The astrophysical origin of the lanthanides is an open question in nuclear astrophysics. Besides the widely studied s , i , and r processes in moderately to strongly neutron-rich environments, an intriguing alternative site for lanthanide production could in fact be robustly proton-rich matter outflows from core-collapse supernovae under specific conditions—in particular, high-entropy winds with enhanced neutrino luminosity and fast dynamical timescales. In this environment, excess protons present after charged-particle reactions have ceased can continue to be converted to neutrons by (anti)neutrino interactions, producing a neutron-capture reaction flow up to A ∼ 200. This scenario, christened the νi process in a recent paper, has previously been discussed as a possibility. Here, we examine the prospects for the νi process through the lenses of stellar abundance patterns, bolometric light curves, and galactic chemical evolution models, with a particular focus on hypernovae as candidate sites. We identify specific lanthanide signatures for which the νi process can provide a credible supplement to the r / i processes.

Ultrafaint dwarf (UFD) galaxies are the smallest and most dark-matter-dominated galaxies, and they preserve signatures of the earliest nucleosynthetic events in the Universe. Because most stars in UFD galaxies formed before reionization, their stellar debris provides a valuable probe of early chemical evolution. We reanalyze a sample of 97 metal-poor halo stars from D. Yong et al. using revised stellar parameters and a homogeneous line-pair selection for Sr and Ba designed to minimize systematic uncertainties. In the [Sr/Ba]-[Ba/Fe] plane, the sample separates into two sequences: an upper branch that follows the Galactic halo trend and a lower branch with UFD-like [Sr/Ba] ratios. A linear selection criterion, [Sr/Ba] < -0.8[Ba/Fe] - 0.7, isolates 25 stars on the lower branch. Among them, 17 stars in Group 1 closely match an isolated dwarf galaxy represented by Model B of Y. Hirai, in which Sr enrichment is dominated by electron-capture supernovae and delayed neutron star mergers. The remaining eight stars in Group 2 exhibit a metallicity-dependent increase in [Sr/Fe] with very small scatter. We suggest that these two chemically distinct populations, to our knowledge, provide the first evidence that accreted halo stars with UFD-like neutron-capture chemistry separate into coherent groups that can be directly compared with specific chemical-evolution models. Their absence of clustering in the L-z-energy plane further highlights the diagnostic power of Sr-Ba abundances for identifying accreted UFD-like relics.

Halo star clusters serve as vital tracers for the formation and evolution of the Andromeda galaxy. In this work, we present physical parameters for 29 M31 halo star clusters, derived from a combination of spectroscopic and photometric data. Low-resolution spectra were acquired using the Beijing Faint Object Spectrograph and Camera spectrograph on the National Astronomical Observatories, Chinese Academy of Sciences Xinglong 2.16 m telescope. For the photometric analysis, we utilized u _SC and v _SAGE bands from the SAGE survey, complemented by archival data from Galaxy Evolution Explorer (near-ultraviolet and far-ultraviolet), PAN-STARRS ( grizy ), and the Two Micron All Sky Survey ( JHK ). Ages and metallicities were determined via ULySS (Vazdekis et al. and pegase-hr) simple stellar population model and the G. Bruzual & S. Charlot stellar population synthesis models. The derived parameters show good agreement with literature values. Notably, for three of these clusters, this study represents the first combined photometric and spectroscopic analysis.

The Chinese Space Station Survey Telescope (CSST) is an upcoming Stage-IV sky survey telescope, distinguished by its large field of view (FoV), high image quality, and multi-band observation capabilities. It can simultaneously conduct precise measurements of the Universe by performing multi-color photometric imaging and slitless spectroscopic surveys. The CSST is equipped with five scientific instruments, i.e., Multi-band Imaging and Slitless Spectroscopy Survey Camera (SC), Multi-Channel Imager (MCI), Integral Field Spectrograph (IFS), Cool Planet Imaging Coronagraph (CPI-C), and THz Spectrometer (TS). Using these instruments, CSST is expected to make significant contributions and discoveries across various astronomical fields, including cosmology, galaxies and active galactic nuclei (AGN), the Milky Way and nearby galaxies, stars, exoplanets, Solar System objects, astrometry, and transients and variable sources. This review aims to provide a comprehensive overview of the CSST instruments, observational capabilities, data products, and scientific potential.

