The epoch of the Gaia-Sausage-Enceladus (GSE) merger, the last major merger experienced by the Milky Way, is crucial for reconstructing the Galaxy's evolutionary history. This event dynamically heated the disk, scattering some stars into the halo and producing the so-called Splash. Yet the observed age distribution of the Splash is systematically older than that of the thick disk from which it is thought to originate, posing a puzzling inconsistency. In this work, we show that this apparent discrepancy can be naturally explained once stellar age uncertainties are taken into account. The GSE truncated the intrinsic age distribution of the Splash at the merger epoch, while measurement errors introduce an Eddington-like bias that systematically shifts the truncated distribution toward older ages. Importantly, the magnitude of this shift depends on both the thick-disk star formation history and the merger epoch, thereby offering a new way to constrain the timing of the merger. By combining the observed Splash age distribution with the peak of thick-disk star formation, we infer a GSE merger epoch of 10.1^+0.2_-0.2 Gyr ago, providing one of the tightest constraints to date. This result offers new insight into the Milky Way's accretion history and opens a path toward more robust reconstructions of its early evolution.

We combine narrow/medium-band filter photometry from the Southern Photometric Local Universe Survey (S-PLUS) DR4 with ultrabroadband filter photometry from Gaia EDR3 to derive fundamental stellar parameters (T-eff,log g , [Fe/H], and ages) and elemental-abundance ratios ([C/Fe] and [alpha/Fe]) for 5.4 million stars in the Galaxy (4.9 million dwarfs and 0.5 million giants), as well as for over 0.7 million red giant stars in the Large and Small Magellanic Clouds (LMC and SMC). The precisions of the abundance estimates range from 0.05 to 0.10 dex for metallicity in the relatively metal-rich range ([Fe/H] > -1.0) to 0.10-0.30 dex in the metal-poor regime ([Fe/H] < -1.0), 0.10-0.20 dex for [C/Fe], and 0.05 dex for [alpha/Fe]. The stellar parameters for LMC and SMC member stars are somewhat less precise than those from the S-PLUS main survey, primarily because of the effect of high reddening. The use of both metallicity- and carbon-sensitive filters provides unbiased measurements of both [Fe/H] and [C/Fe], of particular importance for very low-metallicity ([Fe/H] < -2.0) stars, where carbon enhancement can lead to systematically high estimates of [Fe/H] when only a single metallicity-sensitive filter is employed. Furthermore, multiple narrowband filters enable metallicity estimates down to [Fe/H] similar to -4.0 with an accuracy of around 0.3 dex, exceeding the precision typically achieved by low/medium-resolution spectroscopy. This extensive photometric data set, combined with the other three data sets in this series, will serve as a valuable legacy resource for Milky Way and Magellanic Clouds studies.

It has long been predicted that the Milky Way (MW) will eventually merge with Andromeda (M31), a view reinforced by Hubble Space Telescope measurements indicating a small M31 transverse velocity. However, using updated Gaia-based proper motions (PMs) and including the dynamical influence of the Large Magellanic Cloud and M33, T. Sawala et al. reported an MW-M31 merger probability of similar to 50% within 10 Gyr, leaving the fate of the Local Group (LG) uncertain. Adopting their semianalytic framework, we revisit this problem with the latest and most precise Gaia-based PMs for M31 and M33, corrected for systematic offsets in Gaia astrometry. In our fiducial model, the MW-M31 merger probability rises to 90%, with a median merger time of 6.5-1.5+1.3 Gyr, broadly restoring the classical picture. A sensitivity analysis shows that the merger probability depends strongly on the adopted M31 PM through two channels: a direct effect via the radial-tangential balance of the MW-M31 orbit, and a satellite-mediated effect, where the M31 PM fixes the orbital plane and determines how satellite-induced barycentric reflex motions project onto it, either promoting or suppressing a merger. Given this sensitivity, current measurements, while favoring a high merger probability, remain inconclusive, spanning from 64.7% to 100% across the 2 sigma PM region. Future PM measurements with uncertainty of less than or similar to 2 mu as yr-1 will be required to reach a firm conclusion, i.e., to constrain the probability range within 10% at the 2 sigma level.

