We present a detailed chemical abundance analysis of the three brightest known stars from each of the Turranburra and Willka Yaku stellar streams using high-resolution Magellan/Magellan Inamori Kyocera Echelle spectra. Abundances for 27 elements, ranging from carbon to dysprosium, were derived. Our results support the original classification that Turranburra, with a low average metallicity of [Fe/H] = −2.45 ± 0.07, likely originates from a dwarf galaxy progenitor. Willka Yaku has a low average metallicity of [Fe/H] = −2.35 ± 0.03 with a small scatter in the abundances, consistent with a globular cluster progenitor as suggested by previous studies. Both streams exhibit mild enhancements in neutron-capture elements, with averages of [Eu II/Fe] = 0.47 ± 0.09 for Turranburra and 0.44 ± 0.05 for Willka Yaku, consistent with enrichment from an r-process event. A similar enrichment is observed in other stellar streams, and we further discuss this signature as it relates to the potential enrichment histories of these two streams.

About half the elements heavier than iron in the universe, like silver and gold, are created in the rapid neutron-capture (r-)process. However, today, almost 70 years after the theoretical prediction of this process, it is still highly debated in what type of stellar explosions it can take place. One of the best places to search for answers is in ancient, metal-poor stars formed from the enriched gas. Their chemical makeup is like a time capsule, a direct fingerprint of the elements produced by the stellar generations that came before them. Since the first highly r-process-enhanced star, CS 22892-052 was discovered more than 30 years ago, multiple projects like the Hamburg/ESO r-Process Enhanced Star (HERES) survey, the Chemical Evolution of r-process Elements in Stars (CERES) project, and the r-Process Alliance (RPA) have searched for more r-process-enriched stars in the Milky Way. At the same time, numerous r-process-enriched stars have been discovered in stellar streams and dwarf galaxies. Here we present an overview of recent advances in finding r-process-enriched metal-poor stars and what the detailed chemo-dynamical analysis of these stars can tell us about heavy element nucleosynthesis and the astrophysical site(s) of the r-process.

As part of the ongoing work of the R-Process Alliance (RPA), detailed abundance measurements of 29 heavy elements in three metal-poor stars, 2MASS J14592981−3852558, 2MASS J19445483−4039459, and 2MASS J15211026−0607566, are presented based on an analysis of high-resolution (R ∼ 80,000), high-signal-to-noise “portrait” spectra from the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Clay Telescope at Las Campanas Observatory. The selected targets were identified as r-process-enhanced metal-poor stars in previous RPA snapshot analyses. They have also been linked to possible chemodynamically tagged groups, indicating that the stars may have formed in dwarf galaxies that were later accreted into the Milky Way halo. These stars have also been tentatively linked to the Thamnos structure. The detailed chemical abundances in this work confirm that 2MASS J14592981−3852558 and J15211026−0607566 are r-II stars, while 2MASS J19445483−4039459 is found to lie just below the threshold for r-I status. The r-II stars show signs of slight enhancement in fission fragments compared to 2MASS J19445483−4039459. Based on radioactive age dating with Th, the r-process material in the two r-II stars is found to be old (with ages > 10 Gyr); neither star shows signs of an actinide boost. The varying elemental compositions suggest that these stars likely did not originate in the same environment, though each could be consistent with originating in the Thamnos progenitor.

We present a detailed chemical-abundance and kinematic analysis of four extremely metal-poor (EMP; [Fe/H] ≤ −3.0) stars identified from Gaia BP–RP data in our ongoing search for the most primitive stars. This includes a primary target, Gaia DR3 2563539603865382656 (hereafter G256353), a strongly r-process-enhanced star with [Eu/Fe] = +1.20 and [Ba/Eu] = −0.64. Our results are based on high-resolution, high-signal-to-noise-ratio GHOST spectra from Gemini-South. For the full sample, we statistically match the light-element abundances with those predicted from Population III supernova models. The best-fit model suggests massive progenitors with stellar masses of M_⋆ ∼ 20–30 M_⊙. In addition, we determine orbital histories for all of the stars. We find that Gaia DR3 2887334237669844480 appears to be kinematically associated with Atari, an accreted structure in the Galactic disk. This star has low abundance ratios of strontium ([Sr/Fe] = −1.09) and barium ([Ba/Fe] = −0.37), which support an accretion origin. For G256353, we determine chemical abundances for 15 neutron-capture elements. We compare the observed heavy-element pattern for G256353 with that of the Sun, HD 222925, and two neutron star merger models. The r-process elements in G256353 align reasonably well with HD 222925, the scaled-solar pattern (except for the first peak), and a recent predicted pattern associated with neutron star mergers. This consistency reinforces the universality of the main r-process across diverse astrophysical environments.

