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Detection of actinides in CEMP-rs stars

This study presents the first detection of thorium and marginal detection of uranium in carbon-enhanced metal-poor stars with hybrid r- and s-process signatures (CEMP-rs), confirming that an intermediate neutron-capture process in low-mass, very low-metallicity AGB stars can account for the observed actinide abundances.

Original authors: A. M. Riyas (Department of Physics, University of Calicut, Thenhipalam, Malappuram, Kerala, India), D. Karinkuzhi (Department of Physics, University of Calicut, Thenhipalam, Malappuram, Kerala, India

Published 2026-06-23

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CC BY 4.0

Original authors: A. M. Riyas (Department of Physics, University of Calicut, Thenhipalam, Malappuram, Kerala, India), D. Karinkuzhi (Department of Physics, University of Calicut, Thenhipalam, Malappuram, Kerala, India, Institute of Astronomy and Astrophysics, Université Libre de Bruxelles), S. Van Eck (Institute of Astronomy and Astrophysics, Université Libre de Bruxelles, BLU-ULB, Brussels Laboratory of the Universe, blu.ulb.be), A. Choplin (Institute of Astronomy and Astrophysics, Université Libre de Bruxelles, BLU-ULB, Brussels Laboratory of the Universe, blu.ulb.be), S. Goriely (Institute of Astronomy and Astrophysics, Université Libre de Bruxelles, BLU-ULB, Brussels Laboratory of the Universe, blu.ulb.be), L. Siess (Institute of Astronomy and Astrophysics, Université Libre de Bruxelles, BLU-ULB, Brussels Laboratory of the Universe, blu.ulb.be), A. A. Farha (Department of Physics, University of Calicut, Thenhipalam, Malappuram, Kerala, India)

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant cosmic kitchen. For decades, astronomers have known that this kitchen has two main chefs who cook up heavy elements (like gold, lead, and uranium) from lighter ingredients. One chef, the Slow Chef, works methodically over a long time (the s-process), while the other, the Fast Chef, works in a frantic, explosive burst (the r-process).

Usually, stars show signs of being cooked by just one of these chefs. But there's a weird group of stars called CEMP-rs stars that seem to have a "hybrid" flavor. They taste like they were cooked by both chefs at the same time, which has been a mystery for a long time.

This paper is like a team of food critics (astronomers) going into the kitchen to taste-test these stars and figure out exactly how they were made. Here is what they found, broken down simply:

1. The Mystery Ingredient: The "Heavy" Elements

The critics were specifically looking for the heaviest, rarest ingredients in the cosmic pantry: Thorium and Uranium.

The Problem: These ingredients are like tiny, invisible specks of dust in a giant soup. They are hard to see because they get hidden behind stronger flavors (other elements and carbon molecules) in the star's light.
The Goal: They wanted to see if these stars had these heavy ingredients, which would tell them if the "Fast Chef" (r-process) or the "Slow Chef" (s-process) was responsible.

2. The Investigation: Using a Super-Microscope

The team used a powerful telescope (the VLT in Chile) equipped with a high-resolution camera (UVES) to take a very sharp "photo" of the light coming from three specific stars.

The Result: They successfully found Thorium in all three stars. It was like finding a specific, rare spice in the soup.
The Miss: They tried to find Uranium, but it was too faint to see clearly. They could only say, "It's not there in huge amounts," but couldn't confirm its presence.

3. The Big Discovery: The "Middle Chef"

For a long time, scientists thought Thorium and Uranium could only be made by the "Fast Chef" (the explosive r-process).

The Twist: The team compared their findings with computer simulations of a third type of cooking called the i-process (intermediate process). This happens in aging, low-mass stars (called AGB stars) when they accidentally swallow some hydrogen fuel, causing a middle-ground reaction.
The Match: The computer models showed that this "Middle Chef" (i-process) could actually cook up Thorium and Uranium in the exact amounts the team observed.
The Conclusion: This is the first time actinides (like Thorium) have been found in these specific hybrid stars, and the evidence points to the i-process as the culprit, not a mix of two separate chefs.

4. Why This Matters (and Why It's Tricky)

The paper mentions that Thorium and Uranium are radioactive, meaning they decay over time like a ticking clock. In theory, if you know how much was made and how much is left, you can calculate the star's age.

The Catch: The authors warn that while they found the ingredients, the "recipe" (the nuclear physics) is still a bit fuzzy. The computer models have big margins of error.
The Verdict: Right now, they cannot use these stars as a precise clock to tell us exactly how old the universe is. The "kitchen timer" is still broken. However, finding these elements proves that the "Middle Chef" is a real and powerful force in the universe.

Summary

In short, this paper is a detective story where astronomers found rare, heavy elements in strange stars. They proved that these elements didn't come from the usual "explosive" sources, but rather from a unique, intermediate cooking process inside aging stars. While they can't yet use this to tell the exact age of the stars, they have successfully identified a new way the universe creates its heaviest ingredients.

