Hong-Liang Yan

Supermassive neutron stars ( M ≳ 2   M ⊙ ) provide stringent constraints on the equation of state of dense matter. We report the discovery of a wide binary system composed of a red giant star and an invisible companion with an orbital period P ≈ 1218 ± 3   days and a comparatively low eccentricity e ≈ 0.09 ± 0.01 (with 1σ uncertainty). By combining radial velocity measurements from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope and the Apache Point Observatory Galactic Evolution Experiment (APOGEE) with astrometric data from Gaia DR2 and DR3 catalogs, we derive a Keplerian orbit with an inclination angle i ≈ 73 ∘ ± 11 ∘ . The unseen companion has a mass in the range of 1.8   to   2.8   M ⊙ (spanning the full spread of all estimates), with a best-fit value of 2.20 − 0.13 + 0.14   M ⊙ . This potentially places the unseen companion within the mass gap between neutron stars and black holes. Spectral disentangling using high-resolution near-infrared APOGEE spectra shows no absorption features from an additional component, suggesting the presence of a compact object: either a supermassive neutron star or a low-mass black hole. The wide, low-eccentricity orbit is difficult to explain through standard binary evolution channels, pointing to the need for improved theoretical models to account for such systems.

The post-asymptotic giant branch (post-AGB) star IRAS 14325-6428 exhibits a peculiar heavy-element abundance pattern, characterized by a high [Ba/La] ratio and an anomalous odd-isotope Ba ratio ( f _odd,Ba = 0.25 ± 0.08), which is difficult to explain via the classical slow neutron-capture process ( s -process). We theoretically investigate this abundance pattern by considering the intermediate neutron-capture process ( i -process) nucleosynthesis. The i -process nucleosynthesis calculations are performed with a one-zone network (NucNet Tools) at constant temperature and density, using intershell abundances obtained from a stellar model (1.5 M _⊙ , Z = 0.003) evolved with MESA to the thermally pulsing AGB phase. The observed heavy-element abundances are best reproduced by a model with neutron density N _n = 5 × 10 ¹³ cm ⁻³ and neutron exposure τ = 1.4 mbarn ⁻¹ , followed by dilution with a scaled-solar composition. This model reproduces the high [Ba/La] and [Ba/Ce] ratios and yields a postdilution odd-isotope fraction f _odd,Ba ≈ 0.51, which is supported by the observed value being substantially above the expectations of the pure s -process. The abundance pattern arises from the i -process path approaching the neutron magic number N = 82, where the flow slows, and the material accumulates at ¹³⁵ I, which subsequently β ⁻ decays to ¹³⁵ Ba, enhancing Ba production and reducing the synthesis of heavier species. Combined with its multielement i -process classification, our results provide quantitative support for a dominant i -process origin of the heavy elements in IRAS 14325−6428, likely triggered by a relatively short-lived proton-ingestion episode in its AGB progenitor.

We report [O iii ] narrowband imaging of an extremely large-scale nebula, the Silk Nebula, discovered by the Super Wide-Field [O iii] Narrow-band Survey (SWONS), together with multifiber medium-resolution spectroscopic data covering the H α , [N ii ], [S ii ], and [O iii ] emission lines, acquired with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) as part of the Medium-Resolution Spectroscopic Survey of Nebulae (MRS-N). Based on 2684 high-quality spectra, we measured its physical parameters, including relative line intensities, radial velocities, and full widths at half-maximum. The [S ii ]/H α ratio of the nebula significantly exceeds the shock-excitation threshold of 0.4. Its location in BPT diagnostic diagrams and spatial morphology of [O iii ] both support the optical emission is dominated by shock-heated gas. The oxygen abundance is near solar and spatially uniform, consistent with a shock propagating into the ambient interstellar medium. Because [O iii ] is the only emission line that clearly traces the nebular morphology, we base our distance estimate exclusively on the [O iii ] kinematics, obtaining a fiducial distance of ∼160 pc. At this distance, the nebula has a physical extent of ∼28 pc along the north–south direction. We suggest that the Silk Nebula is a large [O iii ] filament possibly associated with a supernova remnant, although a diffuse-ionized-gas (DIG) or ionized-bubble origin cannot be ruled out with the present data, and one of the largest angular-scale [O iii ] emission structures documented to date. Future deep X-ray observations and shock-modeling efforts are required to firmly establish its nature.

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.

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.

Unlocking all the physical information encoded in low-resolution spectra poses a significant challenge for astronomical survey analysis. Such a task demands modeling spectra and optimizing astrophysical parameters in high-dimensional space as a consequence of line blending. Here we present PhDLspec —a deep learning framework embedded with physical priors for stellar spectrum modeling and analysis. By imposing differential spectra derived from ab initio stellar atmospheric model calculation on a Transformer framework, PhDLspec can rigorously and precisely model stellar spectra by simultaneously taking into account more than 30 physical parameters at a computational speed hundreds of times faster than ab initio model calculation. With such a flexible stellar modeling approach, PhDLspec can effectively derive ∼30 stellar labels from a low-resolution spectrum using affordable optimization techniques. Application to LAMOST spectra ( R ≲ 1800) yields stellar elemental abundances in good agreement with high-resolution spectroscopic surveys, after essential calibrations to correct systematic biases in elemental abundance estimates using wide binaries and reference high-resolution data sets. We provide a catalog of 25 elemental abundances for 116,611 subgiant stars with precise age estimates. The successful application of PhDLspec to LAMOST spectra for high-dimensional parameter determination sheds light on similar challenges faced by other surveys and disciplines.

