arXiv subjects

Zhenxin Lei

Publications and source records attributed to Zhenxin Lei.

At least 19 recordsLinked to original sources

arXiv2026

Characterizing Orbital Parameters of Hot Subdwarf Binaries with Multiple Spectroscopic Surveys

Hot subdwarfs (HSDs) provide critical insights into the physical mechanisms governing binary evolution. In this work, we conduct a systematic analysis of 157 HSDs, selected from Gaia EDR3 and characterized using multi-survey spectroscopic data. Atmospheric parameters of these HSDs are derived via a convolutional neural network (CNN) method and template-matching method. Based on the atmospheric parameters from CNN method, these HSDs exhibit a median mass of $0.45^{+0.19}_{-0.17} M_{\odot}$ and radius of $0.18^{+0.04}_{-0.05} R_{\odot}$, consistent with earlier work. Orbital parameters of 23 systems are determined through the fitting of radial velocity data and light curves, with 11 of them being new solutions. We find that reflection-dominated binaries typically have periods longer than 0.1 d and host low-mass main-sequence companions ($\sim$ 0.2 $M_{\odot}$) with rotation-inflated radii. In contrast, binaries including an HSD and a white dwarf show very short periods ($P < 0.2$ d), with the closest systems hosting more massive white dwarfs. Most of these systems share a similar mass--period distribution with that of post-common-envelope binaries, supporting a common-envelope origin.

astro-ph.SR↗

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.

astro-ph.SR↗

arXiv2026

Deep learning-driven atmospheric parameter prediction for hot subdwarf stars with synthetic and observed spectra

We design a convolutional neural network (CNN) incorporating channel attention and spatial attention mechanisms to predict atmospheric parameters of hot subdwarfs. The experimental dataset comprises spectra at nine distinct signal-to-noise ratio (SNR) levels, with each SNR level containing 11 396 synthetic spectra and 945 observed spectra. The trained deep learning models achieves mean absolute errors (AME) in predicting hot subdwarf atmospheric parameters of 730 K for effective temperature (Teff ), 0.09 dex for surface gravity (log g), and 0.03 dex for helium abundance (log(nHe/nH)), respectively, which reaches the accuracy of traditional spectral fitting methods. Utilizing the trained deep learning models and low-resolution spectra from LAMOST DR12, we confirm 1512 hot subdwarfs from the catalog of hot subdwarf candidates, of which 291 are newly identified. Our results demonstrate that the deep learning model not only achieves accuracy comparable to traditional methods in obtaining hot subdwarf atmospheric parameters, but also far exceeds them in speed and efficiency, making it particularly suitable for the analysis of large datasets of hot subdwarf spectra.

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arXiv2025

MetaCaptioner: Towards Generalist Visual Captioning with Open-source Suites

Generalist visual captioning goes beyond a simple appearance description task, but requires integrating a series of visual cues into a caption and handling various visual domains. In this task, current open-source models present a large performance gap with commercial ones, which limits various applications such as data synthesis. To bridge the gap, this paper proposes CapFlow, a novel multi-agent collaboration workflow. CapFlow demonstrates for the first time that, by capitalizing on open-source models, it is possible to achieve caption quality on par with GPT-4.1 in various domains with an 89.5% reduction in costs. By leveraging CapFlow as the data synthesizer, we produce high-quality visual captions from image and video domains at scale, and obtain a generalist visual captioner via fine-tuning, namely MetaCaptioner. Through extensive experiments, we show that MetaCaptioner not only achieves comparable captioning capabilities with commercial models but also reaches top-tier multimodal performance in the open-source community. We hope CapFlow and MetaCaptioner can benefit future multimodal research by providing a strong and cost-effective visual captioning solution.

cs.CV↗

arXiv2025

The fractions of reflection effect systems detected in different subclasses of hot subdwarfs

