The stellar origins of ⁹⁶Zr excesses in presolar graphites from the Murchison meteorite

¹ Department of Physics, University of Louisiana at Lafayette, Lafayette, LA 70503, USA
² Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA, USA
³ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, HUN-REN, Konkoly Thege M. út 15-17, H-1121 Budapest, Hungary
⁴ CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, Budapest H-1121, Hungary
⁵ NuGrid Collaboration, http://nugridstars.org
⁶ University of Bayreuth, BGI, Universitätsstraße 30, 95447 Bayreuth, Germany
⁷ Istituto Nazionale di Fisica Nucleare – Laboratori Nazionali del Sud, Via Santa Sofia 62, Catania I-95123, Italy
⁸ ELTE Eötvös Loránd University, Institute of Physics and Astronomy, Pázmány Péter sétány 1/A, Budapest 1117, Hungary
⁹ School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
¹⁰ Department of Experimental Physics, Institute of Physics, University of Szeged, Dóm tér 9, H-6720 Szeged, Hungary
¹¹ Department of Physics and Astronomy, University of Victoria, British Columbia, V8W 2Y2, Canada
¹² Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA
¹³ Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
¹⁴ Department of Physics and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130, USA

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Received: 1 August 2025
Accepted: 26 November 2025

Abstract

Context. Zirconium-96 is a stable isotope that can be synthesized under different neutron-rich nucleosynthetic conditions. Astrophysical models predict its production to occur in various stellar environments: from low-to-intermediate-mass asymptotic giant branch (AGB) stars to massive stars and core-collapse supernovae.

Aims. Detections of ⁹⁶Zr excesses, in combination with other isotopic measurements from presolar grains can provide unique constraints on its stellar origin. Presolar grains are microscopic particles found in primitive Solar System materials, which formed in stellar winds and supernova ejecta. The isotopic composition of each grain can provide us a snapshot of the nucleosynthetic processes that took place during the parent star’s lifetime.

Methods. In this study, we measured the stable isotopes of C, N, O, Mo, Zr, and Ru in high-density presolar graphite grains from the Murchison meteorite and found four grains that contain positive isotopic anomalies in ⁹⁶Zr carried by their internal subgrains. We analyzed multi-element isotopic datasets from each grain to explore the source of the observed ⁹⁶Zr excesses.

Results. Comparisons with stellar models indicate that two grains likely condensed in an intermediate-mass AGB star with initial metallicity of Z ≤ 0.014. Their ⁹⁶Zr/⁹⁴Zr ratios also match those predicted for born-again AGB stars undergoing a very late thermal pulse and rapidly accreting white dwarfs. After comparing the relative populations of the aforementioned dust-producing stars, we propose rapidly accreting white dwarfs as a new, and more likely, stellar source for one of the presolar grains. The remaining two grains could have originated in the supernova ejecta of massive stars, due to correlated excesses in the p-nuclides, ^92, 94Mo. Thus, grains with ⁹⁶Zr anomalies can have a variety of stellar origins, in agreement with theoretical studies.

Conclusions. Our study highlights the importance of multi-element analysis in constraining the types of stars where presolar grains have condensed. These data will help improve our understanding of various nucleosynthesis processes in different stellar phases.