arXiv subjects

Gang Zhao

Publications and source records attributed to Gang Zhao.

At least 55 records · Page 3Linked to original sources

arXiv2025

Mock Observations for the CSST Mission: CPI-C -- Instrument Simulation

To support the development of the data processing pipeline and the scientific performance assessment for the Cool Planet Imaging Coronagraph (CPI-C) on the Chinese Space Station Survey Telescope (CSST), we have developed the end-to-end instrument simulation program, CPISM. This paper details the core modules of CPISM that simulate the CPI-C instrument, focusing on the simulation of the high-contrast imaging optical system and the visible-band science camera. We modeled key optical components, such as the transmission apodizing filter, the wavefront corrector, and the focal plane mask using the HCIPy package. A $10^{-8}$ contrast dark hole region, consistent with design specifications, was simulated using the Electric Field Conjugation (EFC) optimization method, and broadband observation effects were considered. For the science camera, which is an electron multiplying charge-coupled device (EMCCD), we established a detailed model encompassing photon collection, charge transfer, electron multiplication (EM), and readout processes, based on test data. This model simulates complex instrumental features including dark current, charge transfer efficiency, clock-induced charge, multiplication noise factor, and various readout effects like striping and drift. We also proposed and validated an improved statistical model for the EM process to enhance simulation efficiency. CPISM can generate simulated images containing rich instrumental details, closely similar to the expected real observational data, thus laying the foundation for the development and verification of CPI-C data processing algorithms and preparations for future scientific research.

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arXiv2025

Mock Observations for the CSST Mission: CPI-C -- Targets for High Contrast Imaging

We introduce CPISM, a simulation program developed for the Cool Planet Imaging Coronagraph (CPI-C) on the China Space Station Telescope (CSST). CPISM supports high-contrast exoplanet imaging by simulating observational conditions and instrumental effects to optimize target selection and observation strategies. The modular design includes target modeling, imaging simulation, observational effects, detector response, and data product generation modules, enabling flexible and realistic synthetic observations. Validation through simulations of a bright star shows strong agreement with theoretical expectations, confirming the program's accuracy. CPISM's modular design allows flexibility, accommodating different stellar and planetary models, and can simulate instrumental noise, cosmic rays, and other observational effects. This tool aids in data processing, signal-to-noise ratio analysis, and high-contrast photometry, contributing to future exoplanet discovery and characterization efforts. The program's outputs will enhance observation planning and scientific return for the CPI-C mission, providing critical insights into exoplanetary systems.

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arXiv2025

Milky Way Mass Through Escape Velocity Curve from LAMOST K Giants

Escape velocity has long been used to constrain the mass of the Dark Matter (DM) halo in the Milky Way (MW). Here we present a study of the escape velocity curve using a sample of high-velocity K giants with full 6D phase-space information and relatively good quality, selected from LAMOST DR8 and cross-matched with Gaia DR3. To expand the high-velocity stars to larger distances, we used radius-dependent criteria of total velocity, that is, $v_{\rm GC}>300\;\rm{km s^{-1}}$ for the solar neighborhood; $v_{\rm GC}>v_{\rm min}\sim0.6\times v_{\rm esc}(r_{\rm GC})$ for outer region. We also selected halo stars based on $v_ϕ-{\rm [Fe/H]}$ information to ensure that the sample is isotropic. We modeled the velocity distribution with traditional power-law models to determine the escape velocity in each radial bin. For the first time, we have directly measured a relatively continuous escape velocity curve that can extend to Galactocentric radii of $\sim 50$ kpc, finding a decline in agreement with previous studies. The escape velocity at the solar position yielded by our measurements is $523.74^{+12.83}_{-13.47}\;\rm{km s^{-1}}$. Combined with the local circular velocity, we estimated the mass of the MW assuming a Navarro-Frenk-White DM profile, which resulted in a total mass of $M_{200,\;\rm{total}}=0.90_{-0.07}^{+0.06}\times 10^{12}\;M_{\odot}$, with a concentration of $c_{200}=13.47_{-1.70}^{+1.85}$. The small uncertainty implies that including the escape velocities beyond the solar neighborhood can result in a more precise mass estimate. Our derived MW mass is consistent with some recent studies using the escape velocity as well as other tracers, which may support a lower mass of the DM halo than in the past.