Extremely metal-poor (EMP; [Fe/H] < -3.0) stars preserve key information about the earliest stages of Galactic chemical evolution. Based on a training set of spectroscopic metallicities, we constructed catalogs of more than 120,000 EMP candidates from large-scale narrow-band photometric surveys. To validate this approach, we obtained high-resolution CFHT/ESPaDOnS spectra for 13 candidates selected from the SkyMapper-based catalog. The follow-up observation confirms that the narrow-band photometric selection is effective in identifying very metal-poor stars and retaining a substantial EMP fraction: four targets are confirmed as EMP stars, and all targets remain very metal-poor with [Fe/H] < -2.4. The photometric metallicities are systematically lower than the spectroscopic values by 0.44 dex, with a relatively small scatter of 0.19 dex, indicating a systematic offset in the photometric metallicity scale that could be reduced through improved calibration. Abundances for more than 20 chemical species are derived, leading to the identification of a new potassium-enhanced r-II star, J2001-1215. By combining chemical abundances with orbital properties, five stars are found to be dynamically consistent with known Galactic substructures, including Gaia–Sausage–Enceladus, Thamnos, and Sequoia. These stars provide high-resolution chemical measurements for the extremely metal-poor regime of these Galactic substructures. These results underscore the role of narrow-band photometry in efficiently selecting EMP candidates for targeted high-resolution spectroscopic follow-up, thereby enabling detailed chemical and dynamical studies of the early Milky Way.

We determine the rotation curve (RC) of the Milky Way in the range of 6

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.

Context. The extent of envelope stripping in progenitor stars is directly reflected in the diversity of spectral features that are observed in stripped-envelope supernovae (SESNe). Aims. Through extensive spectral observation and analysis, we aim to clarify the statistical differences between the subclasses of SESNe. Methods. The Tsinghua supernova group obtained 249 optical spectra of 62 SESNe from 2010 to 2020, covering phases from –16 to over 190 days relative to maximum light. Most spectra were obtained during the photospheric phases after the supernova explosion. For each spectrum, the pseudo-equivalent widths and blueshift velocities of the principal lines were measured. We further investigated the common spectral features by analyzing their velocity and strength correlations across all subtypes. Results. We identified the feature near 6200 Å in SNe Ib as Hα through a comparison with SNe IIb and Ic. This resolves inconsistent interpretations in the literature. Our finding reveals prevalent residual hydrogen in SNe Ib, further supporting a continuous stripping sequence from SNe IIb to Ib. The velocity among different subtypes of stripped-envelope SNe increases, with SNe IIb exhibiting the lowest line velocities, followed by Ib, Ic, and Ic-BL. Typically, the O I lines in SNe Ic/Ic-BL are stronger than those seen in SNe IIb/Ib. In nebular phases, the [Ca II] emission dominates [O I] in SNe IIb/Ib, while [O I] is stronger in SNe Ic, including in the He-rich SN 2016coi. This spectral dichotomy implies that progenitors of SNe Ic (BL) have more massive CO cores and hence higher initial masses.

We report the discovery of DESI-HVS1, a hypervelocity star (HVS) candidate identified from DESI DR1 spectroscopy and Gaia DR3 astrometry. DESI-HVS1 is an old, low-mass, metal-poor F-type star with a mass of 0.8 M-circle dot, an age of similar to 14.1 Gyr, and [Fe/H] = -1.6. It is located at a heliocentric distance of 3.7 7 (- 0.36) (+ 0.39) kpc and has a Galactocentric total velocity of 52 3 (- 47) (+ 46) km s( - 1) , marginally exceeding the local escape speed, corresponding to an unbound probability of P-ub similar to 50%. Backward orbit integrations show that DESI-HVS1 had the closest approach to the Galactic center (GC) of 0.4 0( - 0.11) (+ 0.23) kpc , with a velocity of 68 2( - 35) (+ 22 )km s( - 1) and a flight time of 12.8 9( - 0.74) (+ 0.92 )Myr . The reconstructed orbit exhibits a clear perigalactic turning point and only a single crossing of the Galactic midplane (P-cross > 0.95). These properties suggest that DESI-HVS1 is most naturally explained by the Hills mechanism, although alternative scenarios cannot be entirely ruled out. Its discovery provides the first strong evidence for an old, low-mass HVS candidate consistent with a GC origin, indicating that the apparent dominance of young, massive GC-origin HVSs is likely a consequence of observational selection effects.

