arXiv subjects

Wuming Yang

Publications and source records attributed to Wuming Yang.

At least 19 recordsLinked to original sources

arXiv2026

Photometric Distances for Metal-poor Giants and a Search for Hypervelocity Stars with LAMOST DR13 and Gaia DR3

Hypervelocity stars (HVSs) are stars with velocities high enough to escape the Milky Way, but their identification depends sensitively on distance estimates, particularly for distant giants. In this work, we search for metal-poor HVS candidates by combining LAMOST DR13 spectroscopy with Gaia DR3 astrometry. We calibrate a metallicity-dependent color--absolute-magnitude relation for normal metal-poor giants using a high-quality reference sample, and apply it to derive photometric distances for 41{,}331 stars. The relation reproduces the reference absolute magnitudes with a scatter of 0.24\,mag, corresponding to an intrinsic distance uncertainty of $\sim$9.7\%. Combining these distances with Gaia proper motions and LAMOST radial velocities, we identify 13 initially unbound candidates under the Galactic potential from \citet{McMillan2017}. Spectral inspection indicates that several are chromospherically active binaries or other non-standard systems for which a giant-star calibration is unreliable; removing these contaminants leaves nine metal-poor HVS candidates. Backward orbit integrations suggest that one candidate is most consistent with a disk origin, while three have trajectories suggestive of an association with the Sagittarius stream.

astro-ph.SR↗

arXiv2026

Heterogeneous Knowledge Distillation via Geometry Decoupling and Momentum-Aware Gradient Regulation

Heterogeneous Knowledge Distillation (HKD) aims to transfer knowledge across varying architectures (e.g., from Transformer to CNN) but inherently suffers from severe training instability. We reveal that this instability stems from two highly coupled challenges: massive feature norm discrepancies that cause optimization drag, and severe gradient conflicts between the primary and distillation objectives arising from distinct inductive biases. To achieve stable distillation, we propose SPOFA, a framework built upon a novel Feature and Gradient Dual Stabilization mechanism. Specifically, at the feature level, we introduce a LayerNorm-based decoupling projector that explicitly decouples feature magnitude from direction, creating a bounded and stable space for semantic alignment. At the gradient level, we propose a momentum-driven Exponential Moving Average (MEMA) dynamic scaler. By establishing a robust historical baseline of the optimization trajectory, MEMA actively evaluates instantaneous gradient conflicts and adaptively penalizes harmful distillation signals, guaranteeing stable convergence. Importantly, SPOFA achieves this dual stabilization with an extremely lightweight parameter footprint. Extensive experiments on two mainstream benchmarks demonstrate that SPOFA achieves state-of-the-art accuracy, significantly outperforming computationally expensive methods while introducing only minimal computational overhead compared to standard baselines.

cs.CV↗

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

Additional Evidence for the Existence of a Primordial Disk System

The origin of very metal-poor (VMP; [Fe/H] $\leq -2.0$) stars on planar orbits has been the subject of great attention since their first discovery. However, prior to the release of the Gaia BP/RP (XP) spectra, and large photometric samples such as SkyMapper, SAGES, J-PLUS and S-PLUS, most studies have been limited due to their small sample sizes or strong selection effects. Here, we cross-match photometric metallicities derived from Gaia XP synthetic photometry and geometric distances from Bailer-Jones et al., and select 12,000 VMP stars (1604 dwarfs and 10,396 giants) with available high-quality astrometry. After calculating dynamical parameter estimates using \texttt{AGAMA}, we employ the non-negative matrix factorization technique to the $v_\phi$ distribution across bins in $Z_{\rm max}$ (the maximum height above or below the Galactic plane during the stellar orbit). We find three primary populations of the selected VMP stars: halo, disk system, and the Gaia Sausage/Enceladus (GSE) structure. The fraction of disk-like stars decreases with increasing $Z_{\rm max}$ (as expected), although it is still $\sim 20$\% for stars with $Z_{\rm max}$ $\sim 3 $ kpc. Similar results emerge from the application of the Hayden criterion, which separates stellar populations on the basis of their orbital inclination angles relative to the Galactic plane. We argue that such high fractions of disk-like stars indicate that they are an independent component, rather than originating solely from Galactic building blocks or heating by minor mergers. We suggest that most of these VMP stars are members of the hypothesized ``primordial" disk.

