Jingkun Zhao

White dwarfs (WDs) represent the final evolutionary stage of low- to intermediate-mass stars. They are among the most abundant compact objects in the Milky Way. Large-scale WD surveys are therefore essential for probing stellar evolution and galactic structure. In recent years, large-scale digital sky surveys, such as the Sloan Digital Sky Survey and the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys, have amassed vast archives of photometric images. These datasets provide a rich resource for WD searches. We employ the WD Network to detect and localize WDs directly within 640 × 640 pixels photometric images. The framework enables fully automated identification of WD candidates from photometric images. On the test set, this model achieves a precision of 96.6%, a recall of 94.5%, and an F1 score of 95.5%. Images in the actual search are more diverse, and their distribution may differ from the test set, and these metrics may not directly reflect the model’s performance in the actual search. Using 125,872 photometric images from DESI Legacy Surveys Data Release 9, we detected 18,242 WD candidates. Cross-matching these candidates with DESI Data Release 1 spectra yielded 719 candidate spectra. We then performed manual visual inspections and confirmed 75 new WDs and 7 new CVs, which highlights substantial contamination among the remaining candidates with available spectra. We publicly released this WD candidates catalog online. These results expand the current census of known WDs and provide both a valuable data resource and a practical technical reference for future large-scale astronomical image processing.

Observations reveal a pronounced deficit of white dwarfs (WDs) in open clusters (OCs) relative to theoretical expectations, suggesting that a significant fraction of WDs may have escaped from their parent clusters after formation. In this work, we perform a systematic search for escaped WD candidates from OCs by back tracing the motions of WDs and star clusters from Gaia Data Release 3 catalogs. We identify 476 candidate WDs with kinematics consistent with having escaped from one of 175 OCs. A control-field Monte Carlo test yields a contamination rate of 87.6% ± 4.3%. Given this high contamination rate, we filter the candidates by ensuring each WD’s total age is consistent with its host cluster age, thereby establishing a more reliable follow-up sample of 109 stars. The excluded candidates with anomalous ages are more likely field interlopers or products of accelerated binary evolution. Among these, the low-mass regime exhibits a clear excess over expected field binary merger rates, whereas the high-mass regime remains broadly consistent with binary population synthesis predictions. Finally, comparing escaped WDs with cluster properties, absolute escape counts appear limited by Gaia’s distance-dependent incompleteness. The normalized escape fraction shows little dependence on cluster age, possibly favoring WD loss near formation over gradual evaporation, but is strongly anticorrelated with cluster mass, plausibly because deeper potential wells of massive clusters retain more of their WDs.

We report the discovery of long tidal tails associated with the globular cluster (GC) NGC 5024. A modified matched filter applied to Gaia DR3 data reveals a broad stellar stream spanning α ≈ 230°–175°. The stellar stream overlaps on the sky with the LMS-1 and with the simulated stream of the GC NGC 5053, and all three share similar proper motions and metallicity. Our member candidates may therefore be a mixture of stars from NGC 5024, NGC 5053, and LMS-1. Nevertheless, the trailing tail extends roughly 20° beyond the known LMS-1 stream. Furthermore, the radial velocity (RV) as a function of α is used to distinguish the genuine stream candidates of NGC 5024 from the streams of NGC 5053 and LMS-1. Among the sources in common with DESI DR1, we found two distinct sequences in the RV– α plane, corresponding to the stellar streams of NGC 5024 and NGC 5053 (or LMS-1), respectively. This constitutes the first strong evidence for the existence of extensive tidal tails around NGC 5024.

To identify globular clusters (GCs) belonging to Sagittarius, we conducted verification from multiple aspects and carried out a detailed analysis of member GCs and the associated objectives. From the cluster catalog of E. L. Hunt & S. Reffert, we obtain the candidates of Sagittarius through the geometric space with proper motion ( μ _α , μ _δ ). We conduct further verification to make sure they are genuine members of Sagittarius using parameters such as proper motion, V _gsr , and direction of motion. After three steps of verification, we confirm that 10 GCs belong to Sagittarius. Among them, nine have been reported in previous studies, and our verification further confirms their membership in the Sagittarius system. The remaining one, HSC 61, is a newly identified member. Combining these with 21 credible Sagittarius-associated GCs from D. Minniti et al., our final sample includes 31 Sagittarius-associated GCs. Twenty-six are located in the core, and five are located in the stellar stream. We constructed the metallicity distribution in all of Sagittarius; there is one rich peak in the core, and there is one poor peak in the stream. The reason may be that the outer and poorer GCs of the core were first pulled into the stream. Additionally, we analyze the Sagittarius merger event through energy and angular momentum. Elqui is the most distant stream discovered in the Dark Energy Survey at a distance of 50 kpc, and Malhan thought Elqui belongs to the Sagittarius merger. We present new evidence that Sagittarius and Elqui belong to the same accretion event. Through verification, we have added two new star clusters to this large accretion event of Sagittarius. We found that the Large Magellanic Cloud has a certain influence on the orbits of Elqui and Sagittarius.

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 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.

