List of presentations

Understanding the actinide production through the Th/Eu abundance ratio

Ainun-Nahdhia AzhariAinun-Nahdhia Azhari

While the r-process production, in particular the actinide-to-lanthanide abundance ratio, has been widely studied in metal-poor stars, their evolution in the metal-rich regime is poorly understood. One cause is the limited Th abundance measurement due to the difficulty of detecting the commonly used Th II 4019 Å absorption line in stellar spectra. Our study aims to explore the actinide production in the metal-rich regime, as well as the possibility of deriving the Th abundance using a cleaner absorption line at 5989 Å. We present the [Th/Eu] ratio measurement for a sample of metal-rich disk stars. Our sample covers 89 giants with metallicities -0.7 < [Fe/H] < 0.4 and ages from a few hundred Myr to ~ 14 Gyr. Age information is essential for studying the radioactive element Th, as it allows us to correct for decay and recover the initial production ratios. We derive Th and Eu abundances through spectral fitting of high-resolution spectra (R~80,000) obtained with the High Dispersion Spectrograph (HDS) at the Subaru Telescope, and calculate the initial Th abundance employing stellar age independently determined from seismology. We also demonstrate the viability of using the less-studied Th II 5989 Å line to measure Th abundance, opening the possibility of extending such measurements to larger samples, especially giant stars.

We examine the trend of [Th/Eu] and stellar age, and find that old stars show a higher mean [Th/Eu] initial abundance ratio compared to young stars. Furthermore, a scatter of [Th/Eu] initial abundance ratios is found among old stars in our sample, as also shown in a few previously studied very metal-poor old stars. This finding hints at a non-constant Th/Eu production ratio. Further study of a large sample of very metal-poor and extremely metal-poor stars is required to understand the actinide-to-lanthanide production ratio better.

R-process nucleosynthesis and enrichment by stellar mergers

Aldana Grichener

Interacting massive binaries involving neutron stars (NSs) play a crucial role in r-process nucleosynthesis and enrichment across the Universe. Electromagnetic observations following binary NS mergers have revealed clear signatures of r-process elements, confirming these events as a site of heavy element production. However, despite this groundbreaking progress, it remains uncertain whether binary NS mergers alone can fully account for the observed abundance of r-process elements. Particularly, the long delay between their birth and merger challenges the role of binary neutron star mergers as main r-process enrichment sites of metal-poor stars in our Galaxy and in ultra-faint dwarf galaxies, highlighting the need for additional sources. In this talk, I will present a complementary site: mergers of neutron stars with cores of giant stars during common envelope evolution (CEE). When a NS orbits a giant companion, the companion’s expansion may lead to the engulfment of the NS. This initiates a common envelope evolution (CEE) phase during which orbital energy is transferred to the envelope, leading to a substantial decrease in the orbital separation and ejection of the envelope mass. If the NS merges with the core of the giant during this process, a dense accretion disk can form around the NS from disrupted core material. The disk can then launch jets, and the extreme conditions within both the accretion disk and jets might lead to their neutronization and subsequent r-process nucleosynthesis. I will discuss the physical conditions required for this channel, its potential contribution to r-process enrichment in the Milky Way, and the observational signatures that could distinguish it from other sources.

Heavy element Nucleosynthesis: Insights from Metal-Poor Halo Stars and Globular Clusters

Avrajit Bandyopadhyay

The study of metal-poor stars in the Galactic halo and globular clusters provides critical insights into early nucleosynthesis and the chemical evolution of the Milky Way. In this talk, we investigate the abundance patterns of light, alpha, Fe-peak, and r-process elements across these old stellar populations to probe their formation and enrichment histories. Using high-resolution spectra from the Gran Telescopio Canarias, we examined the chemical signatures of 45 faint halo stars, uncovering distinct trends in r-process abundances that trace contributions from multiple nucleosynthesis sites. Our analysis used Mg and Eu in conjunction with other heavier elements to study the various enrichment pathways of the metal-poor r-process enhanced stars. By incorporating data from all RPA data releases with JINABASE and SAGA Database, we refine constraints on the origin of heavy-elements highlighting variations in production mechanisms. Furthermore, the discovery of a star with possible globular cluster origin at [Fe/H] = –3.0 adds to growing evidence of a lower metallicity threshold for globular clusters. Complementing this, our study of the metal-poor globular cluster NGC 2298 with GHOST spectrograph at Gemini South provides a detailed chemical inventory of over 40 elements, including 20 neutron-capture species. We identify signatures of multiple stellar populations, where first-generation stars exhibit larger dispersion in Fe-peak and neutron-capture elements, including variations in the r-process pattern. Notably, we detect correlations between light, alpha, and neutron-capture elements which reinforces the role of globular clusters as unique chemical tracers. Thus, integrating abundance data from both halo stars and globular clusters, our findings reveal the complex interplay between early nucleosynthesis and Galactic chemical evolution.