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 $1\sigma$. 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

Solar Models and Astrophysical S-factors Constrained by Helioseismic Results and Updated Neutrino Fluxes

The ratio of metal abundance to hydrogen abundance of the solar photosphere, $(Z/X)_{s}$, has been revised several times. Standard solar models, based on these revised solar abundances, are in disagreement with seismically inferred results. Recently, Magg et al. introduced a new value for $(Z/X)_{s}$, which is still in debate in the community. The solar abundance problem or solar modeling problem remains a topic of ongoing debate. We constructed rotating solar models in accordance with various abundance scales where the effects of convection overshoot and enhanced diffusion were included. Among these models, those utilizing Magg's abundance scale exhibit superior sound-speed and density profiles compared to models using other abundance scales. Additionally, they reproduce the observed frequency separation ratios $r_{02}$ and $r_{13}$. These models also match the seismically inferred surface helium abundance and convection zone depth within $1\sigma$ level. Furthermore, the calculated neutrino fluxes from these models agree with detected ones at the level of $1\sigma$. We found that neutrino fluxes and density profile are influenced by nuclear reactions, allowing us to use the combination of detected neutrino fluxes and seismically inferred density for diagnosing astrophysical $S$-factors. This diagnostic approach shows that $S_{11}$ may be underestimated by $2\%$, while $S_{33}$ may be overestimated by about $3\%$ in previous determinations. The $S$-factors favored by updated neutrino fluxes and helioseismic results can lead to significant improvements in solar models.

astro-ph.SR↗

arXiv2022

Distribution of fast radio burst dispersion measures in CHIME/FRB Catalog 1: implications on the origin of FRBs

Recently, CHIME/FRB project published its first fast radio burst (FRB) catalog (hereafter, Catalog 1), which totally contains 536 unique bursts. With the help of the latest set of FRBs in this large-size catalog, we aim to investigate the dispersion measure (DM) or redshift ($z$) distribution of the FRB population, and solution of this problem could be used to clarify the question of FRB origin. In this study, we adopted the M\&E 2018 model, to fit the observed $z$ distribution of FRBs in Catalog 1. In the M\&E 2018 model, we are mostly interested in the $\Phi(z)$ function, i.e., number of bursts per proper time per comoving volume, which is represented by the star formation rate (SFR) with a power-law index $n$. Our estimated value of $n$ is $0.0_{-0.0}^{+0.6}$ ($0.0_{-0.0}^{+2.1}$) at the 68 (95) per cent confidence level, implying that the FRB population evolves with redshift consistent with, or faster than, the SFR. Specially, the consistency of the $n$ values estimated by this study and the SFR provides a potential support for the hypothesis of FRBs originating from young magnetars.

astro-ph.HE↗

arXiv2022

Rotating Solar Models in Agreement with Helioseismic Results and Updated Neutrino Fluxes

Standard solar models (SSMs) constructed in accordance with old solar abundances are in reasonable agreement with seismically inferred results, but SSMs with new low-metal abundances disagree with the seismically inferred results. The constraints of neutrino fluxes on solar models exist in parallel with those of helioseismic results. The solar neutrino fluxes were updated by Borexino Collaboration. We constructed rotating solar models with new low-metal abundances where the effects of enhanced diffusion and convection overshoot were included. A rotating model using OPAL opacities and the Caffau abundance scale has better sound-speed and density profiles than the SSM with the old solar abundances and reproduces the observed $p$-mode frequency ratios $r_{02}$ and $r_{13}$. The depth and helium abundance of the convection zone of the model agree with the seismically inferred ones at the level of $1σ$. The updated neutrino fluxes are also reproduced by the model at the level of $1σ$. The effects of rotation and enhanced diffusion not only improve the model's sound-speed and density profiles but bring the neutrino fluxes predicted by the model into agreement with the detected ones. Moreover, the calculations show that OP may underestimate opacities for the regions of the Sun with $T\gtrsim5\times10^{6}$ K by around $1.5\%$, while OPAL may underestimate opacities for the regions of the Sun with $2\times10^{6}$ K $\lesssim T \lesssim 5\times10^{6}$ K by about $1-2\%$.

