Haining Li

The escape velocity is a key probe of the Milky Way (MW) potential, but standard estimates can be biased by unrelaxed substructures. Using MW analogs from the TNG50 cosmological simulation, we investigate the impact of the dominant, highly radial Gaia–Sausage–Enceladus (GSE) debris on escape-velocity measurements. We introduce an adaptive selection strategy that balances statistical reliability with sample size to identify the power-law decline regime of the local halo velocity distribution. Applying this strategy to 16 MW analogs with prominent GSE-like structures and continuous high-velocity tails, we find that retaining GSE-like stars yields a systematic underestimate, with a recovery fraction of 97 . 5 % − 2.9 % + 2.1 % , while removing GSE-like stars alone increases the recovery fraction to 99 . 4 % − 2.5 % + 2.1 % . The ∼2% underestimation of the escape velocity corresponds to an ∼6% underestimation of the total mass. This improvement persists in LAMOST-like magnitude and footprint selection. GSE-like debris is therefore a key contributor to the systematic underestimation of escape-velocity measurements, with direct implications for the inferred MW mass.

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.

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. ∼54 . ° 0, decl. ∼31 . ° 7, rad ∼3 . ° 9, where APOGEE shows that the metallicity of the stars ranges from [Fe/H] ∼ −1 dex, [ α /M] ∼−0.3 dex, which should be close to the abundances of the accreted gas, to [Fe/H] ∼ 0.3 dex, [ α /M] ∼ 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 ( T _eff , log g , [Fe/H], and ξ _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 [ α /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 α -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.

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.

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 ¹² C/ ¹³ C 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.

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 ∼2500 deg ² 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 ∼18.9 mag at a signal-to-noise ratio ∼10 and an internal photometric precision ≈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 secondary i - and s -processes that follow the r -process in collapsar outflows, characterized by low electron fraction Y _e 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 , γ ) 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 , γ ) 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 , γ ) 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.

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.

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.