We perform a homogeneous analysis of 82 heavy-element-enhanced metal-poor stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) /Subaru sample, including 64 r-process-enhanced stars, two limited-r stars, and 16 stars exhibiting significant or moderate carbon enhancement (comprising nine s-process-enhanced, two r-process-enhanced, two r/s, and three r+s stars). Systematic differences in [X/Eu] between r-I and r-II stars, together with their distinct dynamical properties, suggest that r-II stars likely formed in low-mass dwarf galaxies where contamination from additional nucleosynthetic channels was minimal. We have also discovered a number of peculiar objects. We identify an extremely metal-poor r-II star, J1158+0734 ([Fe/H] = -2.93), whose enhanced Zn abundance ([Zn/Fe] = +0.67) is best explained by yields from a high-energy, massive core-collapse supernova. The surface abundances of s-process-enhanced stars exhibit pronounced dispersion, consistent with enrichment from asymptotic giant branch (AGB) progenitors with diverse properties. Notably, one star, J2256+0215, reveals compelling evidence for additional evolutionary mixing, as indicated by its unusually low carbon abundance ([C/Fe] = +0.08) and extremely low 12C/13C ratio of 2.33. Additionally, we identify three carbon-enhanced metal-poor (CEMP) r+s stars whose abundance patterns reveal combined contributions from both the r- and s-processes. We further propose a modified diagnostic scheme for the identification of CEMP-r+s stars, defined as -0.2 <= [La/Nd] <= 0.2 and -0.5 <= [Eu/Nd] <= 0.0.

Metal-poor stars are crucially important for understanding the early Galaxy, first stars, and the Universe. In this series of papers, we present a homogeneous non-local thermodynamic equilibrium (NLTE) abundance analysis of 12 elements for 103 very metal-poor (VMP)/extremely metal-poor (EMP) stars with metallicity down to -4.3 dex. The sample was selected from the LAMOST survey and observed by the high-resolution spectroscopy of Subaru. In this paper, we present the NLTE abundances and evolution of lithium in these stars. We report different lithium behaviors corresponding to different evolutionary stages and their signatures: (1) The Spite plateau shows a slightly positive slope, indicating increasing lithium abundance with increasing metallicity. Most significantly, it appears to extend to lower metallicities as previously suggested, calling into question the reality of the so-called "meltdown" at low metallicity; (2) we confirm a lithium plateau for lower red giant branch (LRGB) stars with A(Li) = 1.13 dex in our sample, while the lithium abundance drops rapidly to A(Li) < 0.5 as stars continue to evolve to a higher stage. (3) We identify four Li-rich stars in our sample across different evolutionary stages, showing complex and multiple lithium production mechanisms in VMP/EMP stars. These findings suggest that early Galactic lithium enrichment results from a complex interplay between depletion and production processes.

Modified gravity theories such as Modified Newtonian Dynamics (MOND) and Scalar-Tensor-Vector Gravity (STVG) have been proposed as alternatives to dark matter, but decisive tests have been hindered by degeneracies between baryonic structure and gravitational laws. Here we break this degeneracy using independent, high-precision constraints: the Milky Way radial rotation curve, vertical phase-space spirals from Gaia, and a broken-exponential stellar disk. A joint reconstruction of the radial and vertical gravitational fields reveals a structural inconsistency in modified gravity – no model can simultaneously reproduce both observations. Our results strongly disfavor MOND at >13σ and STVG at >4σ. In contrast, dark matter halo models naturally explain the observations, providing a self-consistent test of gravity on galactic scales.

Milky Way halo substructures identified in dynamical space are known to suffer from contamination from the Milky Way in situ stars, which makes their accreted origins uncertain. We present detailed chemical abundances of 35 stars belonging to two sets of dynamically tagged groups, Rg8 and Rg9, to investigate their accreted nature. Both groups are composed of stars with low orbital energy and very retrograde orbits. We find that Rg8 and Rg9 are chemically indistinguishable across 20 elements, from C to Eu, strongly indicating that they belong to the same structure. The iron-abundance distribution of this low- E retrograde group has a prominent peak at [Fe/H] ≈ –2.1, revealing that its main population is very metal-poor (VMP), and a secondary peak at [Fe/H] ≈ –1.5, very likely due to contamination from Milky Way in situ stars. These groups also heavily overlap with the Thamnos substructure in dynamical space, and we thus use them to investigate the chemical properties of Thamnos. The dominant, low-metallicity population provides strong evidence for the ex situ origin of Thamnos, as does its VMP nature. We do not see any evidence of an α -knee in our sample, which is consistent with previous studies. Comparison with the Cetus-Palca stream in the chemical space shows similar abundance distributions, and thus it suggests that the Thamnos progenitor dwarf galaxy had a truncated star formation history due to its early merger with the Milky Way.

