arXiv subjects

Xiao-Wei Liu

Publications and source records attributed to Xiao-Wei Liu.

At least 19 recordsLinked to original sources

arXiv2025

Segmentation and Celestial Mapping of Unobservable Regions in Nighttime All-sky Images for the Mephisto Observations

Accurate identification of unobservable regions in nighttime is essential for autonomous scheduling and data quality control in observations.Traditional methods-such as infrared sensing or photometric extinction-provide only coarse,non-spatial estimates of sky clarity,making them insufficient for real-time decision-making.This not only wastes observing time but also introduces contamination when telescopes are directed toward cloud-covered or moonlight-affected regions.To address these limitations,we propose a deep learning-based segmentation framework that provides pixel-level masks of unobservable areas using all-sky images.Supported by a manually annotated dataset of nighttime images,our method enables precise detection of cloud- and moonlight-affected regions.The segmentation results are further mapped to celestial coordinates through Zenithal Equal-Area projection,allowing seamless integration with observation control systems (OCS) for real-time cloud-aware scheduling.While developed for the Mephisto telescope,the framework is generalizable and applicable to other wide-field robotic observatories equipped with all-sky monitoring.

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arXiv2025

GRB 240825A: Early Reverse Shock and Its Physical Implications

Early multiwavelength observations offer crucial insights into the nature of the relativistic jets responsible for gamma-ray bursts and their interaction with the surrounding medium.We present data of GRB 240825A from 17 space- and ground-based telescopes/instruments, covering wavelengths from NIR/optical to X-ray and GeV, and spanning from the prompt emission to the afterglow phase triggered by Swift and Fermi. The early afterglow observations were carried out by SVOM/C-GFT, and spectroscopic observations of the afterglow by GTC, VLT, and TNG determined the redshift of the burst ($z = 0.659$) later.A comprehensive analysis of the prompt emission spectrum observed by Swift-BAT and Fermi-GBM/LAT reveals a rare and significant high-energy cutoff at ~76 MeV. Assuming this cutoff is due to $γγ$ absorption allows us to place an upper limit on the initial Lorentz factor, $Γ_0 < 245$. The optical/NIR and GeV afterglow light curves be described by the standard external shock model, with early-time emission dominated by a reverse shock (RS) and a subsequent transition to forward shock (FS) emission. Our afterglow modelling yields a consistent estimate of the initial Lorentz factor ($Γ_{\rm 0} \sim 234$). Furthermore, the RS-to-FS magnetic field ratio ($\mathcal{R}_B \sim 302$) indicates that the reverse shock region is significantly more magnetized than the FS region. An isotropic-equivalent kinetic energy of $E_{\text{k,iso}} = 5.25 \times 10^{54}$ erg is derived, and the corresponding $γ$-ray radiation efficiency is estimated to be $η_γ$ = 3.1%. On the other hand, the standard afterglow model can not reproduce the X-ray light curve of GRB 240825A, calling for improved models to characterize all multiwavelength data.

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arXiv2025

Empirically Predicted Absolute Magnitudes for Red Clump Stars in Mephisto and CSST Filters

Red clump (RC) stars are reliable standard candles for studying the structure and evolution of the Milky Way. In this study, we present empirical calibrations of RC absolute magnitudes in the Mephisto (v, g, r, i) and CSST (g, r, i) photometric systems using a high-purity sample of 25,059 RC stars cross-matched between APOGEE and Gaia DR3 XP spectra. Through synthetic photometry and polynomial fitting, we find that RC absolute magnitudes exhibit strong dependencies on effective temperature and metallicity, with the strongest variations observed in bluer bands and progressively decreasing towards redder wavelengths. In particular, the Mephisto v band exhibits the highest sensitivity, with variations reaching up to 2.0 mag across the metallicity range (-1.0 dex <[Fe/H] <0.5 dex) and the temperature range (4500-5200 K). The calibrations achieve high precision for all bands, enabling accurate determination of RC absolute magnitudes and distances. Furthermore, we evaluate the metallicity estimation capabilities of both systems using a Random Forest-based method, achieving a precision of 0.12 dex for Mephisto and 0.14 dex for CSST under typical photometric uncertainties (< 0.01 mag). These results provide robust tools for distance and metallicity determinations, supporting future Galactic structure studies with Mephisto and CSST data.