Deciphering the Origins of the Elements Through Galactic Archeology

Friedrich-Karl Thielemann

based on a joint paper with Khalil Farouqi and Anna Frebel (Eur. Phys. J. A (2025) 61: 207 https://doi.org/10.1140/epja/s10050-025-01668-5)

Low-metallicity stars preserve the signatures of the first stellar nucleosynthesis events in the Galaxy, as their surface abundances reflect the composition of the interstellar medium from the time when they were born. Aside from primordial Big Bang nucleosynthesis, massive stars, due to their short lifetimes, dominate the wind and explosive ejecta into the interstellar medium of the early Galaxy. Most of them will end as core-collapse supernova (CCSN) explosions, and typical ejected abundance distributions, e.g., in terms of the α-element-to-Fe ratios, reflect these contributions. Essentially all CCSNe contribute 56Fe (decaying from radioactive 56Ni). Therefore, low metallicity stars can be used to test whether the abundances of any other elements are correlated with those of Fe, i.e., whether these elements have been co-produced in the progenitor sources or if they require either a different or additional astrophysical origin(s). The present analysis focuses on stars with [Fe/H]<-2, as they probe the earliest formation phase of the Galaxy when only one or very few nucleosynthesis events had contributed their ejecta to the gas from which the lowest metallicity stars form. This was also the era before low and intermediate-mass stars (or type Ia supernovae) could contribute any additional heavy elements.

In the present study we examine Pearson and Spearman correlations of Fe with Li, Be, C, N, Na, Mg, Si, S, Ca, Ti, Cr, Ni, Zn, Ge, Se, Sr, Zr, Ba, Ce, Sm, Eu, Yb, Lu, Hf, Os, Ir, Pb, Th, and U, using high-resolution stellar abundance data from the SAGA and JINA databases. The main goal is to identify which of the observed elements (i) may have been co-produced with Fe in (possibly a variety of) CCSNe, and which elements require (ii) either a completely different, or (iii) at least an additional astrophysical origin.

The Rise of r-Process in Disrupted Dwarf Galaxies

Guilherme Limberg

Multimessenger observations of gravitational wave event GW170817 of a neutron star merger (NSM), have confirmed that these sites are capable of producing copious amounts of heavy elements through the rapid neutron capture process (r-process). However, because NSMs are expected to be delayed by billions of years, it is unclear how much they can actually contribute to the chemical enrichment of galaxies. Prompt sources of r-process, such as exotic kinds of supernovae, are theoretically promising for producing heavy elements in the early universe, but observational evidence is lacking. One way to investigate which is the dominant source of r-process is to look at the chemical evolution of dwarf galaxies, which are relatively simple systems in comparison to the Milky Way. We studied the r-process signature in a pair of disrupted dwarf galaxies in the Milky Way’s halo, the stellar stream Wukong or LMS-1 and the last major merger Gaia-Sausage/Enceladus (GSE). Stars in these substructures are much closer than classical surviving Milky Way satellites and are, hence, much brighter and amenable to high-resolution spectroscopy, which is necessary to detect the heaviest elements. We discovered that both Wukong/LMS-1 and GSE have been enriched in r-process elements predominantly by NSMs. This finding confirms that NSMs not only do happen, but are actually important contributors to the chemical evolution of galaxies. Future observations of additional disrupted and/or surviving dwarf galaxies might finally answer if NSMs alone contribute to all r-process across cosmic history or if prompt sources are also involved.

Beyond the BAD Flag: Uncovering Extremely Metal-Poor and r-Process Stars in APOGEE

Matthew Shetrone (UCO), Vivian Tang (UCSC), Evan Carrasco (UH)

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) has provided an unprecedented view of Galactic chemistry through high-resolution near-infrared spectroscopy. However, the pipeline’s abundance grid truncates at [Fe/H] ≈ –2.5, flagging more metal-poor spectra as “BAD” and removing them from standard analysis. We present a two-pronged effort to recover this hidden population of extremely metal-poor—and potentially r-process-enriched—stars. First, a re-analysis using The Cannon data-driven neural network demonstrates that APOGEE’s BAD-flagged stars can yield reliable stellar labels down to [Fe/H] ≈ –3.5, validating that many lie genuinely beyond the grid edge. Second, optical follow-up with the Shane 3-m and Automated Planet Finder telescopes at Lick Observatory confirms metallicities and abundance patterns for a subset of these stars, including several with r-process enhancement signatures. Together, these approaches establish that APOGEE’s BAD sample contains a valuable, previously overlooked reservoir of metal-poor stars, offering a new pathway to identify r-process tracers in the infrared and expand the survey’s reach into the earliest epochs of Galactic chemical evolution.   This material is based upon work supported by the National Science Foundation under Grant No. 2206514.