The secondary i- and s-processes that follow the r-process in collapsar outflows, characterized by low electron fraction Ye and decelerated expansion, can serve as a major source of rare earth element production. We systematically evaluated the contributions of the r-, i-, and s-processes to the production of rare earth elements in collapsar outflows. A sensitivity analysis of nucleosynthesis yields with respect to (n, gamma) reaction rates involving unstable nuclei near the line of stability highlights dozens of key reactions critical for the synthesis of thulium (Z = 69) and lutetium (Z = 71). These specific (n, gamma) reactions, however, have a negligible impact on the yields predicted in magnetohydrodynamically driven jets or binary neutron star mergers. Our nucleosynthesis models indicate that collapsars can produce elevated Tm/Eu and Lu/Eu abundance ratios, potentially serving as observational signatures distinguishing collapsar nucleosynthesis from others. The robustness of these predictions, however, is highly sensitive to uncertainties in the relevant nuclear reaction rates. We therefore emphasize the need for precise future experimental measurements of the (n, gamma) cross sections for key unstable nuclei. Reducing these nuclear-physics uncertainties will allow collapsar models to predict Tm/Eu and Lu/Eu ratios with greater confidence, thereby improving our interpretation of observed abundances in r-process-enhanced metal-poor stars.

We present a non–local thermodynamic equilibrium (NLTE) abundance analysis of the light neutron-capture elements Sr, Y, and Zr in a sample of 103 very metal-poor stars spanning −4.3 ≲ [Fe/H] ≲ −1.7, based on high-resolution Subaru/HDS spectra and atmospheric parameters from Paper I. This is the first NLTE study of Sr−Y−Zr abundances using a large, homogeneous sample optimized for tracing Galactic chemical evolution (GCE). We find a mild positive trend in [Sr, Y, Zr/Fe] with [Fe/H], consistent with GCE models that incorporate s -process, electron-capture supernovae, compact object mergers, and magnetorotational supernovae, without invoking additional light-element primary processes. We report a statistically significant correlation between [Sr/Fe] and [Na/Fe] in our stellar sample. This finding supports the contribution of fast-rotating massive stars (spinstars) to the simultaneous production of sodium through hydrogen burning and light neutron-capture elements via the weak s -process in low-metallicity environments. We identify five Sr-poor stars exhibiting significantly lower Sr, Y, and Zr abundances than other stars at similar [Fe/H], pointing to chemical inhomogeneities in early star-forming environments. These light neutron-capture elements therefore provide valuable tracers of early nucleosynthetic processes and the Milky Way’s assembly history.

Stellar age is a fundamental quantity for Galactic archaeology, but reliable age estimation for large stellar samples remains challenging. In this work, we develop an uncertainty aware NGBoost framework for stellar age estimation using Gaia XP-derived atmospheric parameters and chemical abundances. Different from the standard NGBoost model, we modify the loss function by incorporating the uncertainties of the training age labels. We further use a Monte Carlo strategy to quantify the influence of input-feature uncertainties on the predicted ages. The resulting model provides age estimates together with uncertainty estimates. Applying this framework to Gaia XP stars, we construct a stellar age catalog containing 15,175,107 stars.

We present the first public data release of the DDO51 band from the Stellar Abundances and Galactic Evolution Survey (SAGES), based on Nanshan One-meter Wide-field Telescope observations obtained between 2023 September and 2024 January. This release initiates the DDO51-band component of the survey, covering similar to 2500 deg2 of the northern sky and including more than 10 million sources. The DDO51 filter is centered near the Mg i b triplet and the adjacent MgH feature, offering sensitivity to stellar surface gravity. The data reduction pipeline incorporates an improved astrometric solution anchored to Gaia DR3 and a photometric calibration strategy tied to synthetic photometry from Gaia XP spectra. These procedures yield a point-source depth of similar to 18.9 mag at a signal-to-noise ratio similar to 10 and an internal photometric precision approximate to 6-7 mmag at the bright end. A preliminary color-color analysis using Gaia broadband photometry confirms the expected sensitivity of the DDO51 band to stellar surface gravity, demonstrating a clear photometric separation between dwarf and giant sequences for late-type stars. This dataset, when combined with existing SAGES photometry in other bands, provides a crucial tool for disentangling the substructures of the Milky Way. All data products from this release are available upon publication.