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arXiv2025

Photometric metallicities of 0.8 million KiDS stars

Accurate determinations of metallicity for large, complete stellar samples are essential for advancing various studies of the Milky Way. In this paper, we present a data-driven algorithm that leverages photometric data from the KiDS and the VIKING surveys to estimate stellar absolute magnitude, effective temperature and metallicities. The algorithm is trained and validated using spectroscopic data from LAMOST, SEGUE, APOGEE, and GALAH, as well as a catalog of very metal-poor stars from the literature, and Gaia EDR3 data. This approach enables us to estimate metallicities, effective temperatures, and g-band absolute magnitudes for approximately 0.8 million stars in the KiDS dataset. The photometric metallicity estimates exhibit an uncertainty of around 0.28 dex when compared to spectroscopic studies, within the metallicity range of -2 dex to 0.5 dex. The photometric effective temperature estimates have an uncertainty of around 149 K, while the uncertainty in the absolute magnitude is approximately 0.36 mag. The metallicity estimates are reliable for values down to about -2 dex. This catalog represents a valuable resource for studying the structure and chemical properties of the Milky Way, offering an extensive dataset for future investigations into Galactic formation and evolution.

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arXiv2025

Finding Quasars Behind the Galactic Plane: Spectroscopic Identifications of ~1300 New Quasars at |b|<=20 degree from LAMOST DR10

Quasars behind the Galactic plane (GPQs) are excellent tracers to probe the chemistry and kinematics of the interstellar/intergalactic medium (ISM/IGM) of the Milky Way along sight lines via absorption line spectroscopy. Moreover, the quasars located at low Galactic latitudes will fill the gap in the spatial distribution of known quasars near the Galactic plane, and can be used to construct an astrometric reference frame for accurate measurements of proper motions (PMs) of stars, and substructures of the Milky Way. We started a survey of background quasars in the low Galactic latitude region since the LAMOST phase II survey in 2017. Quasar candidates have been selected from the optical and infrared photometric data of Pan-STARRS1 and WISE surveys based on their variability and color properties. In this paper, we present a sample of 1982 spectroscopically confirmed GPQs with |b| <= 20 degree based on LAMOST Data Release 10 (DR10). Among them, 1338 are newly discovered. Most GPQs are located around 240<l<90 degree, and the spatial distributions are non-uniform. These GPQs have a magnitude distribution with a peak at i-mag 19.0, and mostly around 18.0-19.5mag. The peak of redshift distributions is around ~1.5, and most GPQs have redshifts between 0.3 and 2.5. Our finding demonstrates the potential discovery space for the GPQs from the spectroscopic surveys and the promising applications for future research.

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arXiv2024

The wave-like disk oscillations of mono-age stellar populations in the Solar neighbourhood from Gaia DR3

The North-South asymmetry in the number density and bulk velocity of stars in the solar neighborhood provides valuable insights into the formation and evolution of the Milky Way disk. Our objective is to investigate the wave-like disk oscillations of mono-age stellar populations in the Solar neighbourhood using data from Gaia Data Release 3. We have selected a comprehensive sample of main sequence turn off stars. The ages of these stars can be accurately determined using isochrone fitting methods. Our findings indicate that the north-south density and mean vertical velocity asymmetries remain consistent across all age groups.The uniformity of perturbations across all subsamples suggests that all populations are responding to the same external influence, which likely affects them irrespective of their age. Moreover, the fact that these perturbations appear consistently implies they could be either ongoing or recent. Regarding vertical velocity dispersions, we observe that older stars exhibit larger dispersions.

