arXiv subjects

Yaqian Wu

Publications and source records attributed to Yaqian Wu.

At least 19 recordsLinked to original sources

arXiv2026

The Stellar Abundances and Galactic Evolution Survey (SAGES). V. The First Data Release of the DDO51 Band

We present the first public data release of DDO51 band from the Stellar Abundances and Galactic Evolution Survey (SAGES), based on Nanshan One-meter Wide-field Telescope (NOWT) observations obtained between 2023 September and 2024 January. This release initiates the DDO51-band component of the survey, covering $\sim$ 2,500 deg$^2$ of the northern sky and including more than 10 million sources. The DDO51 filter is centered near the \ion{Mg}{1}~$b$ 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 $\sim$18.9 mag at S/N$\sim$10 and an internal photometric precision $\approx$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 upon publication will be available.

astro-ph.SR↗

arXiv2026

The Spectroscopic and Photometric Study of a Star Cluster Sample in Andromeda Halo

Halo star clusters serve as vital tracers for the formation and evolution of the Andromeda galaxy. In this work, we present physical parameters for 29 M31 halo star clusters, derived from a combination of spectroscopic and photometric data. Low-resolution spectra were acquired using the BFOSC spectrograph on the NAOC Xinglong 2.16-m telescope. For the photometric analysis, we utilized uSC and vSAGE bands from the SAGE survey, complemented by archival data from GALEX(NUV, FUV), PAN-STARRS(grizy) and the 2MASS(JHK). Ages and metallicities were determined via ULySS (Vazdekis et al. and pegase-hr) SSP model and the Bruzual & Charlot (2003) (BC03) stellar population synthesis models. The derived parameters show good agreement with literature values. Notably, for three of these clusters, this study represents the first combined photometric and spectroscopic analysis.

astro-ph.GA↗

arXiv2026

Using Lithium and Beryllium to Study Structure and Evolution of Rotating Stars: Spite Plateau of Halo Stars

The observed lithum (Li) abundance of Galactic halo stars mainly fall within the range of 2.0--2.4 dex. This nearly constant value, known as the Spite plateau, is approximately a factor of three lower than the value predicted from cosmic microwave background measurements and standard Big Bang Nucleosynthesis (BBN) calculations. This discrepancy -- referred to as the cosmological Li problem -- is considered a potential indication of new physics or astrophysical processes. We employed models incorporating gravitational settling, diffusion, rotation, and magnetic fields to explain the Spite plateau. The rotating models predict that Li abundances in stars with ages of roughly 8--13 Gyr and effective temperatures between 6400 and 5900 K generally fall within 2.0--2.4 dex, forming a well-defined Li plateau, followed by a sharp decline in Li abundance down to about 5200 K. The Li plateau results from the combined effects of variations in convection zone depth, gravitational settling, diffusion, rotation, and magnetic fields. For red giant branch stars with $T_{\mathrm{eff}} \lesssim$ 5200 K, the rotating models predict another Li plateau with an abundance of about 1.0 dex. These results are in good agreement with observations. Moreover, the initial Li abundance of 2.72 dex adopted in the models matches the BBN prediction, implying that the Li problem arises from stellar Li depletion. Furthermore, the rotating models also reproduce the Li and Be distributions of the sample that exhibit the Spite plateau meltdown and Be deviation.

astro-ph.SR↗

arXiv2025

The Stellar Abundances and Galactic Evolution Survey (SAGES). IV. Surface Gravity Estimation and Giant-Dwarf Separation with the DDO51 Filter

Reliable estimation of stellar surface gravity (log $g$) for a large sample is crucial for evaluating stellar evolution models and understanding galactic structure; However, it is not easy to accomplish due to the difficulty in gathering a large spectroscopic data set. Photometric sky survey using a specific filter, on the other hand, can play a substantial role in the assessment of log $g$. The Stellar Abundances and Galactic Evolution Survey (SAGES) utilizes eight filters to provide accurate stellar parameters for $\sim10^{7}$ stars, with its DDO51 intermediate-band filter specifically designed for robust log $g$ determination. In this work, the observed SAGES $u_{\rm SC}$ and $v_{\rm SAGES}$ photometry, the synthetic photometry in $g$, $r$, $i$, and DDO51 bands derived from \textit{Gaia} XP spectra are employed to investigate the importance of the DDO51 filter in the determination of log $g$. We applied machine-learning-based extinction correction and employed XGBoost models, trained on stellar parameters from LAMOST, to predict log $g$ using photometric data. By comparing model predicted log $g$ with LAMOST values, we find that including DDO51 filter improve the accuracies of log $g$ estimates by 21.0\% (from 0.224\,dex to 0.177\,dex) overall, and by 26.5\% (from 0.302\,dex to 0.222\,dex ) for GK-type stars, as compared to those obtained without DDO51. The DDO51 filter is also validated to be particularly effective for metal-poor stars ([Fe/H]$<$-1.0), where it significantly mitigates systematic biases. Our findings highlight the diagnostic power of the SAGES DDO51 filter, providing enhanced stellar characterization vital for future in-depth studies of the Milky Way.