Actinide abundances, Ratios, and Evolution of Metal-Poor Stars

Shivani Shah

Actinides are the heaviest group of r-process elements and require special neutron-rich conditions to be formed. Over the past years, studies have shown that the most easily measured actinide element, thorium (Th), shows significant variations, with respect to lanthanide elements. However, these abundances have been measured by different studies using different sets of atomic data. In this talk, I will discuss the first large homogeneous sample of Th abundances in 49 metal-poor stars. Combined with the literature data, I will discuss the chemical evolution picture that is emerging for Th, along with providing a fresh view on the variation of actinides, relative to lanthanides. I will discuss important future directions needed to further our understanding of the origin of the heaviest elements, and ultimately constraint the properties and sites of r-process.

The journey to explore the r-process: An experimentalist approach

Sivahami Uthayakumaar

Recent observations have shown that the synthesis of heavy elements is more complex than originally thought. It is known that a large fraction of these heavy elements is synthesized through the rapid neutron capture process (r-process), where some potential sites are neutron star mergers or some types of core-collapse supernovae. For nuclei that are involved along the r-process pathway, most nuclear properties such as 𝛽-decay rates, masses, and neutron-capture reaction rates are almost entirely provided by theory, giving large uncertainties in the abundances observed. In this talk, I will detail the experimental approaches taken to explore nuclei involved along the r-process pathway, with particular focus on the neutron capture reactions on exotic nuclei. Highlights of previous experiments that have utilized this method using the SuN detector set up at Argonne National Laboratory and Michigan State University (MSU) will be presented. The Facility for Rare Isotope Beams (FRIB) is now operational at MSU and provides access to r-process isotopes. In this talk, I will present the first experiment completed at FRIB, focusing on providing constraints on neutron-capture involved in the r-process.

Tracing s-, i-, and r-processes through barium isotope ratios in VMP stars.

Tatyana Sitnova

I present a spectroscopic analysis of VMP stars and determine their non-local thermodynamic equilibrium abundances of Ba and Eu, as well as the fractions of the odd Ba isotopes. For different n-capture processes, calculations predict different fractions of odd isotopes (F_odd) and different [Ba/Eu]. For example, F_odd / [Ba/Eu] = 0.10 / 1.25 in s-process, 0.75 / -0.9 in r-process, and 0.60 to 0.80 / > 0.3 in i-process. Determination of F_odd and [Ba/Eu] allows us to estimate a contribution of different n-capture processes to a chemical composition of a given star. This also helps to classify stars with different n-capture sources at play and to further study in detail those sources. By now, I got the following results: (i) in stars with different [Sr,Y,Zr/Eu] ratios, F_odd increases from 0.1 to 0.9 with decreasing [Sr,Y,Zr/Eu], suggesting that the additional Sr synthesis in r-limited stars was due to the early s-process occurring in massive stars; (ii) CEMP-s stars show close to solar or s process F_odd, while CEMP-rs stars show higher F_odd in line with the i-process predictions. The method of F_odd determination requires the Ba II resonance lines to be reliable abundance indicators with equivalent widths of less than 180 mA. This criteria results in a small stellar sample. For example, among publicly available spectra in the Keck and ESO archives, I found 12 CEPM-s,rs stars suitable for my analysis. The high-resolution spectra obtained for 2000 metal-poor stars by the RPA could potentially deliver dozens of stars suitable for reliable F_odd determination and would help to study the properties (masses, rotational velocities and nucleosynthesis) of first stars and a poorly-known I process occurring in low mass AGB stars in binary systems.

Linking nuclear experiments and neutrino-driven winds to the Sr-Ag patterns of limited-r stars

Thanassis Psaltis

The origin of the heavy elements in the first r-process peak, between strontium and silver, remains an open question. Neutrino-driven winds in explosive environments, whether neutron-rich (weak r-process or α-process) or proton-rich (νp-process), present a viable production site. In this talk, I will discuss how different scenarios can be distinguished by constraining nuclear physics uncertainties, particularly the (α,n) reaction rates relevant to the weak r-process, and by comparing nucleosynthesis models to the abundance patterns of limited-r stars. I will also highlight current experimental efforts to measure key (α,n) reaction rates and discuss how new, high-quality spectroscopic data from limited-r stars will provide essential constraints for the next generation of astrophysical models.

The Effect of Global Beta-Decay Properties on the r-Process Dynamics

Yukiya Saito

The heaviest elements in nature are synthesized in the rapid neutron capture process (r-process). The r-process is believed to occur in hot, dense, and neutron-rich environments, such as ejecta from compact binary mergers and some types of core-collapse supernovae. The yield of the nucleosynthesis event sensitively depends on the balance of various kinds of nuclear reactions and decays— neutron capture, photodissociation, beta-decay, fission, and more. Recently, new sets of theoretical beta-decay rates with an axially deformed proton-neutron relativistic quasiparticle RPA (pnRQRPA) using the linear response approach have been calculated and applied to the r-process calculations. In this talk, the effect of the new beta-decay rates on the dynamics of the r-process will be discussed, taking various astrophysical conditions as examples. We will show that beta-decay rates have a significant effect on the speed and amount of relevant nuclear reactions and decays. The results will also be compared with the ones using existing beta-decay rates, such as those based on the finite-range droplet model (FRDM) + QRPA and a spherical pnRQRPA.