Gang Zhao

Peking University, Peping, Beijing, China

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Publications (127)257.46 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Context. White dwarf-main sequence (WDMS) binaries are used to study several different important open problems in modern astrophysics. Aims. The Sloan Digital Sky Survey (SDSS) identified the largest catalogue of WDMS binaries currently known. However, this sample is seriously affected by selection effects and the population of systems containing cool white dwarfs and early-type companions is under-represented.Here we search for WDMS binaries within the spectroscopic data release 1 of the LAMOST (Large sky Area Multi-Object fiber Spectroscopic Telescope) survey. LAMOST and SDSS follow different target selection algorithms. Hence, LAMOST WDMS binaries may be drawn from a different parent population and thus help in overcoming the selection effects incorporated by SDSS on the current observed population. Methods. We develop a fast and efficient routine based on the wavelet transform to identify LAMOST WDMS binaries containing a DA white dwarf and a M dwarf companion, and apply a decomposition/fitting routine to their LAMOST spectra to estimate their distances and measure their stellar parameters, namely the white dwarf effective temperatures, surface gravities and masses, and the secondary star spectral types. Results. We identify 121 LAMOST WDMS binaries, 80 of which are new discoveries, and estimate the sample to be \sim90 per cent complete. The LAMOST and SDSS WDMS binaries are found to be statistically different. However, this result is not due to the different target selection criteria of both surveys, but likely a simple consequence of the different observing conditions. Thus, the LAMOST population is found at considerably shorter distances (\sim50-450 pc) and is dominated by systems containing early-type companions and hot white dwarfs. (abridged)
    08/2014;
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    ABSTRACT: Strongly Mg-enhanced stars with [Mg/Fe] $>$ 1.0 show peculiar abundance patterns and hence are of great interest for our understanding of stellar formation and chemical evolution of the Galaxy. A systematical search for strongly Mg-enhanced stars based on the low-resolution $(R\simeq2000)$ spectra of the Sloan Digital Sky Survey (SDSS) is carried out by finding the best matched synthetic spectrum to the observed one in the region of Mg I b lines around $\lambda$5170\AA$~$via a profile matching method. The advantage of our method is that fitting parameters are refined by reproducing the [Mg/Fe] ratios of 47 stars from very precise high-resolution spectroscopic (HRS) analysis by Nissen and Schuster (2010); and these parameters are crucial to the precision and validity of the derived Mg abundances. As a further check of our method, Mg abundances are estimated with our method for member stars in four Galactic globular clusters (M92, M13, M3, M71) which cover the same metallicity range as our sample, and the results are in good agreement with those of HRS analysis in the literature. The validation of our method is also proved by the agreement of [Mg/Fe] between our values and those of HRS analysis by Aoki et al.(2013). Finally, 33 candidates of strongly Mg-enhanced stars with [Mg/Fe]$>$1.0 are selected from 14850 F and G stars. Follow-up observations will be carried out on these candidates with high-resolution spectroscopy by large telescopes in the near future, so as to check our selection procedure and to perform a precise and detailed abundance analysis and to explore the origins of these stars.
    05/2014;
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    ABSTRACT: With the rapid developments in the exoplanet field, more and more terrestrial exoplanets are being detected. Characterising their atmospheres using transit observations will become a key datum in the quest for detecting an Earth-like exoplanet. The atmospheric transmission spectrum of our Earth will be an ideal template for comparison with future exo-Earth candidates. By observing a lunar eclipse, which offers a similar configuration to that of an exoplanet transit, we have obtained a high resolution and high signal-to-noise ratio transmission spectrum of the Earth's atmosphere. This observation was performed with the High Resolution Spectrograph at Xinglong Station, China during the total lunar eclipse in December 2011. We compare the observed transmission spectrum with our atmospheric model, and determine the characteristics of the various atmospheric species in detail. In the transmission spectrum, O2, O3, O2-O2, NO2 and H2O are detected, and their column densities are measured and compared with the satellites data. The visible Chappuis band of ozone produces the most prominent absorption feature, which suggests that ozone is a promising molecule for the future exo-Earth characterization. The individual O2 lines are resolved and O2 isotopes are clearly detected. Our new observations do not confirm the absorption features of Ca II or Na I which have been reported in previous lunar eclipse observations. However, features in these and some other strong Fraunhofer line positions do occur in the observed spectrum. We propose that these are due to a Raman-scattered component in the forward-scattered sunlight appearing in the lunar umbral spectrum. Water vapour absorption is found to be rather weak in our spectrum because the atmosphere we probed is relatively dry, which prompts us to discuss the detectability of water vapour in Earth-like exoplanet atmospheres.