The earliest assembly of the Milky Way remains poorly understood, yet the spatial, chemical, and kinematic properties of its most metal-poor stars provide a unique fossil record of its proto-Galaxy phase. Understanding how this ancient component formed is essential for linking near-field Galactic archaeology to high-redshift galaxy evolution. We construct the currently largest 3D map of inner-Galaxy metal-poor giants by combining several narrow/medium-band photometric surveys, reaching metallicities down to [Fe/H] ∼ −3.5. Our final sample contains 5,095,676 giants, including 1,717,610 stars with [Fe/H] < −1. Across −4 ≤ [Fe/H] < −1, the density distribution reveals a centrally concentrated, flattened spheroidal component extending to r _gc ∼ 15 kpc, together with a prominent overdensity near X ∼ −5 kpc that is dominated by metal-poor stars on disklike orbits, with a kinematically hot background also present. The selection-function-corrected metallicity distribution function shows a distinct, very metal-poor component around [Fe/H] ∼ −2.7 that becomes most prominent at 1 < r _gc < 3 kpc. Stars with −3.5 ≲ [Fe/H] ≲ −1.4 exhibit weak net rotation and low rotational support within r _gc < 15 kpc. Finally, we briefly note that the centrally enhanced very metal-poor component could be qualitatively consistent with one or more early dissipative build-up episodes (e.g., high- z compaction/“blue-nugget” phases) as one possible interpretation.

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.

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.

Motivated by the vast gap between photometric and spectroscopic data volumes, there is great potential in using 5D kinematic information to identify and study substructures of the Milky Way. We identify substructures in the Galactic halo using 46,575 RR Lyrae stars (RRLs) from Gaia Data Release 3, with photometric metallicities and distances newly estimated by X.-Y. Li et al. Assuming a Gaussian prior distribution of radial velocity, we calculate the orbital distribution characterized by the integrals of motion for each RRL, based on its 3D positions, proper motions, and corresponding errors, and then apply the friends-of-friends algorithm to identify groups moving along similar orbits. We have identified several known substructures, including the Sagittarius stream, the Hercules-Aquila Cloud (HAC), the Virgo Overdensity (VOD), Gaia-Enceladus-Sausage (GES), the Orphan-Chenab stream, Cetus-Palca, the Helmi streams, Sequoia, Wukong, and the Large Magellanic Cloud leading arm, as well as 18 unknown groups. Our findings indicate that the HAC and VOD have kinematic and chemical properties remarkably similar to GES, with most HAC and VOD members exhibiting eccentricity as high as GES, suggesting that they may share a common origin with GES. The ability to identify low-mass and spatially dispersed substructures further demonstrates the potential of our method, which breaks the limit of spectroscopic surveys and is competent for probing substructures throughout the whole Galaxy. Finally, we also identify 18 unknown groups with good spatial clustering and proper-motion consistency, suggesting the possibility of further excavation of Milky Way substructures in the future with only 5D data.

Hypervelocity stars (HVSs) represent a unique class of objects capable of escaping the gravitational pull of the Milky Way due to extreme acceleration events, such as close encounters with the supermassive black hole at the Galactic center (GC), supernova explosions in binary systems, or multibody dynamical interactions. Finding and studying HVSs are crucial to exploring these ejection mechanisms, characterizing central black holes, probing the GC environment, and revealing the distribution of dark matter in our Galaxy. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopic surveys have so far identified four B-type unbound HVSs. To expand this sample with the second-phase LAMOST survey that started in 2018, we conducted a systematic search for early-type HVSs using the LAMOST Data Release 10. We identified 125 early-type high-velocity candidates with total velocities exceeding 300 km s ^−1 . Among them, we report 10 new unbound B- and A-type HVS candidates (designated LAMOST-HVS 5 through LAMOST-HVS 14), tripling the number of unbound HVSs previously identified by LAMOST. Kinematic analyses suggest that these newly discovered HVS candidates likely originated either from the GC or via dynamical interactions. Future high-resolution follow-up observations promise to refine the stellar parameters, distances, and elemental abundances of these candidates, thereby providing deeper insights into their origins and broadening their potential applications across astrophysics.

