Speaker
Description
The intermediate neutron-capture process (i-process) has emerged as one of the most promising yet least understood nucleosynthetic mechanisms responsible for the production of heavy elements beyond iron. Operating at neutron densities intermediate between those of the slow (s-) and rapid (r-) neutron-capture processes (~10¹³–10¹⁵ cm⁻³), the i-process is capable of producing distinctive abundance signatures that cannot be explained by the classical s- or r-process alone. Although theoretical models predict that the i-process may occur during proton-ingestion episodes in evolved stars, its astrophysical site, operating conditions, and contribution to Galactic chemical evolution remain uncertain. Carbon-Enhanced Metal-Poor (CEMP) stars exhibiting simultaneous enhancements of s- and r-process elements (the so-called CEMP-rs stars) provide one of the most compelling observational laboratories for investigating i-process nucleosynthesis, as their heavy-element abundance patterns closely resemble theoretical i-process predictions. In our recent study (A. M. Riyas et al. (2026)), we investigated the presence of very heavy neutron-capture elements, including the actinides, in a sample of CEMP stars using high-resolution, high signal-to-noise near-ultraviolet spectra obtained with UVES on the VLT. We successfully determined the abundances of very heavy r-process elements such as Tb, Ho, Tm, Yb, Os, Ir, and Ta, in approximately seventeen stars and detected actinides in three of them. The key result of this work is the detection of actinides in three CEMP-rs stars, representing the first observational evidence for the presence of actinides in confirmed CEMP-rs stars. The observed abundance patterns are in good agreement with theoretical i-process nucleosynthesis predictions, indicating that the i-process can make a significant contribution to the production of very heavy and actinide elements in CEMP-rs stars. These findings provide important new observational constraints on the operation of the i-process at low metallicities and offer valuable insights into the nucleosynthetic origin of heavy elements in the early Universe.
| Category | Observation |
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