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arXiv2023

Ba-enhanced dwarf and subgiant stars in the LAMOST Galactic surveys

Ba-enhanced stars are interesting probes of stellar astrophysics and Galactic formation history. In this work, we investigate the chemistry and kinematics for a large sample of Ba-enhanced ([Ba/Fe]$>$1.0) dwarf and subgiant stars with $5000 < T_{\rm eff }< 6700$\,K from LAMOST. We find that both stellar internal evolution process and external mass exchange due to binary evolution are responsible for the origins of the Ba-enhancement of our sample stars. About one third of them exhibit C and N enhancement and ultraviolet brightness excess, indicating they are products of binary evolution. The remaining Ba-enhanced stars with normal C and N abundances are mostly warm stars with $T_{\rm eff} > 6000$\,K. They are likely consequences of stellar internal elemental transport processes, but they show very different elemental patterns to the hotter Am/Fm stars. Our results reveal a substantially lack of high-[$α$/Fe] Ba-enhanced stars in the [Fe/H]--[$α$/Fe] plane, which we dub as a {\em high-$α$ desert}. We suggest it is due to a lower efficiency for producing Ba-enhanced stars by low-mass AGB progenitors in binary systems. Our results call for detailed modellings for these Ba-enhanced stellar peculiars, in the context of both stellar internal elemental transport and external mass accretion.

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arXiv2021

Identifications of RR Lyrae stars and Quasars from the simulated data of Mephisto-W Survey

We have investigated the feasibilities and accuracies of the identifications of RR Lyrae stars and quasars from the simulated data of the Multi-channel Photometric Survey Telescope (Mephisto) W Survey. Based on the variable sources light curve libraries from the Sloan Digital Sky Survey (SDSS) Stripe 82 data and the observation history simulation from the Mephisto-W Survey Scheduler, we have simulated the $uvgriz$ multi-band light curves of RR Lyrae stars, quasars and other variable sources for the first year observation of Mephisto-W Survey. We have applied the ensemble machine learning algorithm Random Forest Classifier (RFC) to identify RR Lyrae stars and quasars, respectively. We build training and test samples and extract ~ 150 features from the simulated light curves and train two RFCs respectively for the RR Lyrae star and quasar classification. We find that, our RFCs are able to select the RR Lyrae stars and quasars with remarkably high precision and completeness, with $purity$ = 95.4 per cent and $completeness$ = 96.9 per cent for the RR Lyrae RFC and $purity$ = 91.4 per cent and $completeness$ = 90.2 per cent for the quasar RFC. We have also derived relative importances of the extracted features utilized to classify RR Lyrae stars and quasars.

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arXiv2021

Most "young" $α$-rich stars have high masses but are actually old

Recent observations have revealed a population of $α$-element abundances enhanced giant stars with unexpected high masses ($\gtrsim$1 $M_\odot$) from asteroseismic analysis and spectroscopy. Assuming single-star evolution, their masses imply young ages ($τ<6$Gyr) incompatible with the canonical Galactic chemical evolution scenario. Here we study the chemistry and kinematics of a large sample of such $α$-rich, high-mass red giant branch (RGB) stars drawn from the LAMOST spectroscopic surveys. Using LAMOST and Gaia, we found these stars share the same kinematics as the canonical high-$α$ old stellar population in the Galactic thick disk. The stellar abundances show that these high-$α$ massive stars have $α$- and iron-peak element abundances similar to those of the high-$α$ old thick disk stars. However, a portion of them exhibit higher [(N+C)/Fe] and [Ba/Fe] ratios, which implies they have gained C- and Ba-rich materials from extra sources, presumably asymptotic giant branch (AGB) companions. The results support the previous suggestion that these RGB stars are products of binary evolution. Their high masses thus mimic "young" single stars, yet in fact they belong to an intrinsic old stellar population. To fully explain the stellar abundance patterns of our sample stars, a variety of binary evolution channels, such as, main-sequence (MS) + RGB, MS + AGB, RGB + RGB and RGB + AGB, are required, pointing to diverse formation mechanisms of these seemly rejuvenated cannibals. With this larger sample, our results confirm earlier findings that most, if not all, $α$-rich stars in the Galactic disk seem to be old.