astro-ph.SR↗

arXiv2025

Homogeneous Stellar Atmospheric Parameters and 22 Elemental Abundances for FGK Stars Derived From LAMOST Low-resolution Spectra with DD-Payne

A deep understanding of our Galaxy desires 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 LAMOST low-resolution spectra in a physics-sensible and rigorous manner with DD-Payne, taking labels from high-resolution spectroscopy as 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 (SNR) higher than 20. The [Fe/H] is valid down to $\sim$-4.0, while elemental abundance ratios [X/Fe] are mostly valid for stars with [Fe/H] $\gtrsim-2.0$. Measurement errors in these labels are sensitive to and almost inversely proportional with SNR. For stars with S/N>50, we achieved a typical error of 30 K in Teff, 0.07 dex in $\log g$, $\sim0.05$ 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 non-local thermal equilibrium effect is corrected for the [Fe/H] estimates, the Teff 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.

astro-ph.SR↗

arXiv2025

Potential Impact of the Sagittarius Dwarf Galaxy on the Formation of Young O-rich Stars

The Milky Way underwent significant transformations in its early history, characterised by violent mergers and satellite galaxy accretion. However, recent observations reveal notable star formation events over the past 4 Gyr, likely triggered by perturbations from the Sagittarius dwarf galaxy. Here, we present chemical signatures of this accretion event, using the [Fe/H] (metallicity) and [O/Fe] (oxygen abundance) ratios of thin-disc stars. In the normalised age-metallicity plane, we identify a discontinuous V-shape structure at z$_{\rm max}$ (maximum vertical distance from the disc plane) $<$ 0.4 kpc in the local disc, interrupted by a star formation burst between 4 and 2 Gyr ago. This event is characterised by a significant increase in oxygen abundance, resulting in a distinct [O/Fe] gradient and the formation of young O-rich stars. These stars have larger birth radii, indicating formation in the outer disc followed by radial migration to the Solar neighbourhood. Simulations of late satellite infall suggest that the passage of the Sagittarius dwarf galaxy may have contributed to the observed increase in oxygen abundance in the local disc.

astro-ph.GA↗

arXiv2025

Using Lithium and Beryllium to Study Structure and Evolution of Rotating Stars

The chemical composition of the Sun is still a highly controversial issue. No solar model has yet been able to simultaneously reproduce the solar lithium and beryllium abundances, along with helioseismic results, including the rotation profile. Lithium and beryllium are fragile elements that are highly sensitive to the physical conditions, as well as to transport and mixing processes within and below the convective zone (CZ). Uncovering the transport mechanisms responsible for the depletion of Li and Be in the Sun is crucial for using them as tools to understand stellar interiors and the associated transport and mixing processes. We constructed rotating solar models based on Magg's abundance scale, incorporating the effects of convective overshoot and magnetic fields. The rotating model exhibits superior sound speed and density profile and successfully reproduces the observed ratios $r_{02}$ and $r_{13}$. It also matches the seismically inferred CZ depth, surface helium abundance, and rotation profile, as well as the detected Li and Be abundances and neutrino fluxes within $1σ$. The depletion of Li is dominated by convective overshoot and rotational mixing, while Be depletion is primarily driven by gravitational settling and rotational mixing. The presence of the tachocline accelerates Li depletion but slows down Be depletion. These distinct depletion mechanisms result in the surface abundances of Li and Be evolving differently over time.

astro-ph.SR↗

arXiv2024

Determining Stellar Elemental Abundances from DESI Spectra with the Data-Driven Payne

