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A search for strongly Mg-enhanced stars from the Sloan Digital Sky Survey

Research in Astronomy and Astrophysics 2014年第11期14卷 1423-1437页

作者： Xiang Li Gang Zhao yu-qin chen Hai-Ning Li Key Laboratory of Optical Astronomy National Astronomical ObservatoriesChinese Academyof Sciences University of Chinese Academy of Sciences

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 systematic search for strongly Mg-enhanced stars based on low-resolution （R≈2000） spectra from the Sloan Digital Sky Survey （SDSS） is carried out by finding the synthetic spectrum that best matches the observed one in the region of Mg I b lines around λ5170 ,A 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 the very precise high-resolution spectroscopic （HRS） analysis by Nissen ＆ Schuster; and these parameters are crucial to the precision and validity of the derived Mg abundances. As a further check of our method, Mg abun- dances are estimated with our method for member stars in four Galactic globular clus- ters （M92, M 13, 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 val- idation of our method is also demonstrated by the agreement of [Mg/Fe] between our values and those of HRS analysis by Aoki et al. Finally, 33 candidates of strongly Mg- enhanced stars with [Mg/Fe]〉 1.0 are selected from 14 850 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.

关键词： stars abundance -- stars chemically peculiar -- method data analysis-- techniques spectroscopic analysis

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An Analysis of the Condensation Temperature of Elements of Extrasolar Planetary Systems

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Chinese Journal of Astronomy and Astrophysics 2005年第6期5卷 619-626页

作者： Cong Huang Gang Zhao Hua-Wei Zhang yu-qin chen Department of Astronomy Peking University Beijing 100871 National Astronomical Observatories Chinese Academy of Sciences Beijing 100012

Using high signal-to-noise ratio spectra of extrasolar planet-hosting stars, we obtained the atmospheric parameters, accurate metallicities and the differential abundance for 15 elements （C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Ni and Ba）. In a search for possible signatures of metal-rich material accreting onto the parent stars, we found that, for a given element, there is no significant trend of increasing [X/H] with increasing condensation temperature To. In our sample of planet-harboring stars, the volatile and refractory elements behave similarly, and we can not confirm if there exists any significant dependence on the condensation temperature Tc.

关键词： planetary systems - stars abundance - stars late-type

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Partitioning the Galactic halo with Gaussian Mixture Models

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Research in Astronomy and Astrophysics 2021年第5期21卷 274-286页

作者： Xi-Long Liang yu-qin chen Jing-Kun Zhao Gang Zhao School of Astronomy and Space Science University of Chinese Academy of SciencesBeijing 100049China CAS Key Laboratory of Optical Astronomy National Astronomical ObservatoriesChinese Academy of SciencesBeijing 100101China

The Galactic halo is supposed to form from merging with nearby dwarf galaxies.In order to probe different components of the Galactic halo,we have applied the Gaussian Mixture Models method to a selected sample of metal poor stars with[Fe/H]<−0.7 dex in the APOGEE DR16 catalogue based on four-parameters,metallicity,[Mg/Fe]ratio and spatial velocity(VR,V).Nine groups are identified with four from the halo(group 1,3,4 and 5),one from the thick disk(group 6),one from the thin disk(group 8)and one from dwarf galaxies(group 7)by analyzing their distributions in the([M/H],[Mg/Fe]),(VR,V),(Zmax,eccentricity),(Energy,Lz)and([Mg/Mn],[Al/Fe])coordinates.The rest of the two groups are respectively caused by observational effect(group 9)and the cross section component(group 2)between the thin disk and the thick disk.It is found that in the extremely outer accreted halo(group 1),stars born in the Milky Way cannot be distinguished from those accreted from other galaxies either chemically or kinematically.In the intermediate metallicity of−1.6<[Fe/H]<−0.7 dex,the accreted halo is mainly composed of the Gaia-Enceladus-Sausage substructure(group 5),which can be easily distinguished from group 4(the in-situ halo group)in both chemical and kinematic space.Some stars of group 4 may come from the disk and some disk stars can be scattered to high orbits by resonant effects as shown in the Zmax versus Energy coordinate.We also displayed the spatial distribution of main components of the halo and the ratio of accreted components do not show clear relation to the Galactic radius.