Chemical abundances of stellar streams can be used to determine the nature of a stream’s progenitor. Here we study the progenitor of the recently discovered Leiptr stellar stream, which was previously suggested to be a tidally disrupted halo globular cluster. We obtain high-resolution spectra of five red giant branch stars selected from the Gaia DR2 STREAMFINDER catalog with Magellan/MIKE. One star is a clear non-member. The remaining four stars display chemical abundances consistent with those of a low-mass dwarf galaxy: they have a low mean metallicity, ⟨[Fe/H]⟩ = −2.2; they do not all have identical metallicities; and they display low [α/Fe] ∼ 0 and [Sr/Fe] and [Ba/Fe] ∼ −1. This pattern of low α and neutron-capture element abundances is only found in intact dwarf galaxies with stellar mass ≲ 10⁵ M_⊙. Although more data are needed to be certain, Leiptr’s chemistry is consistent with being the lowest-mass dwarf galaxy stream without a known intact progenitor, possibly in the mass range of ultra-faint dwarf galaxies. Leiptr thus preserves a record of one of the lowest-mass early accretion events into the Milky Way.

Context. Over the past few years, the R-Process Alliance (RPA) has successfully carried out a search for stars that are highly enhanced in elements produced via the rapid neutron-capture (r-) process. In particular, the RPA has identified a number of relatively bright, highly r-process-enhanced (r-II) stars, suitable for observations with the Hubble Space Telescope (HST), facilitating abundance derivation of elements such as gold (Au) and cadmium (Cd). Aims. This paper presents the detailed abundances derived for the metal-poor ([Fe/H] = - 2.55) highly r-process-enhanced ([Eu/Fe] = +1.29) r-II star 2MASS J05383296âÂ?Â? 5904280. Methods. One-dimensional local thermodynamic equilibrium (LTE) elemental abundances were derived via equivalent width and spectral synthesis using high-resolution high signal-to-noise near-UV HST/STIS and optical Magellan/MIKE spectra. Results. Abundances were determined for 43 elements, including 26 neutron-capture elements. In particular, abundances of the rarely studied elements Nb, Mo, Cd, Lu, Os, Pt, and Au are derived from the HST spectrum. These results, combined with RPA near-UV observations of two additional r-II stars, increase the number of Cd abundances derived for r-process-enriched stars from seven to ten and Au abundances from four to seven. A large star-to-star scatter is detected for both of these elements, highlighting the need for more detections enabling further investigations, specifically into possible non-LTE effects.

We study the formation of stars with varying amounts of heavy elements synthesized by the rapid neutron-capture process (r-process) based on our detailed cosmological zoom-in simulation of a Milky Way–like galaxy with an N-body/smoothed particle hydrodynamics code, asura. Most stars with no overabundance in r-process elements, as well as the strongly r-process-enhanced (RPE) r-II stars ([Eu/Fe] > +0.7), are formed in dwarf galaxies accreted by the Milky Way within the 6 Gyr after the Big Bang. In contrast, over half of the moderately enhanced r-I stars (+0.3 < [Eu/Fe] ≤ +0.7) are formed in the main in situ disk after 6 Gyr. Our results suggest that the fraction of r-I and r-II stars formed in disrupted dwarf galaxies is larger the higher their [Eu/Fe] is. Accordingly, the most strongly enhanced r-III stars ([Eu/Fe] > +2.0) are formed in accreted components. These results suggest that non-r-process-enhanced stars and r-II stars are mainly formed in low-mass dwarf galaxies that hosted either none or a single neutron star merger, while the r-I stars tend to form in the well-mixed in situ disk. We compare our findings with high-resolution spectroscopic observations of RPE metal-poor stars in the halo and dwarf galaxies, including those collected by the R-Process Alliance. We conclude that observed [Eu/Fe] and [Eu/Mg] ratios can be employed in chemical tagging of the Milky Way’s accretion history.

