Chlorine and Potassium Enrichment in the Cassiopeia A Supernova Remnant

Abstract

The elements in the universe are synthesized primarily in stars and supernovae, where nuclear fusion favors the production of even-Z elements. In contrast, odd-Z elements are less abundant and their yields are highly dependent on detailed stellar physics, making theoretical predictions of their cosmic abundance uncertain. In particular, the origin of odd-Z elements such as phosphorus (P), chlorine (Cl), and potassium (K), which are important for planet formation and life, is poorly understood. While the abundances of these elements in Milky Way stars are close to solar values, supernova explosion models systematically underestimate their production by up to an order of magnitude, indicating that key mechanisms for odd-Z nucleosynthesis are currently missing from theoretical models. Here we report the observation of P, Cl, and K in the supernova remnant Cassiopeia A using high-resolution X-ray spectroscopy with XRISM, with the detection of K at above the 6$\sigma$ level being the most significant finding. Supernova explosion models of normal massive stars cannot explain the element abundance pattern, especially the high abundances of Cl and K, while models that include stellar rotation, binary interactions or shell mergers agree closely with the observations. Our observations suggest that such stellar activity plays a significant role in supplying these elements to the universe.

29 pages, 9 figures, 2 tables. Published in Nature Astronomy, 4 December 2025

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Cited by in corpus (1)

• Production of heavy $\alpha$-elements and ${}^{44}$Ti in Cas A: comparison to abundances from 1D core-collapse supernova models and evidence for Carbon-Oxygen shell mergers