Hot subdwarfs with close low-mass M-type or brown dwarf companions usually show the reflection effect and often also eclipses. Through inspecting the light variations, we estimate the fractions of reflection effect systems detected in different subclasses of hot subdwarfs and constrain their possible formation channels. We noticed that none of the helium-rich hot subdwarfs were found with the reflection effect. Most of them might form through the merger channel. About 13\% of the He-poor hot subdwarfs on the extreme horizontal branch (EHB) with $T_{\rm eff}$ $\sim$ 25,000 -- 33,000 K are discovered with the reflection effect. Nevertheless, the cooler hot subdwarfs located on the top of the EHB, those grouped on the bottom of the EHB, and those located above the EHB with $T_{\rm eff}$ $\sim$ 45,000 -- 75,000 K exhibit lower fractions of reflection effect systems of about 2\%. These three subclasses of hot subdwarfs might contain higher fractions of single stars. Hot subdwarfs below the EHB also show a low fraction of reflection effect systems. They might have a higher fraction of stars with close WD companions. A similar fraction of reflection effect systems to those on the EHB with $T_{\rm eff}$ $\sim$ 25,000 -- 33,000 K is found in hot subdwarfs located above the EHB with $T_{\rm eff}$ $\sim$ 35,000 -- 45,000 K. We also discovered that hot subdwarfs close to the Galactic Plane might have a higher fraction of stars with the reflection effect than those at higher latitudes.

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arXiv2024

Spike2Former: Efficient Spiking Transformer for High-performance Image Segmentation

Spiking Neural Networks (SNNs) have a low-power advantage but perform poorly in image segmentation tasks. The reason is that directly converting neural networks with complex architectural designs for segmentation tasks into spiking versions leads to performance degradation and non-convergence. To address this challenge, we first identify the modules in the architecture design that lead to the severe reduction in spike firing, make targeted improvements, and propose Spike2Former architecture. Second, we propose normalized integer spiking neurons to solve the training stability problem of SNNs with complex architectures. We set a new state-of-the-art for SNNs in various semantic segmentation datasets, with a significant improvement of +12.7% mIoU and 5.0 efficiency on ADE20K, +14.3% mIoU and 5.2 efficiency on VOC2012, and +9.1% mIoU and 6.6 efficiency on CityScapes.

cs.CV↗

arXiv2024

Radial velocity variability fractions of different types of hot subdwarf stars

Different types of hot subdwarfs may have different origins, which will cause them to present different radial velocity (RV) variability properties. Only 6$\pm$4% of our single-lined He-rich hot subdwarfs that only show spectroscopic features of hot subdwarfs are found to be RV variable, which is lower than the fraction of single-lined He-poor sdB stars (31$\pm$3%). Single-lined sdB stars with effective temperatures ($T_{\rm eff}$) $\sim$ 25,000 $-$ 33,000 K show an RV-variability fraction of 34$\pm$5%, while lower RV-variability fractions are observed for single-lined sdB stars cooler than about 25,000 K (11$\pm$4%), single-lined sdB/OB stars with $T_{\rm eff}$ $\sim$ 33,000 $-$ 40,000 K and surface gravities about 5.7 $-$ 6.0 (13$\pm$3%), as well as single-lined sdO/B stars with $T_{\rm eff}$ $\sim$ 45,000 $-$ 70,000 K (10$\pm$7%). Single-lined hot subdwarfs with $T_{\rm eff}$ $\sim$ 35,000 $-$ 45,000 K located above the extreme horizontal branch (EHB) show a similar RV-variability fraction of 34$\pm$9% as single-lined sdB stars at about 25,000 $-$ 33,000 K. The largest RV-variability fraction of 51$\pm$8% is found in single-lined hot subdwarfs below the canonical EHB. The detected RV-variability fraction of our composite hot subdwarfs with an infrared excess in their spectral energy distributions is 9$\pm$3%, which is lower than that fraction of single-lined hot subdwarfs. Since the average RV uncertainty we measured in the LAMOST spectra is about 7.0 km/s, the lower detected RV-variability fraction for composite hot subdwarfs is expected because the RV amplitudes associated with long-period systems are lower.

astro-ph.SR↗

arXiv2024

MK-like spectral classification for hot subdwarf stars with LAMOST spectra

An MK-like spectral classification has been conducted for 1224 hot subdwarf stars with LAMOST DR9 low-resolution spectra. The whole sample was divided into four categories according to the spectral line characteristics: He-normal, He-weak, He-strong C and He-strong. Each selected spectrum was assigned a spectral class, a luminosity class and an helium class by comparing the line depth and width with standard spectra selected in LAMOST. Relationships between atmospheric parameters and spectral classification were also presented.

