arXiv subjects

Xianhao Ye

Publications and source records attributed to Xianhao Ye.

15 recordsLinked to original sources

arXiv2026

Long Tidal Tails of NGC 5024 Hidden in LMS-1 and NGC 5053 Tidal Streams

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 $α\approx 230^{\circ}-175^{\circ}$. 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 $20^{\circ}$ 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 (Dark Energy Spectroscopic Instrument) DR1, we found two distinct sequences in 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.

astro-ph.GA↗

arXiv2026

Galactic Archaeology with the Subaru `Ōnohi`ula Prime Focus Spectrograph Strategic Program

The recently commissioned Subaru `Ōnohi`ula Prime Focus Spectrograph (PFS) will obtain spectra from nearly 2,400 fibers that cover 1.24 square degrees. The 360 night Subaru Strategic Program for PFS is dedicating approximately one-third of its allocation (130 nights) to study the structure and evolution of galaxies in the Local Group. This Galactic Archaeological survey has three pillars. (1) We will determine whether the mass density profiles of dwarf galaxies are consistent with cusps, as expected for cold dark matter, or cores, as expected from alternative dark matter theories or baryonic feedback. We will deduce the density profiles as a function of radius from modeling of the full line-of-sight velocity and abundance distributions for six dwarf galaxies. Our total sample will consist of 18,000 member stars to beyond the nominal tidal radius of each system. (2) From measurements of the [alpha/Fe] abundance ratio, we will learn the difference in assembly history of the two most massive galaxies in the Local Group: M31 and the Milky Way. We will observe 30,000 member stars over 45 square degrees of M31's halo and outer disk. (3) We will uncover how the most fragile (outer) part of the Milky Way responded to accretion events both in the distant past (such as Gaia-Sausage Enceladus) and in more recent history (such as the Sagittarius dwarf spheroidal galaxy). To support this study, PFS will provide velocities and metallicities--from which, in combination with photometry, we will deduce ages--for tens of thousands of main-sequence stars out to a Galactocentric distance of ~30 kpc.

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arXiv2026

Mapping the Milky Way with Gaia Bp/Rp spectra-IV: the broken and asymmetric density profile of the stellar disk traced by a large sample of red clumps

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 exponential inside R$\sim3$ kpc; (2) a nearly flat plateau from R$\sim3$ to $\sim7$ kpc; (3) an exponential decline beyond the solar radius to around 13 kpc; (4) a sharper exponential drop-off beyond R$\sim$13 kpc. The parameters of these four main components depend on $ϕ$ to some extent. 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 co-rotation radius where the bar's dynamical influence becomes significant. Additionally, we identify a localized density bump in the region $5<R<7$ kpc and $-30^\circ<ϕ<15^\circ$, 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.

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arXiv2026

A Catalog of 486 Barium Stars Identified in GALAH DR4

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.

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arXiv2025

Mapping the Milky Way with Gaia Bp/Rp spectra II: The inner stellar halo traced by a large sample of blue horizontal branch stars

We selected BHB stars based on synthetic photometry and stellar atmosphere parameters inferred from Gaia Bp/Rp spectra. We generated the synthetic SDSS broad-band $ugr$ and Pristine narrow-band CaHK magnitudes from Gaia Bp/Rp data. A photometric selection of BHB candidates was made in the $(u-g, g-r)$ and $(u-\mathrm{CaHK},g-r)$ color-color spaces. A spectroscopic selection in $T_\mathrm{eff}-\log g$ space was applied to remove stars with high surface gravity. The selection function of BHB stars was obtained by using the Gaia DR3 photometry. A non-parametric method that allows the variation in the vertical flattening $q$ with the Galactic radius, was adopted to explore the density shape of the stellar halo. We present a catalog of 44,552 high latitude ($|b|>20^\circ$) BHB candidates chosen with a well-characterized selection function. The stellar halo traced by these BHB stars is more flattened at smaller radii ($q=0.4$ at $r\sim8$ kpc), and becomes nearly spherical at larger radii ($q=0.8$ at $r\sim25$ kpc). Assuming a variable flattening and excluding several obvious outliers that might be related to the halo substructures or contaminants, we obtain a smooth and consistent relationship between $r$ and $q$, and the density profile is best fit with by a single power law with an index $α=-4.65\pm0.04$.