    05/2014;
  • The Astrophysical Journal Letters 03/2014; 784(2):L46. · 6.35 Impact Factor
  • Yinbi Li, Ali Luo, Gang Zhao, Youjun Lu
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    ABSTRACT: Hyper-velocity stars are believed to be ejected out from the Galactic center through dynamical interactions of (binary) stars with the central massive black hole(s). In this paper, we firstly select F and G type main sequence stars from about 370,000 stellar spectra of DR7. Then, we select 369 high velocity stars from main sequence samples using the radial velocity distribution. Finally, we find 13 possible unbound hyper-velocity star candidates from the 369 high velocity stars.
    12/2013;
  • Kefeng Tan, Gang Zhao
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    ABSTRACT: It is now generally believed that the Galaxy was formed through hierarchical merging, which means that different components of the Galaxy may have experienced different chemical evolution histories. Since alpha elements are mainly produced by core collapse supernovae, they are closely associated with the star formation history of the Galaxy. In this regard, Galactic components with different alpha elemental abundance patterns may show different behaviors in beryllium abundances since the production of beryllium is correlated with the cosmic rays and thus the supernovae. A recent study by Nissen & Schuster (2010) has revealed the existence of two distinct halo populations in the solar neighborhood based on the alpha elemental abundances and kinematics of 94 dwarf stars. We determined beryllium abundances for some of these stars and find systematic differences in beryllium abundances between these two halo populations. Our results consolidate the conclusion of two distinct halo populations in the solar neighborhood. Our results also show that beryllium abundance is a very good indicator of star formation rate, and could be used to trace the substructures of the Galactic halo.
    12/2013;
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    ABSTRACT: The Strömgren-Crawford (SC) intermediate-band photometric system is a very powerful and efficient one for the detailed study of stars, and therefore the Milky Way and local universe. However, due to the narrow bandwidth, low efficiency of detectors and serious atmospheric extinction in u band, and high photometric accuracy required, there was only one all sky survey in this system finished a decade ago, which is restricted to stars brighter than 8.3 mag in y (equal to V) in the solar neighborhood. In this context, it is the right time to carry out an all sky survey to a completeness depth of ~19 mag, equivalent to a volume-completed distance 4 kpc for solar-type stars. For stars brighter than 15 mag in V, the expected photometric accuracies are ~0.01mag. With these stars, high precision 3D extinction map can be obtained thanks to the β index. Stellar atmospheric parameters can be determined with accuracies comparable to those from high-resolution spectroscopy. Fundamental parameters like stellar luminosities and distances can be reliably estimated as well. We propose to use the Nanshan 1m telescope to start the survey in early 2014 in the Northern sky. There are several 1-m class telescopes in Chile which can be used to perform the Southern sky survey. This entire survey, when finished, will greatly improve our knowledge on stars and the Galaxy, and will provide us the first 6D map of the Milky Way together with the LAMOST survey and the Gaia mission.
    12/2013;
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    ABSTRACT: We present stellar parameters and abundances of 15 elements (Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, Y and Ba) for 23 field RR Lyrae variables based on high-resolution (R ~ 60 000) and high signal-to-noise (S/N~ 200) spectra obtained using the High Dispersion Spectrograph on the Subaru Telescope. Six stars in the sample have more than one spectrum observed at different pulsation phases. The derived abundance ratios of [X/Fe] for 14 elements (except for Ba) do not vary during the pulsation cycle. An interesting curve of [Ba/Fe] versus phase is detected for the first time and it shows decreasing [Ba/Fe] with increasing temperature at a given metallicity. Combining with data in the literature, abundances of most RR Lyrae stars as a function of [Fe/H] follow the same trends as those of dwarf stars, but [Sc/Fe] and [Y/Fe] ratios of RR Lyrae stars at solar metallicity are lower than those of dwarf stars. The kinematics of RR Lyrae stars indicate that three comparatively metal-rich RR Lyrae stars might originate from the thick disk and they show higher [α/Fe] ratios than RR Lyrae stars with thin disk kinematics. Among 23 RR Lyrae stars, two special objects are found with abnormal abundances; TV Lib has high [α/Fe], [Sc/Fe], [Y/Fe] and [Ba/Fe] ratios while TW Her has solar [α/Fe] but significantly lower [Sc/Fe], [Y/Fe] and [Ba/Fe] ratios as compared with other RR Lyrae stars.