This study explores the density profile of the stellar disk, radially and azimuthally, based on approximately 8.4 million red clump stars selected from Gaia Bp/Rp spectra. After correcting for selection effects and distance uncertainties, we fit the vertical stellar density profile of the Galactic disk with a two-component model consisting of geometrically thin and thick disks. Our derived density profile shows several breaks radially: (1) a steep exponent inside R ∼ 3 kpc; (2) a nearly flat plateau from R ∼ 3 to ∼7 kpc; (3) an exponential decline beyond the solar radius to around 13 kpc; (4) a sharper exponential drop-off beyond R ∼ 13 kpc. The parameters of these four main components depend on ϕ to some extent. The variation of the termination radius of the first component suggests an interaction with the bar/bulge. Besides the typical flaring at R > 6.4 kpc, we find that the thin disk also exhibits a similar and smooth thickening/flaring feature toward the Galactic center at R < 6.4 kpc. The observed inner flaring may indicate heating effects introduced by the Galactic bar, since R = 6.4 kpc lies close to the corotation radius where the bar’s dynamical influence becomes significant. Additionally, we identify a localized density bump in the region 5 < R < 7 kpc and −30° < ϕ < 15°, where a corresponding metallicity bump is also visible near the Galactic plane. This density/metallicity bump may be related to the recently reported bimodal distribution of the guiding radius of super metal-rich stars in the solar vicinity through radial migration.

White dwarfs (WDs), the evolutionary endpoints of most stars, can form through both single-star and binary channels. While single-star evolutionary models enable reliable WD age estimates, binary evolution introduces interactions that can accelerate WD formation and result in a variety of exotic WDs, which may exhibit strong magnetic fields, rapid rotation, or even serve as potential gravitational wave sources. Such systems offer valuable insights into magnetic field generation, angular momentum evolution, and compact object physics. Star clusters, with their approximately coeval populations, allow precise age determination of member WDs. If a WD’s total age derived from single-star evolution exceeds that of its host cluster, it likely indicates a binary origin. In this study, we use Gaia 5D astrometry to identify 439 WD candidates in 117 open clusters, with 244 likely formed via binary evolution. We discuss the possibility of dynamical ejection for WDs meeting only 2D (proper motion space) membership criteria. Spectroscopic observations further reveal a subset with strong magnetic fields and rapid rotation, supporting their binary evolutionary origin.

We present a catalog of 486 Ba stars identified in the GALAH DR4 survey using high-resolution spectra and precise abundance measurements. The sample was selected based on s-process enrichment criteria involving the abundances of Ba and La relative to Eu, and further refined using the signed-distance method, resulting in the largest sample of Ba stars to date, including five newly identified Ba dwarfs. Using astrometric and kinematic parameters from Gaia DR3 and StarHorse, we derived the Galactic velocity components for 367 Ba stars and found that most belong to the thin or thick disk, while 18 exhibit halo-like kinematics. Chemical abundance analysis suggests that most Ba stars are of in situ origin, whereas two stars (4077588766331013248 and 6692980582560946304) display signatures of accreted populations. The E-Lz diagram further shows that star 4077588766331013248 lies within the region of substructure ED-8. The observed decline of [Ba/Fe] and [La/Fe] with increasing metallicity implies that s-process elements originate from nucleosynthetic sites distinct from those of iron-peak elements, while the decreasing [hs/ls] ratio toward higher metallicity indicates higher neutron capture efficiency at lower metallicity. Both giant and dwarf Ba stars-except for 6053735173729807872-are confirmed binaries hosting white dwarf companions. Moreover, we estimated the masses of the former asymptotic giant branch (AGB) companions to these Ba stars based on the good agreement between the FRUITY AGB model predictions and their observed abundance patterns.

We present a catalog of 486 Ba stars identified in the GALAH DR4 survey using high-resolution spectra and precise abundance measurements. The sample was selected based on s- process enrichment criteria involving the abundances of Ba and La relative to Eu, and further refined using the signed-distance method, resulting in the largest sample of Ba stars to date, including five newly identified Ba dwarfs. Using astrometric and kinematic parameters from Gaia DR3 and StarHorse , we derived the Galactic velocity components for 367 Ba stars and found that most belong to the thin or thick disk, while 18 exhibit halo-like kinematics. Chemical abundance analysis suggests that most Ba stars are of in situ origin, whereas two stars (4077588766331013248 and 6692980582560946304) display signatures of accreted populations. The E – L _z diagram further shows that star 4077588766331013248 lies within the region of substructure ED-8. The observed decline of [Ba/Fe] and [La/Fe] with increasing metallicity implies that s- process elements originate from nucleosynthetic sites distinct from those of iron-peak elements, while the decreasing [hs/ls] ratio toward higher metallicity indicates higher neutron-capture efficiency at lower metallicity. Both giant and dwarf Ba stars—except for 6053735173729807872—are confirmed binaries hosting white dwarf companions. Moreover, we estimated the masses of the former asymptotic giant branch (AGB) companions to these Ba stars based on the good agreement between the FRUITY AGB model predictions and their observed abundance patterns.