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arXiv2021

Stellar labels for hot stars from low-resolution spectra - I. the HotPayne method and results for 330,000 stars from LAMOST DR6

We set out to determine stellar labels from low-resolution survey spectra of hot, OBA stars with effective temperature (Teff) higher than 7500K. This fills a gap in the scientific analysis of large spectroscopic stellar surveys such as LAMOST, which offers spectra for millions of stars at R=1800. We first explore the theoretical information content of such spectra for determining stellar labels, via the Cramér-Rao bound. We show that in the limit of perfect model spectra and observed spectra with S/N of 100, precise estimates are possible for a wide range of stellar labels: not only the effective temperature Teff, surface gravity logg, and projected rotation velocity vsini, but also the micro-turbulence velocity, Helium abundance and the elemental abundances [C/H], [N/H], [O/H], [Si/H], [S/H], and [Fe/H]. Our analysis illustrates that the temperature regime of around 9500K is challenging, as the dominant Balmer and Paschen line strength vary little with Teff. We implement the simultaneous fitting of these 11 stellar labels to LAMOST hot-star spectra using the Payne approach, drawing on Kurucz's ATLAS12/SYNTHE LTE spectra as the underlying models. We then obtain stellar parameter estimates for a sample of about 330,000 hot stars with LAMOST spectra, an increase by about two orders of magnitude in sample size. Among them, about 260,000 have good Gaia parallaxes (S/N>5), and more than 95 percent of them are luminous stars, mostly on the main sequence; the rest reflects lower luminosity evolved stars, such as hot subdwarfs and white dwarfs. We show that the fidelity of the abundance estimates is limited by the systematics of the underlying models, as they do not account for NLTE effects. Finally, we show the detailed distribution of vsini of stars with 8000-15,000K, illustrating that it extends to a sharp cut-off at the critical rotation velocity, across a wide range of temperatures.

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arXiv2020

Astronomical Site Monitoring System at Lijiang Observatory

We installed two sets of Astronomical Site Monitoring System(ASMS) at Lijiang Observatory(GMG), for the running of the 2.4-meter Lijiang optical telescope(LJT) and the 1.6-meter Multi-channel Photometric Survey Telescope (Mephisto). The Mephistro is under construction. ASMS has been running on robotic mode since 2017. The core instruments: Cloud Sensor, All-Sky Camera and Autonomous-DIMM that are developed by our group, together with the commercial Meteorological Station and Sky Quality Meter, are combined into the astronomical optical site monitoring system. The new Cloud Sensor's Cloud-Clear Relationship is presented for the first time, which is used to calculate the All-Sky cloud cover. We designed the Autonomous-DIMM located on a tower, with the same height as LJT. The seeing data have been observed for a full year. ASMS's data for the year 2019 are also analysed in detail, which are valuable to observers.

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arXiv2019

Abundance Estimates for 16 Elements in 6 Million Stars from LAMOST DR5 Low-Resolution Spectra

We present the determination of stellar parameters and individual elemental abundances for 6 million stars from $\sim$8 million low-resolution ($R\sim1800$) spectra from LAMOST DR5. This is based on a modeling approach that we dub $The$ $Data$--$Driven$ $Payne$ ($DD$--$Payne$), which inherits essential ingredients from both {\it The Payne} \citep{Ting2019} and $The$ $Cannon$ \citep{Ness2015}. It is a data-driven model that incorporates constraints from theoretical spectral models to ensure the derived abundance estimates are physically sensible. Stars in LAMOST DR5 that are in common with either GALAH DR2 or APOGEE DR14 are used to train a model that delivers stellar parameters ($T_{\rm eff}$, $\log g$, $V_{\rm mic}$) and abundances for 16 elements (C, N, O, Na, Mg, Al, Si, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, and Ba) when applied to LAMOST spectra. Cross-validation and repeat observations suggest that, for ${\rm S/N}_{\rm pix}\ge 50$, the typical internal abundance precision is 0.03--0.1\,dex for the majority of these elements, with 0.2--0.3\,dex for Cu and Ba, and the internal precision of $T_{\rm eff}$ and $\log g$ is better than 30\,K and 0.07\,dex, respectively. Abundance systematics at the $\sim$0.1\,dex level are present in these estimates, but are inherited from the high-resolution surveys' training labels. For some elements, GALAH provides more robust training labels, for others, APOGEE. We provide flags to guide the quality of the label determination and to identify binary/multiple stars in LAMOST DR5. The abundance catalogs are publicly accessible via \href{url}{http://dr5.lamost.org/doc/vac}.