关键词： Galaxy:abundances Galaxy:kinematics Galaxy:structure

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Abundances of 23 field RR Lyrae stars

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Research in Astronomy and Astrophysics 2013年第11期13卷 1307-1329页

作者： Shu Liu Gang Zhao yu-qin chen Yoichi Takeda Satoshi Honda Key Laboratory of Optical Astronomy National Astronomical Observatories Chinese Academy of Sciences Graduate University of Chinese Academy of Sciences National Astronomical Observatory of Japan 2-21-1 Osawa Nishi-Harima Astronomical Observatory Center for Astronomy University of Hyogo

We present stellar parameters and abundances of 15 elements （Na, Mg, A1, 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 [a/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 [a/Fe], [Sc/Fe], [Y/Fe] and [Ba/Fe] ratios while TW Her has solar [a/Fe] but significantly lower [Sc/Fe], [Y/Fe] and [Ba/Fe] ratios as compared with other RR Lyrae stars.

关键词： techniques： spectroscopic -- stars： abundances -- stars： variables： RRLyrae

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Stellar Abundance and Galactic Chemical Evolution through LAMOST Spectroscopic Survey

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Chinese Journal of Astronomy and Astrophysics 2006年第3期6卷 265-280页

作者： Gang Zhao yu-qin chen Jian-Rong Shi Yan-Chun Liang Jin-Liang Hou Li chen Hua-Wei Zhang Ai-Gen Li National Astronomical Observatories Chinese Academy of Sciences Beijing 100012 Shanghai Astronomical Observatory Chinese Academy of Sciences Shanghai 200030 Department of Astronomy School of Physics Peking University Beijing 100871 Department of Physics and Astronomy University of Missouri Columbia MO 65211 USA

A project of a spectroscopic survey of Galactic structure and evolution with a Large sky Area Multi-Object fiber Spectroscopic Telescope （LAMOST） is presented. The spectroscopic survey consists of two observational modes for various targets in our Galaxy. One is a major survey of the Milky Way aimed at a systematic study of the stellar abundance and Galactic chemical evolution through low resolution （R = 1000 2000） spectroscopy. Another is a follow-up observation with medium resolution （R = 10000） spectrographs aimed at detailed studies of the selected stars with different chemical composition, kinematics and dynamics.

关键词： techniques： spectroscopic - stars： abundances - Galaxy： structure - Galaxy：abundances - Galaxy： kinematics and dynamics - Galaxy： evolution

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Halo stream candidates in the LAMOST DR2

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Research in Astronomy and Astrophysics 2015年第8期15卷 1378-1391页

作者： Jing-Kun Zhao Gang Zhao yu-qin chen Ke-Feng Tan Meng-Tian Gao Ming Yang Yong Zhang Yong-Hui Hou Key Laboratory of Optical Astronomy National Astronomical ObservatoriesChinese Academy of Sciences CUG Faculty of Resources China University of GeosciencesWuhan Nanjing Institute of Astronomical Optics & Technology National Astronomical ObservatoriesChinese Academy of Sciences

We have cross-matched the LAMOST DR2 with the WISE, 2MASS and PPMXL catalogs and obtained a sample of 64 819 FGK metal-poor dwarfs with [Fe/H]〈-0.7, distances within 2 kpc from the Sun and reliable kinematics（space velocities,angular momenta and eccentricities）. With a detection strategy for halo streams provided by Klement et al, nine significant ＂phase-space overdensities＂ with stars on very similar orbits are identified from this sample. Among these overdensities, three were previously known and six are new stream candidates. The kinematics and metallicities of these stream candidates are then analyzed; they have typical halo characteristics.We have extracted the most probable members of each halo stream according to their angles with respect to the North Galactic Pole and investigate the distribution of the angular momenta to further verify their existences. Detailed study of elemental abundances for these members based on high resolution and high signal-to-noise spectra from follow-up observations in the near future is of high interest to understand the origin of these streams.