    Research in Astronomy and Astrophysics 11/2013; 13(11):1307-1329. · 1.35 Impact Factor
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    ABSTRACT: We find that Galactic disk stars near the anticenter exhibit velocity asymmetries in both the Galactocentric radial and vertical components across the mid-plane as well as azimuthally. These findings are based on LAMOST spectroscopic velocities for a sample of ~400,000 F-type stars, combined with proper motions from the PPMXL catalog for which we have derived corrections to the zero points based in part on spectroscopically discovered galaxies and QSOs from LAMOST. In the region within 2 kpc outside the Sun's radius and +/-2 kpc from the Galactic midplane, we show that stars above the plane exhibit net outward radial motions with downward vertical velocities, while stars below the plane have roughly the opposite behavior. We discuss this in the context of other recent findings, and conclude that we are likely seeing the signature of vertical disturbances to the disk due to an external perturbation.
    The Astrophysical Journal 09/2013; 777(1). · 6.73 Impact Factor
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    ABSTRACT: The present paper investigates the influence of inner shell photoionization and photoexcitation on the Heα, i.e., the 1s2 to 1s2p transition in He-like ions, and the associated satellite spectra in photoionized plasmas. A comparison of the importance of these processes is made relative to other atomic processes as a function of the electron temperature and irradiation conditions. For the formation of the Heα and the satellite spectra, the K-shell photoionization is found to have significant contribution under low radiation temperature and/or intensity, when lithium- and beryllium-like ions have high abundance, but highly ionized H-like ions are rare.
    High Energy Density Physics 09/2013; 9(3):385–387. · 1.60 Impact Factor
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    ABSTRACT: We present a set of white dwarf-main sequence (WDMS) binaries identified spectroscopically from the Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST, also called the Guo Shou Jing Telescope) pilot survey. We develop a color selection criteria based on what is so far the largest and most complete Sloan Digital Sky Survey (SDSS) DR7 WDMS binary catalog and identify 28 WDMS binaries within the LAMOST pilot survey. The primaries in our binary sample are mostly DA white dwarfs except for one DB white dwarf. We derive the stellar atmospheric parameters, masses, and radii for the two components of 10 of our binaries. We also provide cooling ages for the white dwarf primaries as well as the spectral types for the companion stars of these 10 WDMS binaries. These binaries tend to contain hot white dwarfs and early-type companions. Through cross-identification, we note that nine binaries in our sample have been published in the SDSS DR7 WDMS binary catalog. Nineteen spectroscopic WDMS binaries identified by the LAMOST pilot survey are new. Using the 3$\sigma$ radial velocity variation as a criterion, we find two post-common-envelope binary candidates from our WDMS binary sample.
    The Astronomical Journal 08/2013; 146(4). · 4.97 Impact Factor
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    ABSTRACT: We present new quasars discovered in the vicinity of the Andromeda and Triangulum galaxies with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, also named the Guoshoujing Telescope, during the 2010 and 2011 observational seasons. Quasar candidates are selected based on the available Sloan Digital Sky Survey, Kitt Peak National Observatory 4 m telescope, Xuyi Schmidt Telescope Photometric Survey optical, and Wide-field Infrared Survey Explorer near-infrared photometric data. We present 509 new quasars discovered in a stripe of ~135 deg2 from M31 to M33 along the Giant Stellar Stream in the 2011 pilot survey data sets, and also 17 new quasars discovered in an area of ~100 deg2 that covers the central region and the southeastern halo of M31 in the 2010 commissioning data sets. These 526 new quasars have i magnitudes ranging from 15.5 to 20.0, redshifts from 0.1 to 3.2. They represent a significant increase of the number of identified quasars in the vicinity of M31 and M33. There are now 26, 62, and 139 known quasars in this region of the sky with i magnitudes brighter than 17.0, 17.5, and 18.0, respectively, of which 5, 20, and 75 are newly discovered. These bright quasars provide an invaluable collection with which to probe the kinematics and chemistry of the interstellar/intergalactic medium in the Local Group of galaxies. A total of 93 quasars are now known with locations within 25 of M31, of which 73 are newly discovered. Tens of quasars are now known to be located behind the Giant Stellar Stream, and hundreds are behind the extended halo and its associated substructures of M31. The much enlarged sample of known quasars in the vicinity of M31 and M33 can potentially be utilized to construct a perfect astrometric reference frame to measure the minute proper motions (PMs) of M31 and M33, along with the PMs of substructures associated with the Local Group of galaxies. Those PMs are some of the most fundamental properties of the Local Group.