Technical Summary: Detection of Actinides in CEMP-rs Stars

Problem Statement
Carbon-Enhanced Metal-Poor (CEMP) stars exhibiting hybrid enrichments of slow (s-) and rapid (r-) neutron-capture elements, known as CEMP-rs stars, present a long-standing nucleosynthetic puzzle. While recent studies suggest the intermediate neutron-capture process (i-process) occurring in low-mass, very low-metallicity Asymptotic Giant Branch (AGB) stars can explain the heavy-element patterns in these objects, the origin of actinides (specifically thorium and uranium) remains contentious. Actinide production is traditionally associated with explosive r-process nucleosynthesis in highly neutron-rich environments. Furthermore, detecting these elements in stellar spectra is notoriously difficult due to severe line blending from carbon-bearing molecules and other elements, particularly in carbon-rich stars. Prior to this work, actinide measurements in CEMP-rs stars were largely unexplored, with only tentative detections in a few hybrid-enriched stars.

Methodology
The authors investigated a sample of three CEMP-rs stars (HD 187861, HD 224959, and CS 22891−171) previously analyzed by Karinkuzhi et al. (2021). The study utilized high-resolution, high signal-to-noise spectra obtained with the UVES spectrograph on the ESO VLT (Kueyen), covering wavelengths from ~3280 Å to 6835 Å.

Key methodological steps included:

Stellar Parameters: Parameters ( $T_{\text{eff}}$ , $\log g$ , $\xi$ , [Fe/H]) were refined using Non-Local Thermodynamic Equilibrium (NLTE) corrections based on the TURBOSPECTRUM code and MPIA NLTE tools, iterating until excitation and ionization equilibria were achieved for iron lines.
Spectral Analysis: The authors performed spectral synthesis using the latest non-LTE version of TURBOSPECTRUM. They focused on specific Th II and U II lines, carefully accounting for blending with CN and Fe I features.
Abundance Determination: Thorium abundances were derived from individual lines (e.g., 3469.92 Å, 4019.13 Å, 4086.52 Å), while uranium abundances were constrained via upper limits due to the non-detection of key U II lines.
Theoretical Comparison: The observed abundance patterns were compared against nucleosynthesis predictions from STAREVOL AGB models. These models simulated the i-process during Proton Ingestion Episodes (PIEs) for low-mass (1–2 $M_{\odot}$ ), low-metallicity ( $-3 \le$ [Fe/H] $\le -2$ ) stars. A $\chi^2$ minimization procedure was used to find the best-fitting dilution factors and model parameters, comparing i-process predictions against pure s-process models.

Key Results

Detection of Thorium: Thorium was robustly detected in all three program stars. The derived abundances correspond to [Th/Fe] values in the range of 1.3 to 1.6, which, while at the upper end, are comparable to values measured in r-process-enhanced stars.
Constraints on Uranium: Uranium was not definitively detected in any of the stars. The U II lines at 3859.571 Å, 4050.041 Å, and 4090.132 Å were too weak and blended to yield measurements, resulting only in upper limits.
Nucleosynthetic Origin: The observed abundance patterns, including the presence of thorium, are better reproduced by i-process AGB models than by pure s-process models. The i-process models yield lower reduced $\chi^2$ values for the sample stars. Notably, the s-process models systematically overproduce Sr–Nb elements, whereas the i-process models (specifically for HD 224959) closely match the observed pattern, with minor discrepancies in Iridium and Ytterbium attributed to measurement uncertainties or model limitations.
Chronometry Limitations: While the detection of actinides theoretically allows for cosmochronometry, the authors found that current uncertainties prevent reliable age determination. Theoretical uncertainties in the i-process production ratios (specifically [Th/Eu] and [Th/U]), driven largely by unknown nuclear reaction rates (e.g., the $^{217}\text{Bi}(n,\gamma)^{218}\text{Bi}$ branching rate), introduce age uncertainties exceeding 20–100 Gyr. Additionally, the lack of precise uranium measurements precludes the use of the Th/U ratio, which is typically the most reliable chronometer.

Significance and Claims
This study reports the first confirmed detection of actinides (specifically thorium) in stars classified as CEMP-rs. The primary significance lies in demonstrating that the i-process in low-mass AGB stars is capable of synthesizing actinides without requiring a separate r-process site. This supports the hypothesis that the hybrid chemical signatures of CEMP-rs stars arise from a single nucleosynthetic event (the i-process) rather than a superposition of independent s- and r-process contributions.

While the authors acknowledge that robust quantitative predictions for actinide yields require further progress in nuclear physics and stellar modeling, their findings provide new observational constraints on heavy-element nucleosynthesis. They posit that these detections offer a potential, albeit currently limited, pathway for estimating the time elapsed since the proton-ingestion episode in the progenitor AGB stars and placing lower limits on the ages of the resulting white dwarf remnants, provided nuclear uncertainties are significantly reduced in the future.

1. The Mystery Ingredient: The "Heavy" Elements

2. The Investigation: Using a Super-Microscope

3. The Big Discovery: The "Middle Chef"

4. Why This Matters (and Why It's Tricky)

Summary

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