Context. The astrophysical origin of the rapid neutron-capture process (r-process), responsible for producing roughly half of the elements heavier than iron, remains uncertain. Detailed chemical signatures from the oldest, most metal-poor stars, which act as fossil records of the earliest nucleosynthesis events, can be used to identify the dominant r-process sites.

Aims. We present a homogeneous chemical abundance analysis of ten r-process element-enhanced stars. These old and metal-poor stars are strongly enriched in r-process elements with minimal contamination from other nucleosynthetic sources. By focusing on this chemically pure sample, we aim to investigate intrinsic variations in the r-process abundance patterns and explore their implications for the nature and potential diversity of r-process sites.

Methods. We performed a detailed chemical abundance analysis of high-resolution, high-signal-to-noise spectra. For each star, we inspected over 1400 individual absorption lines using a combination of equivalent width measurements and spectral synthesis. The analysis was conducted under the assumption of 1D local thermodynamic equilibrium and employing the MOOG radiative transfer code.

Results. We derived abundances for 54 chemical species, including 29 neutron-capture (n-capture) elements, covering the full mass range of the r-process abundance pattern. A kinematic analysis reveals that stars likely originated from ten kinematically distinct systems. Based on this assumption, we used the sample to probe the maximum variation expected from ten independent r-process nucleosynthesis events and computed the intrinsic dispersion of each element relative to Zr and Eu for the light and heavy r-process elements, respectively. This exercise resulted in a remarkably low cosmic scatter across the ten r-process sites enriching these stars; for the rare earth and third peak elements, for example, we find σ_[La/Eu] = 0.08 and σ_[Os/Eu] = 0.11 dex, while the scatter between light and heavy elements, σ_[Zr/Eu], is slightly higher at 0.18 dex.

Conclusions. The elemental abundance patterns across the ten independent r-process sites show remarkably small cosmic dispersions. This minimal dispersion suggests a high degree of uniformity in r-process yields across diverse astrophysical environments.

The growing number of Milky Way satellites detected in recent years has introduced a new focus for stellar abundance analysis. Abundances of stars in satellites have been used to probe the nature of these systems and their chemical evolution. However, for most satellites, only centrally located stars have been examined. This paper presents an analysis of three stars in the Tucana V system, one in the inner region and two at ∼10′ (7-10 half-light radii) from the center. We find a remarkable chemical diversity between the stars. One star exhibits enhancements in rapid neutron-capture elements (an r-I star), and another is highly enhanced in C, N, and O but with low neutron-capture abundances (a CEMP-no star). The metallicities of the stars analyzed span more than 1 dex from [Fe/H] = −3.55 to −2.46. This, combined with a large abundance range of other elements like Ca, Sc, and Ni, confirms that Tuc V is an ultrafaint dwarf (UFD) galaxy. The variation in abundances, highlighted by [Mg/Ca] ratios ranging from +0.89 to −0.75, among the stars demonstrates that the chemical enrichment history of Tuc V was very inhomogeneous. Tuc V is only the second UFD galaxy in which stars located at large distances from the galactic center have been analyzed, along with Tucana II. The chemical diversity seen in these two galaxies, driven by the composition of the noncentral member stars, suggests that distant member stars are important to include when classifying faint satellites and that these systems may have experienced more complex chemical enrichment histories than previously anticipated.

We present the discovery of 2MASS J05241392−0336543 (hereafter J0524−0336), a very metal-poor ([Fe/H] = −2.43 ± 0.16), highly r-process-enhanced ([Eu/Fe] = +1.34 ± 0.10) Milky Way halo field red giant star, with an ultrahigh Li abundance of A(Li, 3D, NLTE) = 6.15 ± 0.25 and [Li/Fe] = +7.64 ± 0.25, respectively. This makes J0524−0336 the most lithium-enhanced giant star discovered to date. We present a detailed analysis of the star’s atmospheric stellar parameters and chemical abundance determinations. Additionally, we detect indications of infrared excess, as well as observe variable emission in the wings of the Hα absorption line across multiple epochs, indicative of a potential enhanced mass-loss event with possible outflows. Our analysis reveals that J0524−0336 lies either between the bump and the tip of the red giant branch (RGB), or on the early asymptotic giant branch (e-AGB). We investigate the possible sources of lithium enrichment in J0524−0336, including both internal and external sources. Based on current models and on the observational evidence we have collected, our study shows that J0524−0336 may be undergoing the so-called lithium flash that is expected to occur in low-mass stars when they reach the RGB bump and/or the e-AGB.