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↗

arXiv2020

A Catalog of RV Variable Star Candidates from LAMOST

RV variable stars are important in astrophysics. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopic survey has provided ~ 6.5 million stellar spectra in its Data Release 4 (DR4). During the survey, ~ 4.7 million unique sources were targeted and ~ 1 million stars observed repeatedly. The probabilities of stars being RV variables are estimated by comparing the observed radial velocity variations with the simulated ones. We build a catalog of 80,702 RV variable candidates with probability greater than 0.60 by analyzing the duplicate-observed multi-epoch sources covered by the LAMOST DR4. Simulations and cross-identifications show that the purity of the catalog is higher than 80%. The catalog consists of 77% binary systems and 7% pulsating stars as well as 16% pollution by single stars. 3,138 RV variables are classified through cross-identifications with published results in literatures. By using the 3,138 sources common to both LAMOST and a collection of published RV variable catalogs we are able to analyze LAMOST's RV variable detection rate. The efficiency of the method adopted in this work relies not only on the sampling frequency of observations but also periods and amplitudes of RV variables. With the progress of LAMOST, Gaia and other surveys, more and more RV variables would will be confirmed and classified. This catalog is valuable for other large-scale surveys, especially for RV variable searches. The catalog will be released according to the LAMOST Data Policy via http://dr4.lamost.org.

astro-ph.SR↗

arXiv2019

Rotating Solar Models with Low Metal Abundances as Good as Those with High Metal Abundances

Standard solar models (SSM) constructed in accord with low metal abundances disagree with the seismically inferred results. We constructed rotating solar models with low metal abundances that included enhanced settling and convection overshoot. In one of our rotating models, \textbf{AGSSr2a}, the convection overshoot allowed us to recover the radius of the base of convection zone (CZ) at a level of $1σ$. The rotational mixing almost completely counteracts the enhanced settling for the surface helium abundance, but only partially for the surface heavy-element abundance. At the level of $1σ$, the combination of rotation and enhanced settling brings the surface helium abundance into agreement with the seismically inferred value of $0.2485\pm0.0035$, and makes the model have better sound-speed and density profiles than SSM constructed in accordance with high metal abundances. The radius of the base of the CZ and the surface helium abundance of \textbf{AGSSr2a} are $0.713$ $R_{\odot}$ and $0.2472$, respectively; the absolute values of the relative differences in sound speed and density between it and the Sun are less than $0.0025$ and $0.015$, respectively. Moreover, predicted neutrino fluxes of our model are comparable with the predictions of previous research works.

astro-ph.SR↗

arXiv2018

Estimating The Mean Linewidth And Lifetime of Solar-like Oscillations of Stars

\textbf{Scaling formulas were} deduced to describe the relations between the fundamental stellar parameters and the mean \textbf{linewidth and lifetime} of solar-like oscillations of stars. The mean \textbf{linewidth and} lifetime of solar-like oscillations \textbf{are dependent on the large frequency separation, the effective temperature, and the acoustic impedance ($ρc$) in the photosphere} of stars. The mean lifetime \textbf{can be} approximate to the lifetime of the mode with $ν\simν_{max}$. We compared the results of the scaling relations with the mean lifetimes of solar-like oscillations of stars observed by \textit{Kepler} and \textit{CoRoT}, which shows that the observed mean lifetimes are reproduced well by the scaling relations. The dependence of the mean lifetime on \textbf{the} large frequency separation, the effective temperature, and the acoustic impedance of stars \textbf{indicates} that lifetimes of solar-like oscillations rely on the mass and evolutionary phase of stars. Moreover, our calculations show that the mean lifetimes of $p$-modes of stars can be affected by metallicity abundances.

