R-process Elements Research Articles

Impact of Systematic Modeling Uncertainties on Kilonova Property Estimation

The precise atomic structure and therefore the wavelength-dependent opacities of lanthanides are highly uncertain. This uncertainty introduces systematic errors in modeling transients like kilonovae and estimating key properties such as mass, characteristic velocity, and heavy metal content. Here, we quantify how atomic data from across the literature as well as choices of thermalization efficiency of r-process radioactive decay heating impact the light curve and spectra of kilonovae. Specifically, we analyze the spectra of a grid of models produced by the radiative transfer code Sedona that span the expected range of kilonova properties to identify regions with the highest systematic uncertainty. Our findings indicate that differences in atomic data have a substantial impact on estimates of lanthanide mass fraction, spanning approximately 1 order of magnitude for lanthanide-rich ejecta, and demonstrate the difficulty in precisely measuring the lanthanide fraction in lanthanide-poor ejecta. Mass estimates vary typically by 25%–40% for differing atomic data. Similarly, the choice of thermalization efficiency can affect mass estimates by 20%–50%. Observational properties such as color and decay rate are highly model dependent. Velocity estimation, when fitting solely based on the light curve, can have a typical error of ∼100%. Atomic data of light r-process elements can strongly affect blue emission. Even for well-observed events like GW170817, the total lanthanide production estimated using different atomic data sets can vary by a factor of ∼6.

The Rise of the r-process in the Gaia-Sausage/Enceladus Dwarf Galaxy ∗

Neutron star mergers (NSMs) produce r-process elements after a time-delayed inspiral process. Once a significant number of NSMs are present in a galaxy, r-process elements, such as Eu, are expected to significantly increase with time. Yet, there have been limited observational data in support of Eu increasing within Local Group galaxies. We have obtained high-resolution Magellan/MIKE observations of 43 metal-poor stars in the Gaia-Sausage/Enceladus (GSE) tidally disrupted galaxy with −2.5 < [Fe/H] < −1. For the first time, we find a clear rise in [Eu/Mg] with increasing [Mg/H] within one galaxy. We use a simple chemical evolution model to study how such a rise can result from the interplay of prompt and delayed r-process enrichment events. Delayed r-process sources are required to explain the rise and subsequent leveling off of [Eu/Mg] in this disrupted galaxy. However, the rise may be explained by delayed r-process sources with either short (∼10 Myr) or long (∼500 Myr) minimum delay times. Future studies on the nature of r-process sources and their enrichment processes in the GSE will require additional stars in the GSE at even lower metallicities than the present study.

Element Formation in Radiation-hydrodynamics Simulations of Kilonovae

Understanding the details of r-process nucleosynthesis in binary neutron star merger (BNSM) ejecta is key to interpreting kilonova observations and identifying the role of BNSMs in the origin of heavy elements. We present a self-consistent, two-dimensional, ray-by-ray radiation-hydrodynamic evolution of BNSM ejecta with an online nuclear network (NN) up to a timescale of days. For the first time, an initial numerical relativity ejecta profile composed of the dynamical component and spiral-wave and disk winds is evolved including detailed r-process reactions and nuclear heating effects. A simple model for the jet energy deposition is also included. Our simulation highlights that the common approach of relating in postprocessing the final nucleosynthesis yields to the initial thermodynamic profile of the ejecta can lead to inaccurate predictions. Moreover, we find that neglecting the details of the radiation-hydrodynamic evolution of the ejecta in nuclear calculations can introduce deviations of up to 1 order of magnitude in the final abundances of several elements, including very light and second r-process peak elements. The presence of a jet affects element production only in the innermost part of the polar ejecta, and it does not alter the global nucleosynthesis results. Overall, our analysis shows that employing an online NN improves the reliability of nucleosynthesis and kilonova light-curve predictions.

Do we owe our existence to gravitational waves?

Two heavy elements essential to human biology are thought to have been produced by the astrophysical r-process, which occurs in neutron-rich environments: iodine is a constituent of thyroid hormones that affect many physiological processes including growth and development, body temperature and heart rate, and bromine is essential for tissue development and architecture. Collisions of neutron stars (kilonovae) have been identified as sources of r-process elements including tellurium, which is adjacent to iodine in the periodic table, and lanthanides. Neutron-star collisions arise from energy loss due to gravitational-wave emission from binary systems, leading us to suggest that gravitational waves have played a key role in enabling human life by producing iodine and bromine. We propose probing this proposal by searching in lunar material for live 129I deposited by a recent nearby kilonova explosion.