astro-ph.SR↗

arXiv2023

Mass distribution for single-lined hot subdwarf stars in LAMOST

Masses for 664 single-lined hot subdwarf stars identified in LAMOST were calculated by comparing synthetic fluxes from spectral energy distribution (SED) with observed fluxes from virtual observatory service. Three groups of hot subdwarf stars were selected from the whole sample according to their parallax precision to study the mass distributions. We found, that He-poor sdB/sdOB stars present a wide mass distribution from 0.1 to 1.0 $\mathrm{M}_{\odot}$ with a sharp mass peak around at 0.46 $\rm{M}_{\odot}$, which is consistent with canonical binary model prediction. He-rich sdB/sdOB/sdO stars present a much flatter mass distribution than He-poor sdB/sdOB stars and with a mass peak around 0.42 $\mathrm{M}_{\odot}$. By comparing the observed mass distributions to the predictions of different formation scenarios, we concluded that the binary merger channel, including two helium white dwarfs (He-WDs) and He-WD + main sequence (MS) merger, cannot be the only main formation channel for He-rich hot subdwarfs, and other formation channels such as the surviving companions from type Ia supernovae (SNe Ia) could also make impacts on producing this special population, especially for He-rich hot subdwarfs with masses less than 0.44 $\mathrm{M}_{\odot}$. He-poor sdO stars also present a flatter mass distribution with an inconspicuous peak mass at 0.18 $\mathrm{M}_{\odot}$. The similar mass - $ΔRV_\mathrm{max}$ distribution between He-poor sdB/sdOB and sdO stars supports the scenario that He-poor sdO stars could be the subsequent evolution stage of He-poor sdB/sdOB stars.

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arXiv2022

Hot subdwarf stars identified in LAMOST DR8 with single-lined and composite spectra

222 hot subdwarf stars were identified with LAMOST DR8 spectra, among which 131 stars show composite spectra and have been decomposed, while 91 stars present single-lined spectra. Atmospheric parameters of all sample stars were obtained by fitting Hydrogen (H) and Helium (He) line profiles with synthetic spectra. Two long-period composite sdB binaries were newly discovered by combining our sample with the non-single star data from Gaia DR3. One of the new systems presents the highest eccentricity (i.e., 0.5 +/- 0.09) among known wide sdB binaries, which is beyond model predictions. 15 composite sdB stars fall in the high probability binary region of RUWE-AEN plane, and deserve priority follow-up observations to further study their binary nature. A distinct gap is clearly presented among temperatures of cool companions for our composite-spectra sample. But we could not come to a conclusion whether this feature is connected to the formation history of hot subdwarf stars before their binary natures are confirmed.

astro-ph.SR↗

arXiv2019

Hot subdwarf stars identified in Gaia DR2 with spectra of LAMOST DR6 and DR7 I. Single-lined spectra

182 single-lined hot subdwarf stars are identified by using spectra from the sixth and seventh data release (DR6 and DR7) of the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) survey. We classified all the hot subdwarf stars using a canonical classification scheme, and got 89 sdB, 37 sdOB, 26 sdO, 24 He-sdOB, 3 He-sdO and 3 He-sdB stars, respectively. Among these stars, 108 hot subdwarfs are newly discovered, while 74 stars were reported by previous catalogs. The atmospheric parameters of these stars were obtained by fitting the hydrogen (H) and helium (He) lines with non-local thermodynamic equilibrium (non-LTE) model atmospheres. The atmospheric parameters confirm the two He sequences and the two subgroups of He-sdOB stars in our samples, which were found by previous studies in the T eff -log(nHe/nH) diagram. Our results demonstrate different origins of field hot subdwarf stars and extreme horizontal branch (EHB) stars in globular clusters (GCs), and provide strict observational limits on the formation and evolution models of the different sub-types of these evolved objects. Based on the results, we evaluated the completeness of the Geier et al. (2019) catalog. We found the fraction of hot subdwarf stars is between 10% and 60%, depending on the brightness of the sample. A more accurate estimation for the hot subdwarf fraction can be obtained when similar results from composite spectra will become available.

astro-ph.SR↗

arXiv2019

Searching for Hot Subdwarf Stars in LAMOST DR1-II. Pure spectroscopic identification method for hot subdwarfs