Spectroscopic surveys have identified significant numbers of metal-poor nitrogen-rich (N-rich) field stars. These stars are strong candidates for escapees from globular clusters (GCs), as their distinctive nitrogen enhancement mirrors the chemical patterns observed in some of the members of GCs. As part of the effort to characterize their chemodynamical properties, we derived abundances for up to 25 elements in a sample of 33 N-rich field giant stars (18 of them are studied for the first time) using high-resolution optical spectroscopy. We confirm their elevated abundances of N, Na, and Al, strongly supporting a GC origin. Given that Galactic GCs themselves formed within diverse progenitor galaxies, we sought to identify the ancestral systems of these N-rich field stars. By analyzing their dynamical parameters, we separated the sample into high-energy (HE) and low-energy (LE) groups. The HE group exhibits lower [ α /Fe] and enhanced r -process abundances compared to the LE group. This indicates that the HE stars likely escaped from GCs accreted from massive dwarf galaxies (e.g., Gaia-Sausage-Enceladus), while the LE stars probably originated from in situ GCs. We also find that the chemical pattern of these N-rich stars with [Fe/H] ⪅ −1.0 is similar to that of the high-redshift “N-emitters.” Furthermore, orbital integrations revealed a close encounter between one N-rich field star and the GC NGC 6235. Our work demonstrates the potential of using chemodynamical analyses to trace Galactic assembly through chemical peculiar stars, while highlighting that larger samples and more precise data in the future are crucial to establish definitive origins.

Li-rich giants serve as valuable tracers of stellar evolution and surface enrichment processes, for which a statistically large and homogeneous sample is crucial. Using the massive low-resolution ( R ∼ 1800) spectroscopic dataset from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey, we systematically search for Li-rich giants by determining their stellar lithium abundances through template matching of the Li i 6708 Å absorption line with a grid of synthetic spectra generated from ATLAS9 model atmospheres. The derived abundances are validated against previous high-resolution studies, showing good consistency with a mean absolute error of about 0.15 dex. Therefore, we adopt a threshold of A (Li) > 1.65 dex to select Li-rich candidates, followed by visual inspection to ensure the reliability of each detection. Eventually, more than 20,000 Li-rich giants are identified, corresponding to approximately 2.5% of all giants in LAMOST DR9. We also investigate the occurrence rate of Li-rich giants in different evolutionary stages. This work presents a large and homogeneous catalog of Li-rich giants derived from the LAMOST low-resolution survey, which provides a reliable and valuable dataset for future studies of stellar evolution and lithium enrichment in evolved 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.

Metal-poor stars (MP, [Fe/H]<−1.0) retain the chemical signatures of the early Universe, making their α-element invaluable for tracing the Galactic chemical evolution. Traditional optical spectroscopic methods and machine learning approaches often struggle at low metallicities and neglect the diagnostic power of ultraviolet (UV) spectral features (e.g., OH bands, Mg II/Mg I lines) that are accessible to the forthcoming Chinese Space Station Telescope (CSST). Using over 1.8 × 10 ⁵ simulated CSST spectra, we developed the spectral transformer (SPT)-GloCal model to quantify the impact of UV (2550−4000 Å) low-resolution ( R ≈ 200) spectra on the precision of [O/Fe] and [Mg/Fe] estimates in MP stars. The model improves local attention through score-aware competitive filtering and integrates it with global attention via a learnable gating mechanism. We compared models trained on full spectra versus optical-only spectra. Incorporating UV spectra halved the mean absolute error (MAE) of [O/Fe] predictions from 0.0785 to 0.0367 dex and reduced the scatter ( σ ) from 0.135 to 0.063 dex. For [Mg/Fe], MAE decreased from 0.0010 to 0.0006 dex and σ from 0.054 to 0.0068 dex. Error analyses demonstrated that UV data lead to more stable and accurate estimates, especially at a high log g and low Teff. SPT-GloCal outperforms both its predecessor (SPT) and tree-based regressors, confirming the effectiveness of its global-local attention design. Furthermore, tests on spectra with simulated noise show that the model maintains consistent performance across a range of signal-to-noise ratios (S/N =30−50), with marginal gains over the SPT model at the lower S/N end. UV spectral features, even at low resolution ( R ≈ 200), enhance α-element abundance determinations. The SPT-GloCal framework provides a scalable solution for upcoming space-based surveys. Future work will apply this model to real CSST data, thus extending it to other elements.