astro-ph.SR↗

arXiv2018

The Effects of Binary Stars on the Color-Magnitude Diagrams of Young-Age Massive Star Clusters

Extended main-sequence turnoffs (eMSTO) have been observed in the color-magnitude diagram (CMD) of intermediate-age and young star clusters. The origin of the eMSTO phenomenon is still highly debated. Calculations show that the blue and faint (BF) stars in the CMD of NGC 1866 are hydrogen main sequence (MS) + naked He star systems. The He star derives from the massive star of a binary system. The BF stars and the red and faint MSTO stars belong to the same stellar population. The values of $m_{F336W}$ and $m_{F336W}-m_{F814W}$ of the BF stars are mainly determined by the masses of He stars and H-MS stars, respectively. The behaviors of the BF stars in the CMD are well explained by the H-MS + He-star systems. The BF stars provide a strict restriction on the age of the stellar population. Moreover, the bimodal MS of NGC 1866 can also be reproduced by a younger binary population. The calculations show that part of the blue and bright (BB) MS stars of NGC 1866 are H-MS + He-star systems, H-MS + white dwarf systems, and merged stars in a binary scenario. The H-MS stars of the H-MS + He-star systems for the BB stars are significantly more massive than those of the BF stars. Once the H-MS + He-star systems and their membership in NGC 1866 are confirmed, the extended star-formation histories and the effects of binaries can be confirmed in the young star cluster.

astro-ph.SR↗

arXiv2018

Binary Star Fractions from the LAMOST DR4

Stellar systems composed of single, double, triple or high-order systems are rightfully regarded as the fundamental building blocks of the Milky Way. Binary stars play an important role in formation and evolution of the Galaxy. Through comparing the radial velocity variations from multi-epoch observations, we analyze the binary fraction of dwarf stars observed with the LAMOST. Effects of different model assumptions such as orbital period distributions on the estimate of binary fractions, are investigated. The results based on log-normal distribution of orbital periods reproduce the previous complete analyses better than the power-law distribution. We find that the binary fraction increases with $T_{\rm eff}$ and decreases with [Fe/H]. We first investigate the relation between $α$-elements and binary fraction in such a large sample as the LAMOST. The old stars with high [$α$/Fe] dominate higher binary fraction than young stars with low [$α$/Fe]. At the same mass, former forming stars possess a higher binary fraction than newly forming ones, which may be related with the evolution of the Galaxy.

astro-ph.SR↗

arXiv2017

The effects of the overshooting of the convective core on main-sequence turnoffs of young- and intermediate-age star clusters

Recent investigations have shown that the extended main-sequence turnoffs (eMSTOs) are a common feature of intermediate-age star clusters in the Magellanic Clouds. The eMSTOs are also found in the color-magnitude diagram (CMD) of young-age star clusters. The origin of the eMSTOs is still an open question. Moreover, asteroseismology shows that the value of the overshooting parameter $δ_{\rm ov}$ of the convective core is not fixed for the stars with an approximatelly equal mass. Thus the MSTO of star clusters may be affected by the overshooting of the convective core (OVCC). We calculated the effects of the OVCC with different $δ_{\rm ov}$ on the MSTO of young- and intermediate-age star clusters. \textbf{If $δ_{\rm ov}$ varies between stars in a cluster,} the observed eMSTOs of young- and intermediate-age star clusters can be explained well by the effects. The equivalent age spreads of MSTO caused by the OVCC are related to the age of star clusters and are in good agreement with observed results of many clusters. Moreover, the observed eMSTOs of NGC 1856 are reproduced by the coeval populations with different $δ_{\rm ov}$. The eMSTOs of star clusters may be relevant to the effects of the OVCC. The effects of the OVCC \textbf{are similar to that of rotation in some respects. But the effects cannot result in a significant split of main sequence of young star clusters at $m_{U}\lesssim 21$.} The presence of a rapid rotation can make the split of main sequence of young star clusters more significant.