Employing a new machine learning method, named hierarchical extreme learning machine (HELM) algorithm, we identified 56 hot subdwarf stars in the first data release (DR1) of the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) survey. The atmospheric parameters of the stars are obtained by fitting the profiles of hydrogen (H) Balmer lines and helium (He) lines with synthetic spectra calculated from non-Local Thermodynamic Equilibrium (NLTE) model atmospheres. Five He-rich hot subdwarf stars were found in our sample with their log(nHe/nH) > -1 , while 51 stars are He-poor sdB, sdO and sdOB stars. We also confirmed the two He sequences of hot subdwarf stars found by Edelmann et al. (2003) in Teff - log(nHe/nH) diagram. The HELM algorithm works directly on the observed spectroscopy and is able to filter out spectral properties without supplementary photometric data. The results presented in this study demonstrate that the HELM algorithm is a reliable method to search for hot subdwarf stars after a suitable training is performed, and it is also suitable to search for other objects which have obvious features in their spectra or images.

astro-ph.SR↗

arXiv2019

New hot subdwarf stars identified in Gaia DR2 with LAMOST DR5 spectra

We selected 4593 hot subdwarf candidates from the Gaia DR2 Hertzsprung-Russell (HR) diagram. By combining the sample with LAMOST DR5, we identified 294 hot subdwarf stars, including 169 sdB, 63 sdOB, 31 He-sdOB, 22 sdO, 7 He-sdO and 2 He-sdB stars. The atmospheric parameters (e.g., T_{eff} , log g, log(nHe/nH)) are obtained by fitting the hydrogen (H) and helium (He) line profiles with synthetic spectra. Two distinct He sequences of hot subdwarf stars are clearly presented in the T_{eff} - log g diagram. We found that the He-rich sequence consists of the bulk of sdB and sdOB stars as well as all of the He-sdB, He-sdO and He-sdOB stars in our samples, while all the stars in the He-weak sequence belong to the sdO spectral type, combined with a few sdB and sdOB stars. We demonstrated that the combination of Gaia DR2 and LAMOST DR5 allows one to uncover a huge number of new hot subdwarf stars in our Galaxy.

astro-ph.SR↗

arXiv2019

New hot subdwarf stars identified in Gaia DR2 with LAMOST DR5 spectra -- II

388 hot subdwarf stars have been identified by using the Hertzsprung-Russell (HR) diagram built from the second data release (DR2) of the Gaia mission. By analyzing their observed LAMOST spectra, we characterized 186 sdB, 73 He-sdOB, 65 sdOB, 45 sdO, 12 He-sdO and 7 He-sdB stars. The atmospheric parameters of these stars (e.g., T eff , log g, log(nHe/nH)) are obtained by fitting the hydrogen (H) and helium (He) line profiles with synthetic spectra calculated from non-Local Thermodynamic Equilibrium (non-LTE) model atmospheres. Among these stars, we have 135 new identified hot subdwarfs which have not been cataloged before. Although 253 stars appear in the catalog by Geier et al. (2017) , but only 91 of them have atmospheric parameters. Together with the 294 hot subdwarf stars found by Lei et al. (2018), we identified 682 hot subdwarf stars in total by using the Gaia HR-diagram and LAMOST spectra. These results demonstrate the efficiency of our method to combine large surveys to search for hot subdwarf stars. We found a distinct gap in our He-sdOB stars based on their He abundance, which is also presented in extreme horizontal branch (EHB) stars of the globular cluster (GC) ω Cen. The number fraction of the sample size for the two sub-groups is very different between the two counterparts. However, the distinct gap between the H-sdB stars and He-sdOB stars in ω Cen is not visible in our sample. More interestingly, the He-sdB population with the highest He abundance in our sample is completely missing in ω Cen. The discrepancy between our field hot subdwarf stars and the EHB stas in ω Cen indicate different origins for the two counterparts.

astro-ph.SR↗

arXiv2019

Searching for hot subdwarf stars from the LAMOST spectra. III. classifying the hot subdwarf stars from LAMOST DR4 using deep learning method

Hot subdwarf stars are core He burning stars located at the blue end of the horizontal branch, also known as the extreme horizontal branch. The properties of hot subdwarf stars are important for our understanding of the stellar astrophysics, globular clusters and galaxies. The spectra of hot subdwarf stars can provide us with the detailed information of the stellar atmospheric parameters (such as effective temperature, gravity, and helium abundances), which is important to clarify the astrophysical and statistical properties of hot subdwarf stars. These properties can provide important constraint on the theoretical models of hot subdwarf stars. Searching for hot subdwarf stars from the spectra data obtained by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) can significantly enlarge the sample size of hot subdwarf stars, and help us better study the nature of hot subdwarf stars. In this paper we study a new method of searching for hot subdwarf stars from LAMOST spectra using convolutional neural networks and support vector machine (CNN+SVM). The experiment on the spectra from LAMOST DR4 shows that CNN+SVM can classify the hot subdwarf stars accurately: the accuracy is 88.98$\%$ and the recall is 94.38 $\%$. Our research provides a new machine learning tool for searching for hot subdwarf stars in large spectroscopic surveys.