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arXiv2018

Mapping the emission line strengths and kinematics of Supernova Remnant S147 with extensive LAMOST spectroscopic observations

We present extensive spectroscopic observations of supernova remnant (SNR) S147 collected with the Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST). The spectra were carefully sky-subtracted taking into account the complex filamentary structure of S147. We have utilized all available LAMOST spectra toward S147, including sky and stellar spectra. By measuring the prominent optical emission lines including Hα, [NII]λ6584, and [SII]λλ6717,6731, we present maps of radial velocity and line intensity ratio covering the whole nebula of S147 with unprecedented detail. The maps spatially correlated well with the complex filamentary structure of S147. For the central 2 deg of S147, the radial velocity varies from -100 to 100 km/s and peaks between ~ 0 and 10 km/s. The intensity ratios of Hα/[SII]λλ6717,6731, [SII]λ6717/λ6731 and Hα/[NII]λ6584 peak at about 0.77, 1.35 and 1.48, respectively, with a scatter of 0.17, 0.19 and 0.37, respectively. The intensity ratios are consistent with the literature values. However, the range of variations of line intensity ratios estimated here and representative of the whole nebula, are larger than previously estimated.

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arXiv2017

Quasars in the Galactic Anti-Center Area from the LAMOST DR3

We present a sample of quasars discovered in the area of Galactic Anti-Center (GAC) of $150^{\circ} \leq l \leq 210^{\circ}$ and $|b| \leq 30^{\circ}$, based on the LAMOST Data Release 3 (DR3). The sample contains 151 spectroscopically confirmed quasars. Among them 80 are newly discovered with the LAMOST. All those quasars are very bright, with $i$ magnitudes peaking around 17.5\, mag. All the newly quasars are discovered serendipitously, targeted originally with the LAMOST as stars of bluer colours, except for a few targeted as variable, young stellar object candidates. This bright quasar sample at low Galactic latitudes will help fill the gap in the spatial distribution of known quasars near the Galactic disk that are used to construct astrometric reference frame for the purpose of accurate proper motion measurements, for example, Gaia. They are also excellent tracers to probe the kinematics and chemistry of the interstellar medium of the Milky Way disk and halo via absorption line spectroscopy.

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arXiv2015

On the non-thermal kappa-distributed electrons in planetary nebulae and HII regions: the kappa index and its correlations with other nebular properties

Recently, a suspicion arose that the free electrons in planetary nebulae (PNe) and HII regions might have non-thermal energy distributions. In this scenario, a kappa index is introduced to characterize the electron energy distributions, with smaller kappa values indicating larger deviations from Maxwell-Boltzmann distributions. Assuming that this is the case, we determine the kappa values for a sample of PNe and HII regions by comparing the intensities of [OIII] collisionally excited lines and the hydrogen Balmer jump. We find the average kappa indices of PNe and HII regions to be 27 and 32, respectively. Correlations between the resultant kappa values and various physical properties of the nebulae are examined to explore the potential origin of non-thermal electrons in photoionized gaseous nebulae. However, no positive result is obtained. Thus the current analysis does not lend to support to the idea that kappa-distributed electrons are present in PNe and HII regions.