    The Astronomical Journal 05/2013; 145(6):159. · 4.97 Impact Factor
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    ABSTRACT: We report new detections of thermal emission from the transiting hot Jupiter WASP-43b in the H and Ks-bands as observed at secondary eclipses. The observations were made with the WIRCam instrument on the CFHT. We obtained a secondary eclipse depth of 0.103$_{-0.017}^{+0.017}%$ and 0.194$_{-0.029}^{+0.029}%$ in the H and Ks-bands, respectively. The Ks band depth is consistent with previous measurement in the narrow band centered at 2.09um by Gillon et al. (2012). Our eclipse depths in both bands are consistent with a blackbody spectrum with a temperature of ~1850 K, slightly higher than the dayside equilibrium temperature without day-night energy redistribution. Based on theoretical models of the dayside atmosphere of WASP-43b, our data constrain the day-night energy redistribution in the planet to be $\lesssim 15-25$%, depending on the metal content in the atmosphere. Combined with energy balance arguments our data suggest that a strong temperature inversion is unlikely in the dayside atmosphere of WASP-43b. However, a weak inversion cannot be strictly ruled out at the current time. Future observations are required to place detailed constraints on the chemical composition of the atmosphere.
    The Astrophysical Journal 05/2013; 770(1). · 6.73 Impact Factor
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    ABSTRACT: The driving mechanism of solar flares and coronal mass ejections is a topic of ongoing debate, apart from the consensus that magnetic reconnection plays a key role during the impulsive process. While present solar research mostly depends on observations and theoretical models, laboratory experiments based on high-energy density facilities provide the third method for quantitatively comparing astrophysical observations and models with data achieved in experimental settings. In this article, we show laboratory modeling of solar flares and coronal mass ejections by constructing the magnetic reconnection system with two mutually approaching laser-produced plasmas circumfused of self-generated megagauss magnetic fields. Due to the Euler similarity between the laboratory and solar plasma systems, the present experiments demonstrate the morphological reproduction of flares and coronal mass ejections in solar observations in a scaled sense, and confirm the theory and model predictions about the current-sheet-born anomalous plasmoid as the initial stage of coronal mass ejections, and the behavior of moving-away plasmoid stretching the primary reconnected field lines into a secondary current sheet conjoined with two bright ridges identified as solar flares.
    High Power Laser Science and Engineering. 03/2013; 1(01).
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    ABSTRACT: We selected red horizontal-branch stars along the streams of the Sagittarius dwarf galaxy (Sgr) from sdss DR7 spectroscopic data using a theoretical model. We investigate the metallicity distribution of stars in the Sgr streams. The metallicity gradient is (-1.8 +/- 0.3) × 10⊃3 dex deg-1 in the first wrap of the trailing arm and (-1.5 +/- 0.4) × 10-3 dex deg-1 in the first wrap of the leading arm.
    Proceedings of the International Astronomical Union 02/2013; 8(S289):422-424.
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    ABSTRACT: Using collisional-radiative model codes, we calculate the ionic charge state distributions of Fe plasma produced in a Z-pinch experiment. The influence of the dilution factor (i.e., geometrical reduction factor) α on the simulated ionic charge state distributions is investigated. With the newly diagnosed plasma conditions, involving the dilution factor α and electron temperatures, the obtained ion ratios of Na and F as well as the ionic charge state distributions and average charge state < Z> of Fe are consistently in good agreement with experimental measurements for the first time.
    Journal of the Physical Society of Japan 02/2013; 82(2):4501-. · 2.09 Impact Factor
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    ABSTRACT: We present a close-coupling R-matrix calculation of the electron impact excitation of N-like Ca XIV. GRASP (general-purpose relativistic atomic structure package) was used to obtain the target states. Level energies are reported for the 272 levels of the target ion. The oscillator strengths and radiative rates were calculated for E1, E2, M1, and M2 transitions among these levels. In order to determine the collision strength, the lowest 84 levels were used as the target model, together with 40 partial waves included in the R-matrix calculation. The effective collision strengths were obtained by integrating the collision strengths over a Maxwellian electron energies distribution. Comparisons were made with our FAC and the LB results by Landi and Bhatia (2005, At. Data Nucl. Data Tables, 90, 177). Good agreements are shown for level energies, oscillator strengths and radiative rates, respectively. As to the collision strength, a mass of resonances appears in the R-matrix results, which are important in the determination of effective collision strengths. The discrepancies of the effective collision strengths between results from the R-matrix and from FAC are discussed. Finally, the effective collision strengths for transitions among the first 84 levels are reported. To test our results, we also compared the predicted line ratio with the observed values.