The ratio of [Eu/α] differentiates accreted/in-situ Milky Way stars across metallicities, as indicated by both field stars and globular clusters

Abstract We combine stellar orbits with the abundances of the heavy, r-process element europium and the light, α-element, silicon to separate in-situ and accreted populations in the Milky Way across all metallicities. At high orbital energy, the accretion-dominated halo shows elevated values of [Eu/Si], while at lower energies, where many of the stars were born in-situ, the levels of [Eu/Si] are lower. These systematically different levels of [Eu/Si] in the MW and the accreted halo imply that the scatter in [Eu/α] within a single galaxy is smaller than previously thought. At the lowest metallicities, we find that both accreted and in-situ populations trend down in [Eu/Si], consistent with enrichment via neutron star mergers. Through compiling a large dataset of abundances for 54 globular clusters (GCs), we show that differences in [Eu/Si] extend to populations of in-situ/accreted GCs. We interpret this consistency as evidence that in r-process elements GCs trace the star formation history of their hosts, motivating their use as sub-Gyr timers of galactic evolution. Furthermore, fitting the trends in [Eu/Si] using a simple galactic chemical evolution model, we find that differences in [Eu/Si] between accreted and in-situ MW field stars cannot be explained through star formation efficiency alone. Finally, we show that the use of [Eu/Si] as a chemical tag between GCs and their host galaxies extends beyond the Local Group, to the halo of M31 - potentially offering the opportunity to do Galactic Archaeology in an external galaxy.

Fragmentation in Gravitationally Unstable Collapsar Disks and Subsolar Neutron Star Mergers

Although stable neutron stars (NSs) can in principle exist down to masses M ns ≈ 0.1 M ⊙, standard models of stellar core-collapse predict a robust lower limit M ns ≳ 1.2 M ⊙, roughly commensurate with the Chandrasekhar mass M Ch of the progenitor’s iron core (electron fraction Y e ≈ 0.5). However, this limit may be circumvented in sufficiently dense neutron-rich environments (Y e < 0.5) for which MCh∝Ye2 is reduced to ≲1 M ⊙. Such physical conditions could arise in the black hole accretion disks formed from the collapse of rapidly rotating stars (“collapsars”), as a result of gravitational instabilities and cooling-induced fragmentation, similar to models for planet formation in protostellar disks. We confirm that the conditions to form subsolar-mass NS (ssNS) may be marginally satisfied in the outer regions of massive neutrino-cooled collapsar disks. If the disk fragments into multiple ssNSs, their subsequent coalescence offers a channel for precipitating subsolar mass LIGO/Virgo gravitational-wave mergers that does not implicate primordial black holes. The model makes several additional predictions: (1) ∼Hz frequency Doppler modulation of the ssNS-merger gravitational-wave signals due to the binary’s orbital motion in the disk; (2) at least one additional gravitational-wave event (coincident within ≲hours), from the coalescence of the ssNS-merger remnant(s) with the central black hole; (3) an associated gamma-ray burst and supernova counterpart, the latter boosted in energy and enriched with r-process elements from the NS merger(s) embedded within the exploding stellar envelope (“kilonovae inside a supernova”).

Identification of new gold lines in the 350–1000 nm spectral region using laser produced plasmas

We present results from a pilot study, using a laser-produced plasma, to identify new lines in the 350–1000 nm spectral region for the r-process element gold (Au), of relevance to studies of neutron star mergers. This was achieved via optical-IR spectroscopy of a laser-produced Au plasma, with an Au target of high purity (99.95%) and a low vacuum pressure to remove any air contamination from the experimental spectra. Our data were recorded with a spectrometer of 750 mm focal length and 1200 lines mm−1 grating, yielding a resolution of 0.04 nm. We find 54 lines not previously identified and which are not due to the impurities (principally copper (Cu) and silver (Ag)) in our Au sample. Of these 54 lines, we provisionally match 21 strong transitions to theoretical results from collisional-radiative models that include energy levels derived from atomic structure calculations up to the 6s level. Some of the remaining 33 unidentified lines in our spectra are also strong and may be due to transitions involving energy levels which are higher-lying than those in our plasma models. Nevertheless, our experiments demonstrate that laser-produced plasmas are well suited to the identification of transitions in r-process elements, with the method applicable to spectra ranging from UV to IR wavelengths.