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arXiv2015

On the LSP3 estimates of surface gravity for LAMOST-Kepler stars with asteroseismic measurements

Asteroseismology allows for deriving precise values of surface gravity of stars. The accurate asteroseismic determinations now available for large number of stars in the Kepler fields can be used to check and calibrate surface gravities that are currently being obtained spectroscopically for a huge numbers of stars targeted by large-scale spectroscopic surveys, such as the on-going Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Galactic survey. The LAMOST spectral surveys have obtained a large number of stellar spectra in the Kepler fields. Stellar atmospheric parameters of those stars have been determined with the LAMOST Stellar Parameter Pipeline at Peking University (LSP3), by template matching with the MILES empirical spectral library. In the current work, we compare surface gravities yielded by LSP3 with those of two asteroseismic samples - the largest sample from Huber et al. (2014) and the most accurate sample from Hekker et al. (2012, 2013). We find that LSP3 surface gravities are in good agreement with asteroseismic values of Hekker et al. (2012, 2013), with a dispersion of about 0.2 dex. Except for a few cases, asteroseismic surface gravities of Huber et al. (2014) and LSP3 spectroscopic values agree for a wide range of surface gravities. However, some patterns of differences can be identified upon close inspection. Potential ways to further improve the LSP3 spectroscopic estimation of stellar atmospheric parameters in the near future are briefly discussed. The effects of effective temperature and metallicity on asteroseismic determinations of surface gravities for giant stars are also discussed.

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arXiv2015

Kinematic properties of the dual AGN system J0038+4128 based on long-slit spectroscopy

The study of kiloparsec-scale dual active galactic nuclei (AGN) will provide important clues to understand the co-evolution between the host galaxies and their central supermassive black holes undergoing a merging process. We present long-slit spectroscopy of the J0038$+$4128, a kiloparsec-scale dual AGN candidate discovered by Huang et al. recently, using the Yunnan Faint Object Spectrograph and Camera (YFOSC) mounted on Li-Jiang 2.4-m telescope at Yunnan observatories. From the long-slit spectra, we find that the average relative line-of-sight (LOS) velocity between the two nuclei (J0038$+$4128N and J0038$+$4128S) is about 150 km s$^{-1}$. The LOS velocities of the emission lines from the gas ionized by the nuclei activities and of the absorption lines from stars governed by the host galaxies for different regions of the J0038$+$4128 exhibit the same trend. The same velocities trend indicates that the gaseous disks are co-rotating with the stellar disks in this ongoing merge system. We also find several knots/giant HII regions scattered around the two nuclei with strong star formation revealed by the observed line ratios from the spectra. Those regions are also detected clearly in HST $F336W/U$-band and HST $F555W/V$-band images.

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arXiv2015

Empirical metallicity-dependent calibrations of effective temperature against colours for dwarfs and giants based on interferometric data

We present empirical metallicity-dependent calibrations of effective temperature against colours for dwarfs of luminosity classes IV and V and for giants of luminosity classes II and III, based on a collection from the literature of about two hundred nearby stars with direct effective temperature measurements of better than 2.5 per cent. The calibrations are valid for an effective temperature range 3,100 - 10,000 K for dwarfs of spectral types M5 to A0 and 3,100 - 5,700 K for giants of spectral types K5 to G5. A total of twenty-one colours for dwarfs and eighteen colours for giants of bands of four photometric systems, i.e. the Johnson ($UBVR_{\rm J}I_{\rm J}JHK$), the Cousins ($R_{\rm C}I_{\rm C}$), the Sloan Digital Sky Survey (SDSS, $gr$) and the Two Micron All Sky Survey (2MASS, $JHK_{\rm s}$), have been calibrated. Restricted by the metallicity range of the current sample, the calibrations are mainly applicable for disk stars ([Fe/H]$\,\gtrsim\,-1.0$). The normalized percentage residuals of the calibrations are typically 2.0 and 1.5 per cent for dwarfs and giants, respectively. Some systematic discrepancies at various levels are found between the current scales and those available in the literature (e.g. those based on the infrared flux method IRFM or spectroscopy). Based on the current calibrations, we have re-determined the colours of the Sun. We have also investigated the systematic errors in effective temperatures yielded by the current on-going large scale low- to intermediate-resolution stellar spectroscopic surveys. We show that the calibration of colour ($g-K_{\rm s}$) presented in the current work provides an invaluable tool for the estimation of stellar effective temperature for those on-going or upcoming surveys.

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