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arXiv2025

Where are the Population III star relics in the simulated Milky Way analogues?

Using 6 Milky Way analogues with two different numerical resolutions from the Auriga simulation, we investigate the total mass, spatial distribution and kinematics of the Population III star relics in the Milky Way analogues at $z=0$. These relics (primarily second generation stars) formed over a wide redshift range, from about $z=22$ to $z=4$, with an average formation redshift of $z \sim 10.0$, and comprise about $2\times10^{-5}$ of the entire galactic stellar population. The disk and bulge components host only a small fraction of these relics, contributing less than $12$ percent in total. The stellar halo, in particular the outer stellar halo of which galactic radius $r>30$ kpc, hosts the largest fraction (about 46 percent on average), with an average of one relic star for per $4,000$ to $10,000$ stars, making it a promising region for observational searches. Additionally, around $18$ percent of the Population III star relics are found in satellite galaxies, with smaller and older satellite galaxies tending to contain a higher proportion of these stars. Thus, low-mass and early-formed satellite galaxies are also ideal targets for finding such relics, although some satellite galaxies may lack them entirely. The spatial distribution and kinematics of these stars show good numerical convergence across different simulation resolutions. Our results provide valuable guidance for searches of the Population III star relics and offer insights for interpreting findings from ongoing and future stellar archaeology surveys.

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arXiv2025

Abundance Pattern Fitting with Bayesian Inference: Constraining First Stars' Properties and Their Explosion Mechanism with Extremely Metal-poor Stars

The abundance patterns of extremely metal-poor stars preserve a fossil record of the Universe's earliest chemical enrichment by the supernova explosions from the evolution of first generation of stars, also referred to as Population III (or Pop III). By applying Bayesian inference to the analysis of abundance patterns of these ancient stars, this study presents a systematic investigation into the properties and explosion mechanism of Pop III stars. We apply NLTE corrections to enhance the reliability of abundance measurements, which significantly reduces the discrepancies in abundances between observations and theoretical yields for odd-Z elements, such as Na and Al. Our Bayesian framework also enables the incorporation of explodability and effectively mitigates biases introduced by varying resolutions across different supernova model grids. In addition to confirming a top-heavy ($α=0.54$) initial mass function for massive Pop III stars, we derive a robust mass--energy relation ($E\propto M^2$) of the first supernovae. These findings demonstrate that stellar abundance analysis provides a powerful and independent approach for probing early supernova physics and the fundamental nature of the first stars.

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arXiv2025

Direct Imaging for the Debris Disk around $ε$ Eridani with the Cool-Planet Imaging Coronagraph

We analyze the inner debris disk around $ε$ Eridani using simulated observations with the Cool-Planet Imaging Coronagraph (CPI-C). Using the radiative transfer code MCFOST, we generate synthetic scattered-light images and spectral energy distributions for three disk models that differ in inclination and radial extent, and compare these results with the anticipated performance of CPI-C. CPI-C can resolve disk structures down to $\sim$3 au, offering substantially finer spatial resolution than existing HST/STIS and Spitzer/IRS observations. Recovered inclinations and radial extents closely match the input models, constraining the disk geometry and informing potential planet-disk interactions in the $ε$ Eri system. Although the cold Jupiter-like planet $ε$ Eri b is not detected in our simulations, polarimetric methods may enable detection of its reflected light. These results highlight the capability of next-generation coronagraphs to probe cold dust in nearby planetary systems.

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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.

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arXiv2025

Unveiling Galactic substructures with M Giant stars: A kinematic and chemical study based on LAMOST DR9, Gaia DR3 and APOGEE DR17

Based on the updated M giant star catalog selected from LAMOST DR9, we iden-tify substructures within the integrals-of-motion space through Friends-of-Friends cluster-ing algorithm. We obtain members belonging to several known substructures: the Sagittarius stream, Galactic Anticenter Substructure (GASS), Gaia-Enceladus-Sausage (GES), Splash, and the high-α disk. Furthermore, we also identify two groups which cannot be clearly asso-ciated with previously known substructures. Our findings confirm the existence of metal-rich constituents within the GES, representing newly formed stars that originated from the metal-enriched gas delivered during the GES merger event and subsequently evolved. Additionally, this study further expands the sample of GASS, high-α disk, and Splash stars. Analysis of these metal-rich M giant stars as members of the GES, Splash, and high-α disk compo-nents supports an evolution scenario for the early Milky Way, as proposed by previous stud-ies. In this scenario, stars initially formed in a high-α primordial disk were dynamically heated by the massive accretion event (GES). This process redistributed stellar orbits, creat-ing the Splash population, while the undisturbed portion of the primordial disk persisted as the present-day high-α disk component.