Robust r-process Nucleosynthesis beyond Lanthanides in the Common Envelop Jet Supernovae

The common envelop jet supernovae (CEJSNe) r-process scenario has been proposed as an r-process nucleosynthesis site in the past decade. Jets launched by a neutron star that spirals in inside the core of a red supergiant star in a common envelope evolution supply the proper conditions for forming elements heavier than iron through the rapid neutron-capture process. The present work initially unveils the r-process abundance patterns that result from the density profile in the relatively long-lived jets. The results indicate that the expansion profile unique to the CEJSN scenario can produce the largest ratio of the third r-process peak elements to lanthanides among current r-process scenarios and, in addition, can form quite an amount of lanthanides, about 1% of matters above the first peak, in a single event. The comparison of the ratio of the third-peak elements to the lanthanides with several observed r-enhanced metal-poor stars and with other r-process scenarios suggests that a high mass of third-peak elements is anticorrelated with a high fraction of lanthanides, both in observations and theory. The CEJSN-like scenario plays a significant role in this conclusion since it reproduces the observational features of some particular r-enhanced metal-poor stars where other r-process scenarios encounter problems. Due to the formation of extremely heavy elements, the CEJSN also offers a credible estimation of the age of the most actinide-boosted star by cosmochronometry.

Radioactive Gamma-Ray Lines from Long-lived Neutron Star Merger Remnants

The observation of a kilonova AT2017gfo associated with the gravitational wave event GW170817 provides the first strong evidence that neutron star mergers are dominant contributors to the production of heavy r-process elements. Radioactive gamma-ray lines emitted from neutron star merger remnants provide a unique probe for investigating the nuclide composition and tracking its evolution. In this work, we studied the gamma-ray line features arising from the radioactive decay of heavy nuclei in the merger remnants based on the r-process nuclear reaction network and the astrophysical inputs derived from numerical relativity simulations. The decay chain of 50126 Sn (T 1/2 = 230 kyr) → 51126 Sb (T 1/2 = 12.35 days) → 52126 Te (stable) produces several bright gamma-ray lines with energies of 415, 667, and 695 keV, making it the most promising decay chain during the remnant phase. The photon fluxes of these bright gamma-ray lines reach ∼10−5 γ cm−2 s−1 for Galactic merger remnants with ages less than 100 kyr, which can be detected by the high energy resolution MeV gamma-ray detectors like the MASS mission.

Nucleosynthesis in the Innermost Ejecta of Magnetorotational Supernova Explosions in Three Dimensions

Core-collapse supernova (CCSN) explosions powered by rotation and magnetic fields present an interesting astrophysical site for nucleosynthesis that potentially contributes to the production of r-process elements. Here we present yields of the innermost ejecta in 3D magnetorotational CCSN models simulated using the CoCoNuT-FMT code. Strong magnetic fields tap the rotational energy of the proto−neutron star and lead to earlier and more energetic (∼3 × 1051 erg) explosions than typical neutrino-driven CCSNe. Compared to a reference nonmagnetic model, the ejecta in the magnetorotational models have much more neutron-rich components with Y e down to ∼0.25. Our post-processing calculations with the reaction network SkyNet show significant production of weak r-process elements up to mass number ∼130. We find negligible differences in the synthesis of heavy elements between two magnetorotational models with different initial field strengths of 1010 and 1012 G, in accord with their similar explosion dynamics. The magnetorotational models produce about ∼0.19 and 0.14 M ☉ of radioactive 56Ni, on the low end of inferred hypernova nickel masses. The yields are publicly available at Zenodo (doi: 10.5281/zenodo.10578981) for comparison with stellar abundance patterns, inclusion in modeling galactic chemical evolution, and comparison with other yield calculations. Our results add to the yet-restricted corpus of nucleosynthesis yields from 3D magnetorotational supernova simulations and will help quantify yield uncertainties.

New radiative and collisional atomic data for Sr ii and Y ii with application to Kilonova modelling