Hypervelocity stars (HVSs) are stars with velocities exceeding their local escape velocities. Searching for HVSs and studying their origins can be an important way to study the properties of the Milky Way. In this paper, we utilize precise distances for RR Lyrae stars (RRLs) derived from the period-absolute magnitude–metallicity relation, along with proper motions from Gaia DR3, to conduct a large-volume search for HVSs. Our sample consists of a catalog with 8172 RRLs with metallicity, distance, and radial velocities estimated from the Sloan Digital Sky Survey and LAMOST spectroscopic data and an extended catalog of 135,873 RRLs with metallicity and distance estimated from Gaia photometry. After careful quality cuts, 165 hypervelocity RRL candidates were found. We performed further checks on their light curves and selected the most reliable 87 hypervelocity RRLs. All of them exceed the Milky Way’s escape velocity in the tangential component. Among them, seven stars have tangential velocity over 800 km s ^−1 . We identified two spatially distinct distributions of hypervelocity RRLs: one concentrated toward the Galactic center and another localized around the Magellanic Clouds, suggesting that their origins are likely associated with these regions through the Hills or other mechanisms. Furthermore, we detected a significant number of RRLs associated with dwarf galaxies that exceed the Milky Way’s escape velocity, likely ejected from their host systems. Future Gaia releases and spectroscopic follow-up observations will provide further insight into their ejection origin.

The proper motions (PMs) of M31 and M33 are key to understanding the Local Group's dynamical evolution. However, measurement discrepancies between Gaia blue and red samples, regarding whether the transverse velocity is remarkable, introduce significant ambiguity. In this work, we remeasure the systemic PMs of M31 and M33 using massive supergiant stars from Gaia Data Release 3. Clean disk tracers are selected via color-color diagrams, with foreground contaminants removed through kinematic and astrometric cuts. We identify the discrepancy in M31's blue and red samples as arising from systematic differences between Gaia's five-parameter (5p) and six-parameter (6p) astrometric solutions. The 6p solution, applied to sources lacking accurate color information, relies on a pseudo-color approximation, leading to lower precision and larger uncertainties. Two key limitations of the 6p solution are: (1) degraded astrometric accuracy for very red sources (G(BP) - G(RP) > 2.6); (2) significant PM zero-point offsets. In our sample, red sources are dominated by the 6p solution, while blue sources include a substantial fraction of 5p sources; this mismatch drives the observed discrepancy. By excluding extreme red sources and calibrating PM zero-points separately for 5p and 6p sources using background quasars, we reduce the discrepancy, bringing blue and red measurements into agreement within 1 sigma. We ultimately report the most robust Gaia-based PMs using high-quality 5p sources. For M31, we obtain (mu(& lowast;)(alpha), mu(delta))(M31) = (45.9 +/- 8.1, - 20.5 +/- 6.6)mu as yr(-1), consistent with, but more precise than, the HST result. For M33, we find that (mu(& lowast;)(alpha), mu(delta))(M33) = (45.3 +/- 9.7, 26.3 +/- 7.3) mu as yr(-1), which agrees with the VLBA measurement within 1.5 sigma. These results support a first infall scenario for M33.

We present systematic identifications of supergiants in M31/M33 based on the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopic survey. Radial velocities of nearly 5000 photometrically selected M31/M33 supergiant candidates have been properly derived from the qualified spectra released in LAMOST DR10. By comparing their radial velocities with those predicted from the rotation curve of M31, as well as utilizing Gaia astrometric measurements to exclude foreground contaminations, 199 supergiant members in M31, including 168 "Rank1" and 31 "Rank2," have been successfully identified. This sample contains 62 blue supergiants (BSGs, all "Rank1"), 134 yellow supergiants (YSGs, 103 "Rank1" and 31 "Rank2") and three red supergiants (RSGs, all "Rank1"). For M33, we identify 84 supergiant members (56 "Rank1" and 28 "Rank2"), which include 28 BSGs (all "Rank1"), 53 YSGs (25 "Rank1" and 28 "Rank2") and three RSGs (all "Rank1"). So far, this is one of the largest supergiant samples of M31/M33 with full optical wavelength coverage (3700 < lambda < 9100 & Aring;). This sample is valuable for understanding star formation and stellar evolution under different environments.