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arXiv2025

The Pristine survey: XXV. The very metal-poor Galaxy: Chemodynamics through the follow-up of the Pristine-Gaia synthetic catalogue

The Pristine-\textit{Gaia} synthetic catalogue provides reliable photometric metallicities for $\sim$30 million FGK stars using the Pristine survey model and Gaia XP spectra. We perform the first low-to-medium-resolution spectroscopic follow-up of bright (G<15) and distant (up to 35 kpc) very and extremely metal-poor (V/EMP, [Fe/H]<-2.5) red giant branch stars from this. We use Isaac Newton Telescope/Intermediate Dispersion Spectrograph (INT/IDS) observations centred around the calcium triplet region ideal for V/EMP stars. We find that 76\% of our stars indeed have [Fe/H]<-2.5 with these inferred spectroscopic metallicities and only 3\% are outliers with [Fe/H] > -2.0. We report a success rate of 77\% and 38\% in finding stars with [Fe/H]<-2.5 and -3.0 respectively. This will allow for 10,000-20,000 homogeneously analysed EMP stars using the WEAVE survey follow-up of Pristine EMP candidates. We associate 20\%, 46\%, and 34\% of the stars to be confined to the disc plane, or to have inner and outer halo orbits, respectively. We also associate these V/EMP stars to known accretion events such as Gaia-Enceladus-Sausage (GES), LMS-1/Wukong, Thamnos, Helmi streams, Sagittarius, Sequoia, etc. For the stars that orbit close to the disc plane, we find that the prograde region with low vertical action is overdense with a significance of 4$σ$ as compared to its retrograde counterpart. We also find three new (brightest) members of the most metal-poor stellar stream, C-19, one of which is 50$^\circ$ away from the main body of the stream. Our measured mean metallicity, velocity dispersion, and stream width are consistent with the literature, but our results favour a higher distance ($\sim$21.5 kpc) for the stream. We publish a catalogue (and 1D spectra) of 215 V/EMP stars from this spectroscopic follow-up and showcase the power of chemokinematic analysis of V/EMP end.

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arXiv2025

Mapping the Milky Way with Gaia Bp/Rp spectra I: Systematic flux corrections and atmospheric parameters for 68 million stars

Gaia Bp/Rp spectra for over two hundred million stars have great potential for mapping metallicity across the Milky Way. We aim to construct an alternative catalog of atmospheric parameters from Gaia Bp/Rp spectra by fitting them with synthetic spectra based on model atmospheres, and provide corrections to the Bp/Rp fluxes according to stellar colors, magnitudes, and extinction. We use GaiaXPy to obtain calibrated spectra and apply FERRE to match the corrected Bp/Rp spectra with models and infer atmospheric parameters. We train a neural network using stars in APOGEE to predict flux corrections as a function of wavelength for each target. Based on the comparison with APOGEE parameters, we conclude that our estimated parameters have systematic errors and uncertainties in $T_{\mathrm{eff}}$, $\log g$, and [M/H] about $-38 \pm 167$ K, $0.05 \pm 0.40$ dex, and $-0.12 \pm 0.19$ dex, respectively, for stars in the range $4000 \le T_{\mathrm{eff}} \le 7000$ K. The corrected Bp/Rp spectra show better agreement with both models and Hubble Space Telescope CALSPEC data. Our correction increases the precision of the relative spectrophotometry of the Bp/Rp data from $3.2\% - 3.7\%$ to $1.2\% - 2.4\%$. Finally, we have built a catalog of atmospheric parameters for stars within $4000 \le T_{\mathrm{eff}} \le 7000$ K, comprising $68,394,431$ sources, along with a subset of $124,188$ stars with $\mathrm{[M/H]} \le -2.5$. Our results confirm that the Gaia Bp/Rp flux calibrated spectra show systematic patterns as a function of wavelength that are tightly related to colors, magnitudes, and extinction. Our optimization algorithm can give us accurate atmospheric parameters of stars with a clear and direct link to models of stellar atmospheres, and can be used to efficiently search for extremely metal-poor stars.