ABSTRACT The spectra of singly ionized Strontium and Yttrium (Sr ii and Y ii) have been proposed as identifications of certain spectral features in the AT2017gfo spectrum. With the growing demand for non-local-thermodynamic-equilibrium (NLTE) simulations of Kilonovae, there is an increasing need for atomic data for these and other r-process elements. Our goal is to expand upon the current set of atomic data for r-process elements, by presenting transition probabilities and Maxwellian-averaged effective collision strengths for Sr ii and Y ii. The Breit–Pauli and darcR-matrix codes are employed to calculate the appropriate collision strengths, which are thermally averaged according to a Maxwellian distribution to calculate excitation and de-excitation rates. The tardis and ColRadPy packages are subsequently used to perform local thermodynamic equilibrium (LTE) and NLTE modelling, respectively. A complete set of transition probabilities and effective collision strengths involving levels for Sr ii and Y ii have been calculated for temperature ranges compatible with kilonova plasma conditions. Forbidden transitions were found to disagree heavily with the Axelrod approximation, an approximation which is currently employed by other models within the literature. Theoretically important spectral lines are identified with both LTE and NLTE modelling codes. LTE simulations in tardis reveal no new significant changes to the full synthetic spectra. NLTE simulations in ColRadPy provide indications of which features are expected to be strong for a range of regimes, and we include luminosity estimates. Synthetic emission spectra over kilonova densities and temperatures reveal potentially interesting spectral lines in the NIR.

VLBA Astrometry of the Galactic Double Neutron Stars PSR J0509+3801 and PSR J1930–1852: A Preliminary Transverse Velocity Distribution of Double Neutron Stars and its Implications

The mergers of double neutron star (DNS) systems are believed to drive the majority of short γ-ray bursts (SGRBs), while also serving as production sites of heavy r-process elements. Despite being key to (i) confirming the nature of the extragalactic SGRBs, (ii) addressing the poorly understood r-process enrichment in the ultrafaint dwarf galaxies (UFDGs), and (iii) probing the formation process of DNS systems, the space velocity distribution of DNSs is still poorly constrained, due to the small number of DNSs with well-determined astrometry. In this work, we determine new proper motions and parallaxes of two Galactic DNSs, PSR J0509+3801 and PSR J1930−1852, using the Very Long Baseline Array, and we estimate the transverse velocities v ⊥ of all 11 isolated Galactic DNSs having proper-motion measurements in a consistent manner. Our correlation analysis reveals that the DNS v ⊥ is tentatively correlated with three parameters: spin period, orbital eccentricity, and companion mass. With the preliminary v ⊥ distribution, we obtain the following findings. First, the refined v ⊥ distribution is confirmed to agree with the observed displacements of the localized SGRBs from their host galaxy birth sites. Second, we estimate that around 11% and 25% of DNSs remain gravitationally bound to UFDGs with escape velocities of 15 and 25 km s−1, respectively. Hence, the retained DNSs might indeed be responsible for the r-process enrichment confirmed so far in a few UFDGs. Finally, we discuss how a future ensemble of astrometrically determined DNSs may probe the multimodality of the v ⊥ distribution.

Location of the neutron drip line for Sn and its impact on r-process abundances

Nuclear physical inputs are critical for predicting the abundance of r-process elements. Extensive sensitivity studies have recently been performed to gauge the impact of the individual properties of nuclei on the r-process. In this work, we investigate the impact of the large uncertainties in the theoretical predictions of the masses of neutron-rich Sn isotopes and the location of the neutron drip line on the abundance of r-process abundances. The uncertainties in the predicted r-process abundances are obtained through large-scale network calculations by simultaneously varying the masses and reaction rates of Sn within the predicted mass uncertainties. The calculations use a generally indicative astrophysical trajectory of neutron-star mergers and are based on three different mass models. The results indicate that the large uncertainty in the location of the neutron drip line for Sn can significantly and asymmetrically affect the predicted abundances of nuclei after the second peak, in particular around A=140−155, sometimes accompanied by a delayed freeze-out.

Joint r-process Enrichment by Supernovae with a Metallicity Threshold and Neutron Star Mergers

The enrichment history of r-process elements has been imprinted on the stellar abundances that change in accordance with increasing metallicity in galaxies. Close examination of the [Eu/Fe] feature caused by stars in nearby galaxies, including the Large Magellanic Cloud (LMC), shows its perplexity. The decreasing trend of the [Eu/Fe] feature is followed by a nearly constant value; this trend is generally attributed to an onset of the delayed Fe release from Type Ia supernovae (SNe Ia), which is the same interpretation of the [α/Fe] feature. However, this feature appears in the LMC at [Fe/H] of approximately −0.7, which is significantly higher than that for the [α/Fe] case (≈−2). This result potentially indicates the presence of an overlooked property of the r-process site that remains unseen in the study of the Milky Way. Here, we propose that this [Eu/Fe]-knee feature is created by a fade-out of core-collapse SNe producing r-process elements; these elements along with neutron star mergers (NSMs) promote the r-process enrichment under the condition for this specific SNe such that their occurrence is limited to a low-metallicity environment. This metallicity threshold for the occurrence rate of r-process SNe at a subsolar is nearly identical to that for long gamma-ray bursts whose origin may be connected to fast-rotating massive stars. Moreover, we reason that the contribution of Eu from NSMs is crucial to maintain a high [Eu/Fe] at an early stage in dwarf galaxies by a balance with Fe from SNe Ia; both enrichments via NSMs and SNe Ia proceed with similar delay time distributions.