astro-ph.IM↗

arXiv2016

A possible formation channel for blue hook stars in globular cluster - II. Effects of metallicity, mass ratio, tidal enhancement efficiency and helium abundance

Employing tidally enhanced stellar wind, we studied in binaries the effects of metallicity, mass ratio of primary to secondary, tidal enhancement efficiency and helium abundance on the formation of blue hook (BHk) stars in globular clusters (GCs). A total of 28 sets of binary models combined with different input parameters are studied. For each set of binary model, we presented a range of initial orbital periods that is needed to produce BHk stars in binaries. All the binary models could produce BHk stars within different range of initial orbital periods. We also compared our results with the observation in the Teff-logg diagram of GC NGC 2808 and ω Cen. Most of the BHk stars in these two GCs locate well in the region predicted by our theoretical models, especially when C/N-enhanced model atmospheres are considered. We found that mass ratio of primary to secondary and tidal enhancement efficiency have little effects on the formation of BHk stars in binaries, while metallicity and helium abundance would play important roles, especially for helium abundance. Specifically, with helium abundance increasing in binary models, the space range of initial orbital periods needed to produce BHk stars becomes obviously wider, regardless of other input parameters adopted. Our results were discussed with recent observations and other theoretical models.

astro-ph.SR↗

arXiv2015

A possible formation channel for blue hook stars in globular cluster

The formation mechanism for blue hook (BHk) stars in globular clusters (GCs) is still unclear. Following one of the possible scenario, named late hot flash scenario, we proposed that tidally enhanced stellar wind in binary evolution may provide the huge mass loss on the red giant branch (RGB) and produce BHk stars. Employing the detailed stellar evolution code, Modules for Experiments in Stellar Astrophysics (MESA), we investigated the contributions of tidally enhanced stellar wind as a possible formation channel for BHk stars in GCs. We evolved the primary stars with different initial orbital periods using the binary module in MESA (version 6208) from zero age main-sequence (ZAMS) to post horizontal branch (HB) stage, and obtained their evolution parameters which are compared with the observation. The results are consistent with observation in the color-magnitude diagram (CMD) and the logg-Teff plane for NGC 2808, which is an example GC hosting BHk stars. However, the helium abundance in the surface for our models is higher than the one obtained in BHk stars. This discrepancy between our models and observation is possibly due to the fact that gravitational settling and radiative levitation which are common processes in hot HB stars are not considered in the models as well as the fact that the flash mixing efficiency may be overestimated in the calculations. Our results suggested that tidally enhanced stellar wind in binary evolution is able to naturally provide the huge mass loss on the RGB needed for late hot flash scenario and it is a possible and reasonable formation channel for BHk stars in GCs.

astro-ph.SR↗

arXiv2014

Effect of Binary Fraction on Horizontal Branch Morphology under Tidally Enhanced Stellar Wind

Tidally enhanced stellar wind may affect horizontal branch (HB) morphology in globular clusters (GCs) by enhancing the mass loss of primary star during binary evolution. Lei et al. (2013a, 2013b) studied the effect of this kind of wind on HB morphology in details, and their results indicated that binary is a possible secondparameter (2P) candidate in GCs. Binary fraction is an very important fact in the tidally-enhanced-stellar-wind model. In this paper, we studied the effect of binary fraction on HB morphology by removing the effects of metallicity and age. Five different binary fractions (i.e., 10%, 15%, 20%, 30% and 50%) are adopted in our model calculations. The synthetic HB morphologies with different binary fractions are obtained at different metallicities and ages. We found that, due to the great influence of metallicity and age, the effect of binary fraction on HB morphology may be masked by these two parameters. However, when the effects of metallicity and age are removed, the tendency that HB morphologies become bluer with increasing of binary fractions is clearly presented. Furthermore, we compared our results with the observation by Milone et al. (2012). Our results are consistent well with the observation at metalrich and metal-poor GCs. For the GCs with intermediate metallicity, when the effect of age on HB morphology is removed, a weak tendency that HB morphologies become bluer with increasing of binary fractions is presented in all regions of GCs, which is consistent with our results obtained in this metallicity range.

astro-ph.SR↗