael J. Pellin2,3,4,6
The Astrophysical Journal Letters 844 L12 Link to Article [https://doi.org/10.3847/2041-8213/aa7d4c]
1Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA
2Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
3Chicago Center for Cosmochemistry, Chicago, IL, USA
4The Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
5Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
6Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA

We report Mo isotopic data of 27 new presolar SiC grains, including 12 14N-rich AB (14N/15N > 440, AB2) and 15 mainstream (MS) grains, and their correlated Sr and Ba isotope ratios when available. Direct comparison of the data for the MS grains, which came from low-mass asymptotic giant branch (AGB) stars with large s-process isotope enhancements, with the AB2 grain data demonstrates that AB2 grains show near-solar isotopic compositions and lack s-process enhancements. The near-normal Sr, Mo, and Ba isotopic compositions of AB2 grains clearly exclude born-again AGB stars, where the intermediate neutron-capture process (i-process) takes place, as their stellar source. On the other hand, low-mass CO novae and early R- and J-type carbon stars show 13C and 14N excesses but no s-process enhancements and are thus potential stellar sources of AB2 grains. Because both early R-type carbon stars and CO novae are rare objects, the abundant J-type carbon stars (10%–15% of all carbon stars) are thus likely to be a dominant source of AB2 grains.

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