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arXiv2025

Looking for Signs of Unresolved Binarity in the Continuum of LAMOST Stellar Spectra

We describe an attempt to derive the binarity rate of samples of 166 A-, F-, G-, and K-type stars from LAMOST DR5 and 1000 randomly selected presumably single stars from Gaia DR3 catalogs. To this end, we compared continua of the observed spectra with the continua of synthetic spectra in the 3700 to 9097 Angstrom range. The latter spectra were reduced to the LAMOST set of wavelengths, while the former ones were smoothed. Next, we searched for every observed star the nearest synthetic spectrum using a four-parameter representation - effective temperature, gravity, [Fe/H], and a range of interstellar absorption values. However, rms deviations of observed spectra from synthetic ones appeared to be not sufficient to claim that any of the stars is a binary. We conclude that comparison of the intensity of pairs of spectral lines remains the best way to detect binarity.

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arXiv2025

Asteroseismology of Carbon-Deficient Red Giants: Merger Products of Hierarchical Triple Systems?

Carbon-deficient giants (CDGs) are a rare and chemically peculiar class of stars whose origins remain under active investigation. We present an asteroseismic analysis of the entire known CDG population, selecting 129 stars observed by $Kepler$, K2, and TESS to obtain seismic constraints. We detect solar-like oscillations in 43 CDGs. By measuring $ν_{\rm max}$ and applying seismic scaling relations, we determine precise masses for these stars, finding that 79\% are low-mass ($M \lesssim 2~M_\odot$). The luminosity distribution is bimodal, and the CDGs separate into three chemically and evolutionarily distinct groups, characterized by clear trends in sodium and CNO abundances, $α$-element enhancement, and kinematics. We find that two of these groups are only distinguished by their initial $α$-element abundances, thus effectively reducing the number of groups to two. Lithium enrichment is common across all groups, linking CDGs to lithium-rich giants and suggesting a shared evolutionary origin. We find that spectroscopic $\log g$ is systematically offset from seismic values. Group~1 CDG patterns are most consistent with formation through core He-flash mixing, while the more massive and more chemically processed Groups~2 and 2$α$ likely formed through mergers involving helium white dwarfs, possibly in hierarchical triples. Pollution from AGB stars appears very unlikely, given the unchanged [C+N+O] abundance across all groups.

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arXiv2025

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

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

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arXiv2025

Identification of OB associations using the LAMOST-Gaia OB star sample

OB associations, as an intermediate stage between Galactic clusters and field stars, play an important role in understanding the star formation process, early stellar evolution, and Galactic evolution. In this work, we construct a large sample of OB stars with 6D phase space parameters ($l, b, d, V_{\rm los}, pmra ,pmdec$) by combining the distances from Bailer-Jones et al. (2021), radial velocities derived from low-resolution spectra of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), and proper motions from the \textit{Gaia} Data Release 3 (DR3). This sample includes 19,933 OB stars, most of which are located within 6\,kpc of the Sun. Using 6D phase space parameters and friends-of-friends clustering algorithm, we identify 67 OB associations and 112 OB association candidates, among them, 49 OB associations and 107 OB association candidates are newly identified. The Galactic rotation curve derived using 67 OB association members is relatively flat in the range of Galactocentric distances 7$<$$R$$<$13\,kpc. The angular rotation velocity at solar Galactocentric distance of $R_\odot$ =8.34\,kpc is $Ω_0$ = 29.05$\pm$0.55\,km\,s$^{-1}$\,kpc$^{-1}$. The spatial distribution of the 67 OB associations indicates that they are mainly located at low Galactic latitudes and near spiral arms of the Milky Way. Additionally, we estimate the velocity dispersions and sizes of these 67 OB associations. Our results show that the velocity dispersions decrease as Galactocentric distances increase, while their sizes increase as Galactocentric distances increase.