The Effects of r-Process Enrichment in Hydrogen-rich Supernovae

Core-collapse supernovae (SNe) are candidate sites for rapid neutron capture process (r-process) nucleosynthesis. We explore the effects of enrichment from r-process nuclei on the light curves of hydrogen-rich SNe and assess the detectability of these signatures. We modify the radiation hydrodynamics code, SuperNova Explosion Code, to include the approximate effects of opacity and radioactive heating from r-process elements in the supernova (SN) ejecta. We present models spanning a range of total r-process masses M r and their assumed radial distribution within the ejecta, finding that M r ≳ 10−2 M ⊙ is sufficient to induce appreciable differences in their light curves as compared to ordinary hydrogen-rich SNe (without any r-process elements). The primary photometric signatures of r-process enrichment include a shortening of the plateau phase, coinciding with the hydrogen-recombination photosphere retreating to the r-process-enriched layers, and a steeper post-plateau decline associated with a reddening of the SN colors. We compare our r-process-enriched models to ordinary SNe models and observational data, showing that yields of M r ≳ 10−2 M ⊙ are potentially detectable across several of the metrics used by transient observers, provided that r-process-rich layers are mixed at least halfway to the ejecta surface. This detectability threshold can roughly be reproduced analytically using a two-zone (kilonova-within-an-SN) picture. Assuming that a small fraction of SNe produce a detectable r-process yield of M r ≳ 10−2 M ⊙, and respecting constraints on the total Galactic production rate, we estimate that ≳103–104 SNe need be observed to find one r-enriched event, a feat that may become possible with the Vera Rubin Observatory.

Impact of Ejecta Temperature and Mass on the Strength of Heavy Element Signatures in Kilonovae

A kilonova, the electromagnetic emission produced by compact binary mergers, is formed through a delicate interplay of physical processes, involving r-process nucleosynthesis and interactions between heavy elements and photons through radiative transfer. This complexity makes it difficult to achieve a comprehensive understanding of kilonova spectra. In this study, we aim to enhance our understanding and establish connections between physical parameters and observables through radiative-transfer simulations. Specifically, we investigate how ejecta temperature and element mass influence the resulting kilonova spectrum. For each species, the strength of its line features depends on these parameters, leading to the formation of a distinct region in the parameter space, dubbed the resonance island, where the line signature of that species is notably evident in the kilonova spectrum. We explore its origin and applications. Among explored r-process elements (31 ≤ Z ≤ 92), we find that four species—SrII, YII, BaII, and CeII—exhibit large and strong resonance islands, suggesting their significant contributions to kilonova spectra at specific wavelengths. In addition, we discuss potential challenges and future perspectives in observable heavy elements and their masses in the context of the resonance island.

Heavy element abundances in galactic globular clusters

Context. Globular clusters are considered key objects for understanding the formation and evolution of the Milky Way. In this sense, the characterisation of their chemical and orbital parameters can provide constraints on chemical evolution models of the Galaxy. Aims. We use the heavy element abundances of globular clusters to trace their overall behaviour in the Galaxy, with the aim to analyse potential relations between the hot H-burning and s-process elements. Methods. We measured the content of Cu I and s- and r-process elements (Y II, Ba II, La II, and Eu II) in a sample of 210 giant stars in 18 galactic globular clusters from high-quality UVES spectra. These clusters span a broad metallicity range and the sample is the largest that has been uniformly analysed to date, with respect to heavy elements in globular clusters. Results. The Cu abundances did not show a considerable spread in the sample, nor any correlation with Na, indicating that the Na nucleosynthesis process does not affect the Cu abundance. Most GCs closely follow the Cu, Y, Ba, La, and Eu field stars’ distribution, revealing a similar chemical evolution. The Y abundances in mid-metallicity regime GCs (−1.10 dex &lt; [Fe/H] &lt; −1.80 dex) display a mildly significant correlation with the Na abundance, which ought to be further investigated. Finally, we do not find any significant difference between the n-capture abundances among GCs with either Galactic and extragalactic origins.