The Milky Way promotes the formation of young stellar objects (YSOs) by accreting nearby metal-poor gas. The degree of mixing varies, resulting in different metallicity of the stars. We found a pre-main-sequence star cluster at R.A. similar to 54 .degrees 0, decl. similar to 31 .degrees 7, rad similar to 3 .degrees 9, where APOGEE shows that the metallicity of the stars ranges from [Fe/H] similar to -1 dex, [alpha/M] similar to-0.3 dex, which should be close to the abundances of the accreted gas, to [Fe/H] similar to 0.3 dex, [alpha/M] similar to 0.1 dex, the abundances of the Milky Way's local gas, arranged in a straight line. In the EBHIS H I survey data, there is a accretion gas stream located near local standard of rest velocity -46.3 km s-1, a gas cavity produced during star formation near -3.8 km s-1, and a local gas structure near 6.5 km s-1. Our target cluster is composed of two subclusters, and the accretion gas stream happens to bifurcate and connect to the two subclusters, respectively, thus providing strong evidence. The YSO cluster found in this work serves as a very interesting laboratory for star formation.

We present a chemodynamical analysis of three hypervelocity stars (HVS) selected from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope medium-resolution spectroscopic survey and Gaia. The high-resolution follow-up observation was conducted with Subaru, enabling precise stellar atmospheric parameter determination (Teff, log g, [Fe/H], and xi t) and detailed abundance measurements for elements Na, Mg, Ca, Ti, Ba, and Eu. Non-local thermodynamic equilibrium corrections were applied for most species. Orbital properties were computed using GalPy, integrating 1000 Monte Carlo realizations of phase-space coordinates backward for 13 Gyr under the Galactic potential. Our results reveal distinct origins of the three objects: J 1436+3537 exhibits low [alpha/Fe], and high [Ba/Fe] and [Eu/Fe] ratios, consistent with an accreted origin. Its galactocentric velocity exceeds the Galactic escape velocity; J 1334+1400 shows thick disk-like alpha-abundances and a bulge-proximal orbit, supporting an in situ ejection; J 1718+2418 displays the typical chemistry and kinematics of the thick disk. This study demonstrates that the HVS formation mechanisms are complex, and coupling chemical abundances with orbital histories is helpful to constrain their origins.

Based on 6.1  fb^{-1} of e^{+}e^{-} annihilation data collected at center-of-mass energies from 4.600 to 4.843 GeV with the BESIII detector at the BEPCII collider, a partial wave analysis of Λ_{c}^{+}→Λπ^{+}η is performed, and branching fractions and decay asymmetry parameters of intermediate processes are determined. The process Λ_{c}^{+}→Λa_{0}(980)^{+} is observed for the first time, and evidence for the pentaquark candidate Σ(1380)^{+} decaying into Λπ^{+} is found with statistical significance larger than 3σ with mass and width fixed to theoretical predictions. The branching fraction product B[Λ_{c}^{+}→Λa_{0}(980)^{+}]B[a_{0}(980)^{+}→π^{+}η] is determined to be (1.05±0.16_{stat}±0.05_{syst}±0.07_{ext})%, which is larger than theoretical calculations by 1-2 orders of magnitude. Here the third (external) systematic is from B(Λ_{c}^{+}→Λπ^{+}η). Finally, we precisely obtain the absolute branching fraction B(Λ_{c}^{+}→Λπ^{+}η)=(1.94±0.07_{stat}±0.11_{syst})%.

Escape velocity has long been used to constrain the mass of the dark matter (DM) halo in the Milky Way (MW). Here, we present a study of the escape velocity curve using a sample of high-velocity K giants with full 6D phase-space information and of relatively good quality, selected from LAMOST DR8 and cross-matched with Gaia DR3. To expand the high-velocity stars to larger distances, we used radius-dependent criteria of total velocity, i.e., v(GC) > 300 km s(-1) for the solar neighborhood and v(GC)>v(min)similar to 0.6xv(esc)(r(GC)) for the outer region. We also selected halo stars based on v(phi) - [Fe/H] information to ensure that the sample is isotropic. We modeled the velocity distribution with traditional power-law models to determine the escape velocity in each radial bin. For the first time, we have directly measured a relatively continuous escape velocity curve that can extend to Galactocentric radii of similar to 50 kpc, finding a decline in agreement with previous studies. The escape velocity at the solar position yielded by our measurements is 523.74(-13.47)(+12.83)kms(-1) . Combined with the local circular velocity, we estimated the mass of the MW assuming a Navarro-Frenk-White DM profile, which resulted in a total mass of M-200,M-total=0.90(-0.07)(+0.06)x10(12)M(circle dot) , with a concentration of c(200)=13.47(-1.70)(+1.85) . The small uncertainty implies that including the escape velocities beyond the solar neighborhood can result in a more precise mass estimate. Our derived MW mass is consistent with some recent studies using the escape velocity as well as other tracers, which may support a lower mass of the DM halo than in the past.