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arXiv2025

Li-enrichment in red clump giants: Clues for past binary interaction or merger events

To understand the underlying mechanisms for high lithium abundances among core He-burning or red clump (RC) giants, we analyzed a sample of 5227 RC giants of mass M $\leq$ 2~M$_{\odot}$ using spectra and asteroseismic data. We found 120 RC giants ($\sim$2~$\%$) with a lower limit of A(Li) = 0.7~dex, a factor of 40 more than their predecessors close to the RGB tip. Of the 120 RC giants, we could measure actual rotations for 16 RC giants using stellar spots from the Kepler light curve analysis. We found that most of the high rotation RC giants are also very high Li-rich RC giants, and the rotation seems to decline rapidly with Li abundance depletion, suggesting that both the high rotation and high Li abundance are transient phenomena and associated with a single source. Further, we found a significantly high occurrence of 15~$\%$ and 12~$\%$ of Li-rich RC giants among extremely low-mass RC giants and RC giants with anomalous [C/N] ratios, respectively. The extremely low mass, fast rotation and anomalous [C/N] values of RC giants are attributed to their past binary interaction/merger history. The results pose a question of whether the binary interaction/merger is a prerequisite along with the He-flash for Li-enhancement among RC giants.

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arXiv2025

Chemical abundances of seven stars in the GD-1 stream

We present the first detailed chemical abundances for seven GD-1 stream stars from Subaru/HDS spectroscopy. Atmospheric parameters were derived via color calibrations ($T\rm_{eff}$) and iterative spectroscopic analysis. LTE abundances for 14 elements ($α$, odd-Z, iron-peak, n-capture) were measured. Six stars trace the main orbit, one resides in a `blob'. All exhibit tightly clustered metallicities ([Fe/H] = -2.38, {\bf intrinsic dispersion smaller than 0.05 dex, average uncertainty is about 0.13 dex}). While one star shows binary mass transfer signatures, the other six display consistent abundance patterns (dispersions $<$ uncertainties). Their iron-peak elements (Sc, Cr, Mn, Ni) match Milky Way halo stars. In contrast, Y and Sr are systematically lower than halo stars of similar [Fe/H]. Significantly, six stars show consistently enhanced [Eu/Fe] $\sim$ 0.60 ($σ$ = 0.08). A tight Ba-Eu correlation (r = 0.83, p=0.04) exists, with [Ba/Fe] = -0.03 $\pm$ 0.05, indicating a common r-process origin. This extreme chemical homogeneity strongly supports an origin from a single disrupted globular cluster. The lack of light-element anti-correlations may stem from our sample size or the progenitor's low mass.

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arXiv2025

Chasing the formation history of the Galactic metal-poor disc

In our previous work, we identified $\sim100,000$ metal-poor stars ([Fe/H] $<$ -1.0) from the LAMOST Survey. This work estimates their chemical abundances and explores the origin and evolution of the Galactic metal-poor disc. Our chemo-dynamical analysis reveals four main populations within the metal-poor disc: (1) a primordial disc older than 12 Gyr with [Fe/H] $>$ -1.5; (2) debris stars from the progenitor galaxy of Gaia-Sausage-Enceladus (GSE), but now residing in the Galactic disc; (3) the metal-poor tail of the metal-rich, high-$α$ disc formed 10-12 Gyr ago, with metallicity lower limit extending to -2.0; (4) the metal-poor tail of the metal-rich, low-$α$ disc younger than 8 Gyr, reaching a lower metallicity limit of -1.8. These results reveal the presence of a primordial disc and show that both high-$α$ and low-$α$ discs reach lower metallicities than previously thought. Analysis of merger debris reveals that Wukong, with extremely low metallicity, likely originate from merger events distinct from GSE. Additionally, three new substructures are identified: ShangGu-1, characterized by unusual [Fe/H]-eccentricity correlations; ShangGu-2, possibly heated disc stars; and ShangGu-3, which can be divided into four subgroups based on differing orbital directions, with two aligning with the previously known Nyx and Nyx-2.