astro-ph.SR↗

arXiv2016

Estimate the radius of the convective core of main-sequence stars from observed oscillation frequencies

The determination of the size of the convective core of main-sequence stars is usually dependent on the construction of models of stars. Here we introduce a method to estimate the radius of the convective core of main-sequence stars with masses between about 1.1 and 1.5 $M_{\odot}$ from observed frequencies of low-degree p-modes. A formula is proposed to achieve the estimation. The values of the radius of the convective core of four known stars are successfully estimated by the formula. The radius of the convective core of KIC 9812850 estimated by the formula is $\mathbf{0.140\pm0.028}$ $R_{\odot}$. In order to confirm this prediction, a grid of evolutionary models were computed. The value of the convective-core radius of the best-fit model of KIC 9812850 is $0.149$ $R_{\odot}$, which is in good agreement with that estimated by the formula from observed frequencies. The formula aids in understanding the interior structure of stars directly from observed frequencies. The understanding is not dependent on the construction of models.

astro-ph.SR↗

arXiv2016

A Large Convective-Core Overshoot in \textit{Kepler} Target KIC 11081729

The frequency ratios $r_{01}$ and $r_{10}$ of KIC 11081729 decrease firstly and then increase with the increase in frequency. For different spectroscopic constraints, all models with overshooting parameter $δ_{\mathrm{ov}}$ less than 1.7 can not reproduce the distributions of the ratios. However, the distributions of the ratios can be directly reproduced by models with $δ_{\mathrm{ov}}$ in the range of about $1.7-1.8$. The estimations of mass and age of the star can be affected by spectroscopic results, but the determination of the $δ_{\mathrm{ov}}$ is not dependent on the spectroscopic results. A large overshooting of convective core may exist in KIC 11081729. The characteristics of $r_{01}$ and $r_{10}$ of KIC 11081729 may result from the effects of the large overshooting of convective core. The distributions of $r_{01}$ and $r_{10}$ of different stars with a convective core can be reproduced by the function $B(ν_{n,1})$. If the value of the critical frequency $ν_{0}$ is larger than the value of frequency of maximum oscillation power $ν_{max}$, a star may have a small convective core and $δ_{\rm ov}$. But if the value of $ν_{0}$ is less than that of $ν_{max}$, the star may have a large convective core and $δ_{\mathrm{ov}}$. The function aids in determining the presence of convective core and the size of the convective core including overshooting region from observed frequencies. The determination is not dependent on the calculation of stellar models.

astro-ph.SR↗

arXiv2016

Solar models with new low-metal abundances

In the last decade, the photospheric abundances of the Sun had been revised several times by many observers. The standard solar models (SSM) constructed with the new low-metal abundances disagree with helioseismic results and detected neutrino fluxes. The solar model problem has been puzzled some stellar physicists for more than ten years. Rotation, enhanced diffusion, convection overshoot, and magnetic fields are used to reconcile the new abundances with helioseismology. The \textbf{too} low-helium \textbf{subsurface abundance} in enhanced diffusion models can be improved by the mixing caused by rotation and magnetic fields. The problem of the depth of the convective zone in rotating models can be resolved by convection overshoot. Consequently the Asplund-Grevesse-Sauval rotation model including overshooting (AGSR) reproduces the seismically inferred sound-speed and density profiles, and the convection zone depth as well as the Grevesse and Sauval (GS98) model computed before. But this model fails to reproduce the surface helium abundance which is 0.2393 ($2.6$ $σ$ away from the seismic value) and neutrino fluxes. The magnetic model called AGSM keeps the agreement of the AGSR and improves the prediction of the surface helium abundance. The observed separation ratios $r_{02}$ and $r_{13}$ are reasonably reproduced by AGSM. Moreover, neutrino fluxes calculated by this model are not far from the detected neutrino fluxes and the predictions of previous works.

astro-ph.SR↗