The Sun is depleted in refractory elements compared to nearby solar twins, which may be linked to the formation of giant or terrestrial planets. Here we present high-resolution, high signal-to-noise spectroscopic data for 17 solar-like stars hosting planets, obtained with Magellan II/MIKE, to investigate whether this depletion is related to planet formation. We derive stellar parameters, including stellar atmosphere, age, radius, mass, and chemical abundances for 22 elements from carbon to europium through line-by-line differential analysis. Our uncertainties range from 0.01 dex for Fe and Si to 0.08 dex for Sr, Y, and Eu. By comparing the solar abundances to those of the 17 stars, we investigate the differential abundance ([X/Fe] solar –[X/Fe] star ) versus condensation temperature ( T c ) trend. In particular, we apply Galactic chemical evolution corrections to five solar twins within the full sample. Our results conform to previous studies that the Sun is relatively depleted in refractory compared to volatile elements. For both five solar twins and the rest of the solar-like stars, we find that all stars hosting known gas giant planets exhibit negative T c trend slopes, suggesting that the Sun is relatively depleted in refractory elements compared to similar giant planet-hosting stars. Additionally, we find no correlation between T c trend slopes and the total mass of detected terrestrial planets in each system, suggesting that terrestrial planet formation may not be the cause of refractory element depletion in the Sun.

Unveiling the properties of the progenitors that formed the tidal structures of the Milky Way (MW) is essential for exploring the MW's assembly history. Such studies further afford a unique opportunity to dissect the formation and evolution of dwarf galaxies by contrasting surviving and disrupted systems. However, reconstructing the intrinsic structure of progenitors solely from the positions and velocities of their tidal debris remains a formidable challenge. Here, we decode the chemo-kinematical signatures embedded in the stellar streams of the Sagittarius (Sgr) dwarf galaxy through N-body simulations. For the first time, we reproduce a vertical metallicity gradient in the Sgr streams, which we attribute to a disk-dominated progenitor with a Λ-like metallicity profile across the disk. Remarkably, similar metallicity profiles are identified in MaNGA survey data for several field dwarf galaxies, suggesting this feature is not unique to Sgr. The vertical metallicity gradient of stellar streams preserves extra detailed galactic fossil records, enabling quantitative reconstruction of the peculiar structure of the satellite galaxy. The similar metallicity profile of Sgr to MW opens a question on a possible special coevolution.

Using 7.9fb−1 of e+e− collision data collected at s=3.773 GeV with the BESIII detector at the BEPCII collider, we search for the massless dark photon with the flavor-changing neutral current processes D0→ωγ′ and D0→γγ′ for the first time. No significant signals are observed, and the upper limits at the 90% confidence level on the massless dark photon branching fraction are set to be 1.1×10−5 and 2.0×10−6 for D0→ωγ′ and D0→γγ′, respectively. These results provide the most stringent constraint on the new physics energy scale associated with cuγ′ coupling in the world, with the new physics energy scale related parameter |C|2+|C5|2<8.2×10−17 GeV−2 at the 90% confidence level. Published by the American Physical Society 2025

A deep understanding of our Galaxy calls for detailed decomposition of its stellar populations via their chemical fingerprints. This requires precise stellar abundances of many elements for a large number of stars. Here, we present an updated catalog of stellar labels derived from Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) low-resolution spectra in a physics-sensible and rigorous manner with DD-Payne , taking labels from high-resolution spectroscopy as a training set. The catalog contains atmospheric parameters for 6.4 million stars released in LAMOST DR9, and abundances for 22 elements, namely, C, N, O, Na, Mg, Al, Si, Ca, Ti, Cr, Mn, Fe, Ni, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, and Eu, for nearly 3.6 million stars with spectral signal-to-noise ratio (S/N) higher than 20. The [Fe/H] is valid down to ≃ −4.0, while elemental abundance ratios [X/Fe] are mostly valid for stars with [Fe/H] ≳ −2.0. Measurement errors in these labels are sensitive to and almost inversely proportional with S/N. For stars with S/N > 50, we achieved a typical error of 30 K in T eff , 0.07 dex in log g , ∼0.05 dex in abundances for most elements with atomic number smaller than Sr, and 0.1–0.2 dex for heavier elements. Homogenization to the label estimates is carried out via dedicated internal and external calibration. In particular, the nonlocal thermal equilibrium effect is corrected for the [Fe/H] estimates, the T eff is calibrated to the infrared flux method scale, and the log g is validated with asteroseismic measurements. The elemental abundances are internally calibrated using wide binaries, eliminating systematic trend with effective temperature. The catalog is publicly available.