JWST detection of a supernova associated with GRB 221009A without an r-process signature

Identifying the sites of r-process nucleosynthesis, a primary mechanism of heavy element production, is a key goal of astrophysics. The discovery of the brightest gamma-ray burst (GRB) to date, GRB 221009A, presented an opportunity to spectroscopically test the idea that r-process elements are produced following the collapse of rapidly rotating massive stars. Here we present James Webb Space Telescope observations of GRB 221009A obtained +168 and +170 rest-frame days after the gamma-ray trigger, and demonstrate that they are well described by a SN 1998bw-like supernova (SN) and power-law afterglow, with no evidence for a component from r-process emission. The SN, with a nickel mass of approximately 0.09 M⊙, is only slightly fainter than the brightness of SN 1998bw at this phase, which indicates that the SN is not an unusual GRB-SN. This demonstrates that the GRB and SN mechanisms are decoupled and that highly energetic GRBs are not likely to produce significant quantities of r-process material, which leaves open the question of whether explosions of massive stars are key sources of r-process elements. Moreover, the host galaxy of GRB 221009A has a very low metallicity of approximately 0.12 Z⊙ and strong H2 emission at the explosion site, which is consistent with recent star formation, hinting that environmental factors are responsible for its extreme energetics.

Extending the chemical reach of the H3 survey: detailed abundances of the dwarf-galaxy stellar stream Wukong/LMS-1

ABSTRACT We present the first detailed chemical-abundance analysis of stars from the dwarf-galaxy stellar stream Wukong/LMS-1 covering a wide metallicity range ($-3.5 \lt \rm [Fe/H] \lesssim -1.3$). We find abundance patterns that are effectively indistinguishable from the bulk of Indus and Jhelum, a pair of smaller stellar streams proposed to be dynamically associated with Wukong/LMS-1. We confirmed a carbon-enhanced metal-poor star ($\rm [C/Fe] \gt +0.7$ and $\rm [Fe/H] \sim -2.9$) in Wukong/LMS-1 with strong enhancements in Sr, Y, and Zr, which is peculiar given its solar-level [Ba/Fe]. Wukong/LMS-1 stars have high abundances of α elements up to $\rm [Fe/H] \gtrsim -2$, which is expected for relatively massive dwarfs. Towards the high-metallicity end, Wukong/LMS-1 becomes α-poor, revealing that it probably experienced fairly standard chemical evolution. We identified a pair of N- and Na-rich stars in Wukong/LMS-1, reminiscent of multiple stellar populations in globular clusters. This indicates that this dwarf galaxy contained at least one globular cluster that was completely disrupted in addition to two intact ones previously known to be associated with Wukong/LMS-1, which is possibly connected to similar evidence found in Indus. From these ≥3 globular clusters, we estimate the total mass of Wukong/LMS-1 to be ${\approx }10^{10} \, \mathrm{M}_\odot$, representing ∼1 per cent of the present-day Milky Way. Finally, the [Eu/Mg] ratio in Wukong/LMS-1 continuously increases with metallicity, making this the first example of a dwarf galaxy where the production of r-process elements is clearly dominated by delayed sources, presumably neutron-star mergers.

Detection of the Actinide Th in an r-process-enhanced Star with Accretion Origin

The thorium and six second-peak r-process element (56 ≤ Z ≤ 72) abundances are determined for the α-poor star LAMOST J1124+4535 based on a high-resolution spectrum obtained with the High Dispersion Spectrograph on the Subaru Telescope. The age of J1124+4535 is 11.3 ± 4.4 Gyr using thorium and other r-process element abundances. J1124+4535 is confirmed to be a Galactic halo metal-poor ([Fe/H] = −1.27 ± 0.1) star with extreme r-process element overabundance ([Eu/Fe] = 1.13 ± 0.08) and α element deficiency ([Mg/Fe] = −0.31 ± 0.09) by the LAMOST-Subaru project. Along with the subsolar α-to-iron ratios (e.g., [Mg/Fe], [Si/Fe], [Ca/Fe]), the relatively low abundances of Na, Cr, Ni, and Zn in J1124+4535 show a significant departure from the general trends of the Galactic halo but are in good agreement with those of dwarf galaxies. The chemical abundances and kinematics of J1124+4535 suggest it was formed in the late stage of star formation in a dwarf galaxy that has been disrupted by the Milky Way. The star formation of its progenitor dwarf galaxy lasted more than 2 Gyr and has been affected by a rare r-process event before the occurrence of accretion event.