Stellar abundances for a large number of stars are key information for the study of Galactic formation history. Large spectroscopic surveys such as DESI and LAMOST take median-to-low resolution ($R\lesssim5000$) spectra in the full optical wavelength range for millions of stars. However, line blending effect in these spectra causes great challenges for the elemental abundances determination. Here we employ the DD-PAYNE, a data-driven method regularised by differential spectra from stellar physical models, to the DESI EDR spectra for stellar abundance determination. Our implementation delivers 15 labels, including effective temperature $T_{\rm eff}$, surface gravity $\log g$, microturbulence velocity $v_{\rm mic}$, and abundances for 12 individual elements, namely C, N, O, Mg, Al, Si, Ca, Ti, Cr, Mn, Fe, Ni. Given a spectral signal-to-noise ratio of 100 per pixel, internal precision of the label estimates are about 20 K for $T_{\rm eff}$, 0.05 dex for $\log~g$, and 0.05 dex for most elemental abundances. These results are agree with theoretical limits from the Crámer-Rao bound calculation within a factor of two. The Gaia-Enceladus-Sausage that contributes the majority of the accreted halo stars are discernible from the disk and in-situ halo populations in the resultant [Mg/Fe]-[Fe/H] and [Al/Fe]-[Fe/H] abundance spaces. We also provide distance and orbital parameters for the sample stars, which spread a distance out to $\sim$100 kpc. The DESI sample has a significant higher fraction of distant (or metal-poor) stars than other existed spectroscopic surveys, making it a powerful data set to study the Galactic outskirts. The catalog is publicly available.

astro-ph.GA↗

arXiv2023

On the Migration Origin of the Hercules Moving Group with GAIA, LAMOST, APOGEE, and GALAH Surveys

Using Gaia DR3 data and the wavelet transformation technique, we study the substructures of the Hercules moving group (HMG): Hercules 1 (H1) and Hercules 2 (H2). Spectroscopic survey data from LAMOST, APOGEE, and GALAH are used to obtain metallicities and ages of stars belonging to the HMG. Our analysis leads to several key findings as follows: ($a$) the HMG is on average richer in metallicity than the Galactic disk, with H2 being metal richer than H1; ($b$) the HMG likely has a radial metallicity gradient distinct from that of the disk; ($c$) the HMG is on average older than the disk, with H2 being older than H1; ($d$) the HMG likely has a radial age gradient distinct from that of the disk; and ($e$) the metallicity and age distributions of the HMG depend mainly on the Galactic radius but show no dependence on the azimuthal velocity. Taken all together, we conclude that the HMG is composed primarily of stars undergoing radial migration. We suggest that the HMG is associated with a higher-order dynamical resonance of the bar of the Galaxy.

astro-ph.GA↗

arXiv2023

Age of FGK Dwarfs Observed with LAMOST and GALAH: Considering the Oxygen Enhancement

Varying oxygen abundance could impact the modeling-inferred ages. This work aims to estimate the ages of dwarfs considering observed oxygen abundance. To characterize 67,503 LAMOST and 4,006 GALAH FGK-type dwarf stars, we construct a grid of stellar models which take into account oxygen abundance as an independent model input. Compared with ages determined with commonly-used $α$-enhanced models, we find a difference of $\sim$9% on average when the observed oxygen abundance is considered. The age differences between the two types of models are correlated to [Fe/H] and [O/$α$], and they are relatively significant on stars with [Fe/H] $\lesssim$ -0.6 dex. Generally, varying 0.2 dex in [O/$α$] will alter the age estimates of metal-rich (-0.2 $<$ [Fe/H] $<$ 0.2) stars by $\sim$10%, and relatively metal-poor (-1 $<$ [Fe/H] $<$ -0.2) stars by $\sim$15%. Of the low-O stars with [Fe/H] $<$ 0.1 dex and [O/$α$] $\sim$ -0.2 dex, many have fractional age differences of $\geq$ 10%, and even reach up to 27%. The fractional age difference of high-O stars with [O/$α$] $\sim$ 0.4 dex reaches up to -33% to -42% at [Fe/H] $\lesssim$ -0.6 dex. We also analyze the chemical properties of these stars. We find a decreasing trend of [Fe/H] with age from 7.5-9 Gyr to 5-6.5 Gyr for the stars from the LAMOST and GALAH. The [O/Fe] of these stars increases with decreasing age from 7.5-9 Gyr to 3-4 Gyr, indicating that the younger population is more O-rich.

astro-ph.SR↗

arXiv2023

Characterising abundance-age relations of GALAH stars using oxygen-enhanced stellar models