We present fundamental atmospheric parameters ( T eff and log g ) and metallicities ([M/H]) for 507,513 M dwarf stars using low-resolution spectra ( R ∼ 1800) from LAMOST DR10. By employing Cycle-StarNet, an innovative domain adaptation approach, we successfully bridge the gap between theoretical PHOENIX synthetic spectra and observed LAMOST spectra, enabling parameter measurements even for lower signal-to-noise (S/N) data (S/N > 5). The fitting residual analysis shows a reduction from 2.0 times to 1.68 times the flux uncertainty. Comparing with available literature values, we find systematic offsets and precisions of 12 ± 70 K in T eff , −0.04 ± 0.17 dex in log g , and −0.06 ± 0.20 dex in [M/H]. The precision improves for higher-quality spectra (S/N > 50) to 47 K, 0.12 dex, and 0.14 dex respectively. The metallicity consistency between wide binaries shows a scatter of 0.24 dex, improving to 0.15 dex at S/N > 50. We provide a comprehensive catalog including stellar parameters, spectral classifications, activity indicators, and binary/variability flags, establishing a resource for studies of the most numerous stellar population. The complete catalog can be accessed at doi:10.5281/zenodo.14030249.

Very metal-poor (VMP) stars provide a record of the chemical composition and dynamics of the early Galaxy. Based on the high-resolution and high signal-to-noise ratio spectra from the Subaru Telescope for 103 VMP stars, in this series of papers we homogeneously investigate the nonlocal thermodynamic equilibrium (NLTE) abundances of important astrophysical elements. This sample covers a wide metallicity range from [Fe/H] ∼ −1.7 dex down to −4.3 dex, including 13 objects with [Fe/H] ≤ −3.0 dex. Here, we present a set of homogeneous stellar atmospheric parameters, including the effective temperature, surface gravity, metallicity, and microturbulence velocity with the spectroscopic method, and the NLTE line formation for both Fe i and Fe ii in the classical one-dimensional model atmospheres have been considered. The NLTE effects of the Fe i lines range from ∼0.03 dex to ∼0.3 dex, and increase with decreasing metallicity. In addition, they depend on the surface gravity, which generally increases with decreasing log g . The largest NLTE effects can be found for the giants of log g ∼ 2.5 dex. For dwarfs and subgiants, our final effective temperatures are consistent with those derived from the T IRFM scales of A. Alonso et al. with a mean difference of 7.1 ± 100.2 K, while for giants, our results are slightly lower than those from the T IRFM scales of A. Alonso et al. with a mean difference of −69.5 ± 94.1 K. For dwarfs and subgiants, the spectroscopically derived surface gravity is consistent with that estimated based on the Gaia DR3 parallax; however, for giants, the former leads to a 0.2 dex lower surface gravity.

We present a catalog of 8440 candidate very metal-poor (VMP; [Fe/H] ≤ −2.0) main-sequence turn-off (MSTO) and red giant stars in the Milky Way, identified from low-resolution spectra in LAMOST DR10. More than 7000 of these candidates are brighter than G ∼ 16, making them excellent targets for high-resolution spectroscopic follow-up with 4–10 m class telescopes. Unlike most previous studies, we employed an empirical calibration to estimate metallicities from the equivalent widths of the calcium triplet lines, taking advantage of the high signal-to-noise ratio in the red arm of LAMOST spectra. We further refined this calibration to improve its reliability for more distant stars. This method enables robust identification of VMP candidates with metallicities as low as [Fe/H] = −4.0 among both MSTO and red giant stars. Comparisons with metal-poor samples from other spectroscopic surveys and high-resolution follow-up observations confirm the accuracy of our estimates, showing a typical median offset of ∼0.1 dex and a standard deviation of ∼0.2 dex.