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arXiv2023

Detection of multiple phase space overdensities of GSE stars by orbit integration

In N-body simulations, nearly radial mergers can form shell-like overdensities in the sky position and phase space ($r-v_r$) due to the combination of dynamical friction and tidal stripping. The merger event of Gaia-Sausage-Enceladus has provided a unique opportunity to study the shells in the phase space. To search for them, we integrate the orbits of 5949 GSE-related halo K giants from the LAMOST survey and record their positions at all time intervals in $r-v_r$ diagram. After the subtraction of a smoothed background, we find six significant and complete thin chevron-like overdensities. The apocenters $r_\mathrm{apo}$ of stars in the six chevrons are around 6.75, 12.75, 18.75, 25.25, 27.25, and 30.25 kpc. These chevrons reveal the multiple pile-ups of GSE stars at different apocenters. The application of a different Milky Way mass $M_\mathrm{vir}$ will change the opening angles of these chevrons, while leave their apocenters almost unchanged. By comparing with a recent study of the phase space overdensities of local halo stars from Gaia RVS survey, our results are more inclined to a medium $M_\mathrm{vir}$ of $10^{12}\,M_\odot$. The application of a non-axisymmetric Galactic potential with a steadily rotating bar has a blurring effect on the appearance of these chevron-like overdensities, especially for the chevrons with $r_\mathrm{apo} > 20$ kpc.

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arXiv2022

Revisit of open clusters UPK 39, UPK 41 and PHOC 39 : a new binary open cluster found

We investigate the three open clusters near Aquila Rift cloud, named as UPK 39 (\texttt{c1} hereafter), UPK 41 (\texttt{c2} hereafter) in Sim et al. (2019) and PHOC 39 (\texttt{c3} hereafter) in Hunt \& Reffert (2021), respectively. Using photometric passpands, reddening, and extinction from Gaia DR3, we construct the color-absolute-magnitude diagram (CAMD). Using isochrone fits their ages are estimated as $6.3\pm0.9$, $8.1\pm1.4$ and $21.8\pm2.2$ Myr, respectively. Their proper motions and radial velocities, estimated using data from Gaia and LAMOST are very similar. From their orbits, relative distances among them at different times, kinematics, ages, and metallicities, we conclude that \texttt{c1} and \texttt{c2} are primordial binary open cluster, which are likely to have been formed at the same time, and \texttt{c3} may capture \texttt{c1}, \texttt{c2} in the future.

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arXiv2021

Extended Tidal Tails of IC 4756 detected by {\it Gaia} EDR3

We report the discovery of emerged tidal tails around open cluster IC 4756 ($\sim$ 1 Gyr) based on 644 members identified from {\it Gaia} EDR3. Three-dimensional spatial positions, two-dimensional tangential velocities $\left( x, y, z, κ\cdot μ_α^{*}/\varpi, κ\cdot μ_δ/\varpi \right)$ are utilized to determine the co-moving member candidates of IC 4756. Using a Bayesian method, we correct the distance for each cluster member. Two tidal tails extend up to 180 pc and display a S-shape in $X^{\prime}Y^{\prime}$ space (Cartesian coordinates focused on cluster center). A clean sequence of our members in Color-Absolute-Magnitude Diagram (CAMD) indicates the coeval population and matches perfectly with the PARSEC isochrone with age from Bossini et al. (2019). Mass segregation is detected in this cluster as well. Finally, we derive the tidal radius and core radius of IC 4756 about $12.13$ pc and $4.33 \pm 0.75$ pc, respectively.