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arXiv2025

The Rotating Bulge and Halo in the Milky Way: Evidence of Angular Momentum Transferred from the Decelerating Bar

Recent observations indicate that both the Milky Way bulge and inner halo exhibit angular momentum, although the origin and evolution of this prograde signature remain ambiguous. One plausible scenario involves secular evolution induced by the central bar and spiral arms. In this study, we identified a component consisting of 1,175,737 stars with net rotation through the application of a neural network (NN) method. To investigate the composition of this rotating sample and the origin of its rotation, we conducted a test particle simulation incorporating an equilibrium axisymmetric background potential together with a central decelerating bar. The test particles were generated using a distribution function (DF) model derived from observational constraints. Our results indicate that the decelerating bar transfers angular momentum to the pseudo-stars, and the rotational profile from our simulation shows strong agreement with observational data. These findings suggest that the rotating sample identified by our NN model predominantly comprises bulge, halo, and thick disk stars, and that the central decelerating bar is pivotal in shaping the inner Galaxy's kinematics through angular momentum transfer.

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arXiv2025

Main-sequence Turnoff Stars as Probes of the Ancient Galactic Relic: Chemo-dynamical Analysis of a Pilot Sample

The main-sequence turnoff (MSTO) stars well preserve the chemical properties where they were born, making them ideal tracers for studying the stellar population. We perform a detailed chemo-dynamical analysis on moderately metal-poor ($-2.0<\mathrm{[Fe/H]}<-1.0$) MSTO stars to explore the early accretion history of the Milky Way. Our sample includes four stars observed with high-resolution spectroscopy using CFHT/ESPaDOnS and 163 nearby MSTO stars selected from the SAGA database with high-resolution results. Within the action-angle spaces, we identified Gaia-Sausage-Enceladus (GSE, 35), stars born in the Milky Way (in situ, 31), and other substructures (21). We find that both GSE and in-situ stars present a similar Li plateau around $A(\mathrm{Li)}\sim 2.17$. GSE shows a clear $α$-knee feature in Mg at $\mathrm{[Fe/H]}\sim-1.60\pm 0.06$, while the $α$-elements of in-situ stars remain nearly constant within the metallicity range. The iron-peak elements show little difference between GSE and in-situ stars except for Zn and Ni, which decrease in GSE at $\mathrm{[Fe/H]}>-1.6$, while they remain constant in in-situ stars. Among heavy elements, GSE shows overall enhancement in Eu, with [Ba/Eu] increasing with the metallicity, while this ratio remains almost constant for in-situ stars, suggesting the contribution of longer time-scale sources to the $s$-process in GSE. Moreover, for the first time, we present the $r$-process abundance pattern for an extremely $r$-process enhanced ($r$-II) GSE star, which appears consistent with the solar $r$-process pattern except for Pr. Further investigation of larger GSE samples using high-resolution spectra is required to explore the reason for the significantly higher Pr in the GSE r-II star.

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arXiv2025

The Shape and Mass of the Galactic Dark Matter Halo from the Axisymmetric Jeans Model

We explore the density profile, shape, and virial mass of the Milky Way's dark matter halo using K giants (KG) from LAMOST and SDSS/SEGUE, as well as blue horizontal branch (BHB) stars from SDSS. Incorporating Gaia DR3 proper motions, we first investigate the velocity ellipsoid distribution within the $(R, |z|)$ space. The ellipsoids projected onto the $(v_R, v_z)$ plane exhibit near-spherical alignment. We then probe the underlying dark matter distribution using the axisymmetric Jeans equations with multi-Gaussian expansion (MGE) and the spherically aligned Jeans anisotropic modelling (JAM${\rm sph}$), allowing for different flattened dark matter density models. For each model, we apply two fitting approaches: fitting the KGs and BHBs separately or fit them simultaneously as two dynamical tracers in one gravitational potential. We find consistent results on the dark matter density profiles, $r_{200}$, and $M_{200}$ within a 1-$σ$ confidence region for models constrained by KGs, BHBs, and both. We find the strongest consistency between KGs and BHBs in constraining dark matter profiles for models incorporating radially varying halo flattening ($q(r_{\rm gc})$), which suggests the Milky Way's dark matter halo shape evolves with Galactocentric distance ($r_{\rm gc}$). Specifically, the halo flattening parameter $q_h$ decreases within $r_{\rm gc} < 20$ kpc and increases for $r_{\rm gc} > 20$ kpc. In this model, $M_{\rm tot} (< 60~{\rm kpc}) = 0.533^{+0.061}_{-0.054} \times 10^{12}$ $M_{\odot}$, $r_{200}$ is $188\pm15$ kpc, with $M_{200}$ estimated at $0.820^{+0.210}_{-0.186} \times 10^{12} M_{\odot}$.

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