    Publications- Astronomical Society of Japan 12/2012; · 2.44 Impact Factor
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    ABSTRACT: We present an online catalog of distance determinations for 4781 K giants, most of which are members of the Milky Way's stellar halo. Their spectra from SDSS/SEGUE provide metallicities with accuracies \Delta [Fe/H]\approx\pm0.2 dex and giant-dwarf distinction. The distance moduli are derived from a comparison of each star's apparent magnitude with the absolute magnitude of empirically calibrated color-luminosity fiducials, at the observed (g-r)_0 color and spectroscopic [Fe/H]. We employ a probabilistic approach that makes it straightforward to propagate the errors in metallicities, magnitudes, and colors properly into distance uncertainties. We also fold in prior information about the giant-branch luminosity function and different metallicity distributions of the SEGUE K-giant targeting sub-categories. We show that the metallicity prior plays little role in the distance estimates, but that neglecting the luminosity prior would lead to a systematic distance modulus bias of up to 0.2 mag. We find a median distance precision of 12%, with distance estimates most precise for the least metal-poor stars near the tip of the red-giant branch. We use globular and open clusters to verify the precision and accuracy of our distance estimates. The stars in our publicly available catalog are up to 110 kpc distant from the Galactic center, with 270 stars beyond 50 kpc, forming the largest sample of distant tracers in the Galactic halo.
    The Astrophysical Journal 11/2012; · 6.73 Impact Factor
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    ABSTRACT: We examine the metallicity distribution of the Galactic thick disk using F, G, and K dwarf stars selected from the Sloan Digital Sky Survey, Data Release 8. Using the large sample of dwarf stars with proper motions and spectroscopically determined stellar parameters, metallicity gradients in the radial direction for various heights above the Galactic plane and in the vertical direction for various radial distances from the Galaxy center have been found. In particular, we find a vertical metallicity gradient of -0.113 +/- 0.010 (-0.125 +/- 0.008) dex/kpc using an isochrone (photometric) distance determination in the range 1 < |Z| < 3 kpc, which is the vertical height range most consistent with the thick disk of our Galaxy. In the radial direction, we find metallicity gradients between +0.02 and +0.03 dex/kpc for bins in the vertical direction between 1 < |Z| < 3 kpc. Both of these results agree with similar values determined from other populations of stars, but this is the first time a radial metallicity gradient for the thick disk has been found at these vertical heights. We are also able to separate thin and thick disk stars based on kinematic and spatial probabilities in the vertical height range where there is significant overlap of these two populations. This should aid further studies of the metallicity gradients of the disk for vertical heights lower than those studied here but above the solar neighborhood. Metallicity gradients in the thin and thick disks are important probes into possible formation scenarios for our Galaxy and a consistent picture is beginning to emerge from results using large spectroscopic surveys, such as the ones presented here.
    The Astronomical Journal 10/2012; 144(6). · 4.97 Impact Factor
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    ABSTRACT: The aim of the present paper is to investigate the influence of inner-shell photoionization and photoexcitation on He{sub {alpha}} and its satellite's spectra in photoionized plasmas. An analysis is carried out on the relative importance of the various atomic processes in photoionized plasmas as a function of the electron temperature and irradiation conditions. In particular, we investigate the influence of K-shell photoionization of Li-like ions on the He{sub {alpha}} spectrum and of Be-like ions on the He{sub {alpha}} satellites. It is found that in photoionized plasmas these inner-shell processes contribute significantly under low radiation temperature and/or intensity, when Li- and Be-like ions are highly abundant but highly ionized H-like ions are rare. A short discussion is presented about the parameter space in which the excited 1s2p state has statistical or non-statistical distributions, and how such distributions affect the emission spectrum.
    The Astronomical Journal 10/2012; 144(4). · 4.97 Impact Factor

Publication Stats

317 Citations
257.46 Total Impact Points

Institutions

  • 2014
    • Peking University
      • Department of Astronomy
      Peping, Beijing, China
  • 2001–2013
    • Chinese Academy of Sciences
      • • Purple Mountain Observatory
      • • Shanghai Astronomical Observatory
      Peping, Beijing, China
    • China Academy of Chinese Medical Sciences
      Peping, Beijing, China
  • 2012
    • The Astronomical Observatory of Brera
      Merate, Lombardy, Italy
  • 2010–2012
    • Northeast Institute of Geography and Agroecology
      • Shanghai Astronomical Observatory
      Peping, Beijing, China
  • 2011
    • Tokyo Institute of Technology
      • Earth and Planetary Sciences Department
      Edo, Tōkyō, Japan