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arXiv2021

Tracing the Origin of Moving Groups. III. Detecting Moving Groups in The LAMOST DR7

We revisit the moving groups (MGs) in the solar neighborhood with a sample of 91969 nearby stars constructed from LAMOST DR7. Using the wavelet technique and Monte Carlo simulations, five MGs together with a new candidate located at $V \simeq$ -130 km s$^{-1}$ are detected simultaneously in $V-\sqrt{U^2+2V^2}$ space. Taking into account the other known MGs, we conclude that MGs in the Galactic disk are spaced by approximately 15 $\sim$ 25 km s$^{-1}$ along $V$ velocity. The origin of detected MGs is analysed through the distributions of [Fe/H]$-$[Mg/Fe] and ages. Our results support attributing the origin to the continuous resonant mechanisms probably induced by the bar or spiral arms of the Milky Way.

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arXiv2020

Nearly 30,000 late-type main-sequence stars with stellar age from LAMOST DR5

We construct a sample of nearly 30,000 main-sequence stars with 4500K $<T\rm_{eff}<$ 5000K and stellar ages estimated by the chromospheric activity$-$age relation. This sample is used to determine the age distribution in the $R-Z$ plane of the Galaxy, where $R$ is the projected Galactocentric distance in the disk midplane and $Z$ is the height above the disk midplane. As $|Z|$ increases, the percentage of old stars becomes larger. It is known that scale-height of Galactic disk increases as $R$ increases, which is called flare. A mild flare from $R$ $\sim$ 8.0 to 9.0 kpc in stellar age distribution is found. We also find that the velocity dispersion increases with age as confirmed by previous studies. Finally we present spiral-shaped structures in $Z-\upsilon_{Z}$ phase space in three stellar age bins. The spiral is clearly seen in the age bin of [0, 1] Gyr, which suggests that a vertical perturbation to the disk probably took place within the last $\sim$ 1.0 Gyr.

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arXiv2020

Estimating Atmospheric Parameters of DA White Dwarf Stars with Deep Learning

The determination of atmospheric parameters of white dwarf stars (WDs) is crucial for researches on them. Traditional methodology is to fit the model spectra to observed absorption lines and report the parameters with the lowest $χ^2$ error, which strongly relies on theoretical models that are not always publicly accessible. In this work, we construct a deep learning network to model-independently estimate Teff and log g of DA stars (DAs), corresponding to WDs with hydrogen dominated atmospheres. The network is directly trained and tested on the normalized flux pixels of full optical wavelength range of DAs spectroscopically confirmed in the Sloan Digital Sky Survey (SDSS). Experiments in test part yield that the root mean square error (RMSE) for Teff and log g approaches to 900 K and 0.1 dex, respectively. This technique is applicable for those DAs with Teff from 5000 K to 40000 K and log g from 7.0 dex to 9.0 dex. Furthermore, the applicability of this method is verified for the spectra with degraded resolution $\sim 200$. So it is also practical for the analysis of DAs that will be detected by the Chinese Space Station Telescope (CSST).

astro-ph.IM↗

arXiv2020

A new moving group in the Local Arm

We present a new moving group clustered in kinematics, spatial position and elemental abundances. Its spatial position is around the center of the Local Arm of the Milky Way. A convergent point method was taken to select candidate member stars.\textbf{ Among 206 candidate member stars, 74 are pre-main-sequence stars and some of them have stellar disks.} We presume those pre-main sequence stars belong to Orion nebula. We suggest this moving group is caused by density wave of the Local Arm passing by.

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arXiv2019

Stellar chromospheric activity and age relation from open clusters in the LAMOST Survey

We identify member stars of more than 90 open clusters in the LAMOST survey. With the method of Fang et al.(2018), the chromospheric activity (CA) indices logR'CaK for 1091 member stars in 82 open clusters and logR'Hα for 1118 member stars in 83 open clusters are calculated. The relations between the average logR'CaK, logR'Hα in each open cluster and its age are investigated in different Teff and [Fe/H] ranges. We find that CA starts to decrease slowly from logt = 6.70 to logt = 8.50, and then decreases rapidly until logt = 9.53. The trend becomes clearer for cooler stars. The quadratic functions between logR' and logt with 4000K < Teff < 5500K are constructed, which can be used to roughly estimate ages of field stars with accuracy about 40% for logR'CaK and 60% for logR'Hα.

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