Main Sequence Turn-off stars (MSTO) and subgiant stars are good tracers of galactic populations. We present a study of 41,034 MSTO and subgiant stars from the GALAH survey. Using a grid of stellar models that accounts for the variation of O abundances, we determine their ages with a median age uncertainty of $\sim$9.4 per cent. Our analysis reveals that the ages of high-O stars based on O-enhanced models (OEM models) are smaller than those determined with $α$-enhanced models, resulting in a mean fractional age difference of -5.3 per cent at [O/$α$] = 0.2 and -11.0 per cent at [O/$α$] = 0.4. This age difference significantly impacts the age distribution of thick disc and halo stars, leading to a steeper downward trend in the [Fe/H]-age plane from 8 Gyr to 14 Gyr, indicating a shorter formation time-scale and a faster chemical-enhanced history for these populations. We confirm the V-shape of the normalized age-metallicity distribution $p$($τ$$\mid$[Fe/H]) of thin disc stars, which is presumably a consequence of the second gas infall. Additionally, we find that the halo stars in our sample can be divided into two sequences, a metal-rich sequence (Splash stars) and a metal-poor sequence (accreted stars), with the Splash stars predominantly older than 9 Gyr and the accreted halo stars older than 10 Gyr. Finally, we observe two distinct sequences in the relations between various chemical abundances and age for disc stars, namely a young sequence with ages $<$ $\sim$8 Gyr and an old sequence with ages $>$ $\sim$8 Gyr.

astro-ph.GA↗

arXiv2022

Discovery of post-mass-transfer helium-burning red giants using asteroseismology

A star expands to become a red giant when it has fused all the hydrogen in its core into helium. If the star is in a binary system, its envelope can overflow onto its companion or be ejected into space, leaving a hot core and potentially forming a subdwarf-B star. However, most red giants that have partially transferred envelopes in this way remain cool on the surface and are almost indistinguishable from those that have not. Among $\sim$7000 helium-burning red giants observed by NASA's Kepler mission, we use asteroseismology to identify two classes of stars that must have undergone dramatic mass loss, presumably due to stripping in binary interactions. The first class comprises about 7 underluminous stars with smaller helium-burning cores than their single-star counterparts. Theoretical models show that these small cores imply the stars had much larger masses when ascending the red giant branch. The second class consists of 32 red giants with masses down to 0.5 M$_\odot$, whose implied ages would exceed the age of the universe had no mass loss occurred. The numbers are consistent with binary statistics, and our results open up new possibilities to study the evolution of post-mass-transfer binary systems.

astro-ph.SR↗

arXiv2022

Ages of Main-Sequence Turn-Off Stars from the GALAH Survey

Main sequence turn-off (MSTO) stars are good tracers of Galactic populations since their ages can be reliably estimated from atmospheric parameters. Based on the GALAH survey, we use the Yale Rotation Evolution Code to determine ages of 2926 MSTO stars with mean age uncertainty $\sim$10\% considering the variation of C and O abundances. Ages of CO-poor stars are systematically affected by $\sim$10\% due to the C and O abundances, globally shifting to $\sim$0.5 Gyr older compared to the results using solar metal-mixture. Of the stars with \mbox{[Fe/H] $\sim$0.3-0.5} or \mbox{[O/Fe]~$\leq$~-0.25}, many have fractional age differences~$\geq$~20\%, and even reach up to 36\%. The age-metallicity relation appears to possibly exist two distinct sequences: a young sequence of stars with age mostly $<$ 7 Gyr, and a relatively older sequence of stars with age mostly $>$ 7 Gyr, overlapping at 5 Gyr $\leq$~age~$\leq$ 7 Gyr. Moreover, the trends of abundances to age ratios show two corresponding sequences, especially in [O/Fe]-age plane. We also find that [Y/Mg] is a good chemical clock in disk populations. The young sequence and the old sequence can not be separated based on chemistry or kinematics, therefore stellar age is an important parameter to distinguish these two sequences in our sample.

astro-ph.SR↗

arXiv2021

High-α-Metal-Rich stars in the LAMOST-MRS survey and its connection with the galactic bulge

We report the detection of a large sample of high-$α$-metal-rich stars on the low giant branch with $2.6 0.2$ than the comparison group. These properties strongly indicate its connection with the outer bar/bulge region at $R=3-5$ kpc. A tentative interpretation of this special group is that its stars were formed in the X-shaped bar/bulge region, close to its corotation radius, where radial migration is the most intense, and brings them to present locations at 9 kpc and beyond. Low eccentricities and slightly outward radial excursions of its stars are consistent with this scenario. Its kinematics (cold) and chemistry ($[α/Fe]$ $\sim 0.1$) further support the formation of the instability-driven X-shaped bar/bulge from the thin disk.

