We present a strictly line-by-line differential analysis of a moderately r-process-enhanced star (r-I: HD 107752) with respect to a strongly r-process-enhanced star (r-II: CS 31082-0001) to investigate the possible common origin of their heavy-element nucleosynthesis with high-precision abundances. This study employs European Southern Observatory data archive high-resolution and high-signal-to-noise spectra taken with the Ultraviolet and Visual Echelle Spectrograph Very Large Telescope spectrograph. Considering only the lines in common in both spectra, we estimate differential abundances of 16 light/Fe-peak elements and 15 neutron-capture elements. Abundances of O, Al, Pr, Gd, Dy, Ho, Er, and a detection of Tm in HD 107752, are presented for the first time. We found three distinct features in the differential-abundance pattern. Nearly equal abundances of light elements up to Zn are present in both stars, indicating a common origin for these elements; in addition to no noticable odd-even differential pattern. Regarding the neutron-capture elements, the r-I star exhibits mildly depleted light r-process elements and more depleted heavier r-process elements relative to r-II star. We also show that, among the r-I and r-II stars, the ratio of lighter-to-heavier r-process elements (e.g., [(Sr, Y, Zr)/Eu]) exhibits a decreasing trend with respect to the overall r-process enhancement, forming a continuous sequence from the r-I to the r-II stars. Finally, we discuss the necessity of multiple sites for the formation of r-I stars.

We report the discovery of a new subclass of carbon-enhanced metal-poor (CEMP) stars, characterized by high absolute carbon abundances (A(C) > 7.39) and extremely low metallicity ([Fe/H] <= -3.1) but notably lacking enhancements in neutron-capture elements, thus falling under the CEMP-no category. This population emerged from a detailed analysis of low-resolution spectroscopic data obtained from the Sloan Digital Sky Survey and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, where the observed frequency trends with the decreasing metallicity of CEMP-s (s-process-enhanced) and CEMP-no (no neutron-capture enhanced) stars deviated from established expectations. In contrast to earlier findings, we observe a rise in high-A(C) stars below [Fe/H] = -3.1, which we interpret as a distinct group not accounted for in traditional CEMP classifications. Following the Yoon-Beers group classification, we define these stars as Group IV. Statistical modeling confirms their presence as a separate peak in the A(C) distribution, and available radial velocity data suggest that about 30% of Group IV stars may be binaries, indicating possible binary-related formation mechanisms. This discovery challenges the current CEMP-no star formation pathways and implies the existence of alternative or hybrid enrichment scenarios in the early Universe. High-resolution spectroscopic follow-up of Group IV candidates will be crucial for identifying their progenitors and understanding their evolutionary implications.

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. Evidence suggests that the Milky Way (MW) underwent a major collision with the Gaia–Sausage/Enceladus (GSE) dwarf galaxy around cosmic noon. While GSE has since been fully disrupted, it brought in ex situ stars and dynamically heated in situ stars into the halo. In addition, the gas-rich merger may have triggered a burst of in situ star formation, potentially giving rise to a chemically distinct stellar component. Aims. We investigated the region of phase space where stars formed during the GSE merger likely reside, and retain distinct chemical and dynamical signatures. Methods. Building on our previous investigation of metallicity ([Fe/H]) and vertical angular momentum (LZ) distributions, we analysed spectroscopic samples from GALAH, APOGEE, SDSS, and LAMOST, combined with Gaia kinematics. We focused on high proper-motion stars as effective tracers of the phase-space volume likely influenced by the GSE merger. To correct for selection effects, we incorporated metallicity estimates derived from SDSS and SMSS photometry. Results. Our analysis reveals that low-α stars with GSE-like kinematics exhibit bimodality in [Na/Fe] and [Al/Fe] at −1.0 ≲ [Fe/H] ≲ −0.4. One group follows the low light-element abundances of GSE stars, while another exhibits enhanced values. These low-α, high-Na stars have eccentric orbits but are more confined to the inner MW. Eos overlaps with a high-eccentricity subset of these stars, implying that it constitutes a smaller structure nested within the broader population. After correcting for sampling biases, we estimated a population ratio of approximately 1:10 between the low-α, high-Na stars and the GSE debris. Conclusions. These results suggest that the low-α, high-Na stars formed in a compact region, likely fuelled by gas from the GSE progenitor, analogous to clumpy star-forming clouds seen in high-redshift galaxies. Such stars may trace the first sparks of more extensive merger-driven starburst activity. © The Authors 2025.

The circular speed curve of the Milky Way provides a key constraint on its mass distribution, reflecting the axisymmetric component of the gravitational potential. This is especially critical in the inner Galaxy (R less than or similar to 4 kpc), where nonaxisymmetric structures, such as the stellar bar and nuclear stellar disk, strongly influence dynamics. However, significant discrepancies remain between circular speed curves inferred from stellar dynamical modeling and those derived from the terminal-velocity method applied to gas kinematics. To investigate this, we perform three-dimensional hydrodynamic simulations including cooling, heating, star formation, and feedback, under a realistic gravitational potential derived from stellar dynamical models calibrated to observational data. This potential includes the Galactic bar, stellar disks, dark matter halo, nuclear stellar disk, and nuclear star cluster. We generate synthetic longitude-velocity diagrams and apply the terminal-velocity method to derive circular speeds. The simulated gas reproduces the observed terminal-velocity envelope, including a steep inner rise. We find this feature arises from bar-driven noncircular motions, which cause the terminal-velocity method to overestimate circular speeds by up to a factor of 2 at R similar to 0.4 kpc, and enclosed mass by up to a factor of 4. These results suggest that inner gas-based rotation curves can significantly overestimate central mass concentrations. The steep inner rise in gas-derived circular speeds does not require a massive classical bulge but can be explained by bar-induced streaming motions. Rather than proposing a new mechanism, our study provides a clear, Milky Way-specific demonstration of this effect, emphasizing the importance of dynamical modeling that explicitly includes noncircular motions for accurate mass inference in the inner Milky Way.

A major goal of computational astrophysics is to simulate the Milky Way Galaxy with sufficient resolution down to individual stars. However, the scaling fails due to some small-scale, short-timescale phenomena, such as supernova explosions. We have developed a novel integration scheme of N-body/hydrodynamics simulations working with machine learning. This approach bypasses the short timesteps caused by supernova explosions using a surrogate model, thereby improving scalability. With this method, we reached 300 billion particles using 148,900 nodes, equivalent to 7,147,200 CPU cores, breaking through the billion-particle barrier currently faced by state-of-the-art simulations. This resolution allows us to perform the first star-by-star galaxy simulation, which resolves individual stars in the Milky Way Galaxy. The performance scales over 104 CPU cores, an upper limit in the current state-of-the-art simulations using both A64FX and X86-64 processors and NVIDIA CUDA GPUs. © 2025 Copyright held by the owner/author(s).