astro-ph.GA↗

arXiv2021

Exploring the Galactic Anticenter substructure with LAMOST & Gaia DR2

We characterize the kinematic and chemical properties of 589 Galactic Anticenter Substructure Stars (GASS) with K-/M- giants in Integrals-of-Motion space. These stars likely include members of previously identified substructures such as Monoceros, A13, and the Triangulum-Andromeda cloud (TriAnd). We show that these stars are on nearly circular orbits on both sides of the Galactic plane. We can see velocity($V_{Z}$) gradient along Y-axis especially for the south GASS members. Our GASS members have similar energy and angular momentum distributions to thin disk stars. Their location in [$α$/M] vs. [M/H] space is more metal poor than typical thin disk stars, with [$α$/M] \textbf{lower} than the thick disk. We infer that our GASS members are part of the outer metal-poor disk stars, and the outer-disk extends to 30 kpc. Considering the distance range and $α$-abundance features, GASS could be formed after the thick disk was formed due to the molecular cloud density decreased in the outer disk where the SFR might be less efficient than the inner disk.

astro-ph.GA↗

arXiv2020

Age-metallicity dependent stellar kinematics of the Milky Way disc from LAMOST and Gaia

We investigate the stellar kinematics of the Galactic disc in 7 $<$ $R$ $<$ 13\,kpc using a sample of 118\,945 red giant branch (RGB) stars from LAMOST and Gaia. We characterize the median, dispersion and skewness of the distributions of the 3D stellar velocities, actions and orbital parameters across the age-metallicity and the disc $R$ -- $Z$ plane. Our results reveal abundant but clear stellar kinematic patterns and structures in the age -- metallicity and the disc $R$ -- $Z$ plane. The most prominent feature is the strong variations of the velocity, action, and orbital parameter distributions from the young, metal-rich thin disc to the old, metal-poor thick disc, a number of smaller-scale structures -- such as velocity streams, north-south asymmetries, and kinematic features of spiral arms -- are clearly revealed. Particularly, the skewness of $V_ϕ$ and $J_ϕ$ reveals a new substructure at $R\simeq12$\,kpc and $Z\simeq0$\,kpc, possibly related to dynamical effects of spiral arms in the outer disc. We further study the stellar migration through analysing the stellar orbital parameters and stellar birth radii. The results suggest that the thick disc stars near the solar radii and beyond are mostly migrated from the inner disc of $R\sim4 - 6$\,kpc due to their highly eccentrical orbits. Stellar migration due to dynamical processes with angular momentum transfer (churning) are prominent for both the old, metal-rich stars (outward migrators) and the young metal-poor stars (inward migrators). The spatial distribution in the $R$ -- $Z$ plane for the inward migrators born at a Galactocentric radius of $>$12\,kpc show clear age stratifications, possibly an evidence that these inward migrators are consequences of splashes triggered by merger events of satellite galaxies that have been lasted in the past few giga years.

astro-ph.GA↗

arXiv2020

Variations of helioseismic parameters due to magnetic field generated by a flux transport model

The change of sound speed has been found at the base of the convection during the solar cycles, which can be used to constrain the solar internal magnetic field. We aim to check whether the magnetic field generated by the solar dynamo can lead to the cyclic variation of the sound speed detected through helioseismology. The basic configuration of magnetic field in the solar interior was obtained by using a Babcock-Leighton (BL) type flux transport dynamo. We reconstructed one-dimensional solar models by assimilating magnetic field generated by an established dynamo and examined their influences on the structural variables. The results show that magnetic field generated by the dynamo is able to cause noticeable change of the sound speed profile at the base of the convective zone during a solar cycle. Detailed features of this theoretical prediction are also similar to those of the helioseismic results in solar cycle 23 by adjusting the free parameters of the dynamo model.

astro-ph.SR↗

arXiv2020

Endogenous Treatment Effect Estimation with some Invalid and Irrelevant Instruments

Instrumental variables (IV) regression is a popular method for the estimation of the endogenous treatment effects. Conventional IV methods require all the instruments are relevant and valid. However, this is impractical especially in high-dimensional models when we consider a large set of candidate IVs. In this paper, we propose an IV estimator robust to the existence of both the invalid and irrelevant instruments (called R2IVE) for the estimation of endogenous treatment effects. This paper extends the scope of Kang et al. (2016) by considering a true high-dimensional IV model and a nonparametric reduced form equation. It is shown that our procedure can select the relevant and valid instruments consistently and the proposed R2IVE is root-n consistent and asymptotically normal. Monte Carlo simulations demonstrate that the R2IVE performs favorably compared to the existing high-dimensional IV estimators (such as, NAIVE (Fan and Zhong, 2018) and sisVIVE (Kang et al., 2016)) when invalid instruments exist. In the empirical study, we revisit the classic question of trade and growth (Frankel and Romer, 1999).

econ.EM↗