关键词： Galaxy： kinematics and dynamics

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High-α-Metal-Rich stars in the LAMOST-MRS survey and its connection with the galactic bulge

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Research in Astronomy and Astrophysics 2021年第6期21卷 424-432页

作者： Hao-Peng Zhang yu-qin chen Gang Zhao Jing-Kun Zhao Xi-Long Liang Hai-Ning Li Ya-Qian Wu Ali Luo Rui Wang CAS Key Laboratory of Optical Astronomy National Astronomical ObservatoriesChinese Academy of SciencesBeijing 100101China School of Astronomy and Space Science University of Chinese Academy of SciencesBeijing 100049China

We report the detection of a large sample of high-α-metal-rich stars on the low giant branch with 2.60.2 than the comparison group.These properties strongly indicate its connection with the outer bar/bulge region at R=3-5 kpc.A tentative interpretation of this special group is that its stars were formed in the X-shaped bar/bulge region,close to its corotation radius,where radial migration is the most intense,and brings them to present locations at 9 kpc and beyond.Low eccentricities and slightly outward radial excursions of its stars are consistent with this scenario.Its kinematics(cold)and chemistry([α/Fe]~0.1)further support the formation of the instability-driven X-shaped bar/bulge from the thin disk.

关键词： stars:late type stars:abundances stars:fundamental parameters Galaxy:disk Galaxy:evolution Galaxy:structure

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Chemical abundance analysis of 19 barium stars

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Research in Astronomy and Astrophysics 2016年第1期16卷 159-174页

作者： Guo-Chao Yang Yan-Chun Liang Monique Spite yu-qin chen Gang Zhao Bo Zhang Guo-qing Liu yu-Juan Liu Nian Liu Li-Cai Deng Francois Spite Vanessa Hill Cai-Xia Zhang College of Physics and Information Engineering Hebei Normal University Shijiazhuang 050024 China Key Laboratory of Optical Astronomy National Astronomical Observatories Chinese Academy of Sciences Beijing 100012 China University of Chinese Academy of Sciences Beijing 100049 China GEPI Observatoire de Paris-Meudon 92195 Meudon France Physics Department and Tsinghua Center for Astrophysics Tsinghua University Beijing 100084 China Astronomy Department Beijing Normal University Beijing 100875 China Department of Physics Shijiazhuang University Shijiazhuang 050035 China

We aim at deriving accurate atmospheric parameters and chemical abundances of 19 barium（Ba） stars, including both strong and mild Ba stars, based on the high signal-to-noise ratio and high resolution Echelle spectra obtained from the 2.16 m telescope at Xinglong station of National Astronomical Observatories, Chinese Academy of Sciences. The chemical abundances of the sample stars were obtained from an LTE, plane-parallel and line-blanketed atmospheric model by inputting the atmospheric parameters（effective temperatures Teff, surface gravities log g, metallicity [Fe/H] and microturbulence velocity ξt） and equivalent widths of stellar absorption lines. These samples of Ba stars are giants as indicated by atmospheric parameters, metallicities and kinematic analysis about UVW velocity. Chemical abundances of 17 elements were obtained for these Ba stars. Their Na, Al, α- and iron-peak elements（O, Na, Mg, Al, Si, Ca,Sc, Ti, V, Cr, Mn, Ni） are similar to the solar abundances. Our samples of Ba stars show obvious overabundances of neutron-capture（n-capture） process elements relative to the Sun. Their median abundances of[Ba/Fe], [La/Fe] and [Eu/Fe] are 0.54, 0.65 and 0.40, respectively. The Y I and Zr I abundances are lower than Ba, La and Eu, but higher than the α- and iron-peak elements for the strong Ba stars and similar to the iron-peak elements for the mild stars. There exists a positive correlation between Ba intensity and [Ba/Fe].For the n-capture elements（Y, Zr, Ba, La）, there is an anti-correlation between their [X/Fe] and [Fe/H]. We identify nine of our sample stars as strong Ba stars with [Ba/Fe]〉0.6 where seven of them have Ba intensity Ba=2-5, one has Ba=1.5 and another one has Ba=1.0. The remaining ten stars are classified as mild Ba stars with 0.17〈[Ba/Fe]〈0.54.

关键词： methods： observational - stars： abundances - stars： atmospheres - stars： chemically peculiar- stars： kinematics and dynamics - stars： late-type

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A comparison of stellar atmospheric parameters from the LAMOST and APOGEE datasets

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Research in Astronomy and Astrophysics 2015年第8期15卷 1125-1136页

作者： yu-qin chen Gang Zhao Chao Liu Jing Ren yun-Peng Jia Jing-Kun Zhao A-Li Luo yue Wu Yong Zhang Yong-Hui Hou yue-Fei Wang Ming Yang Key Laboratory of Optical Astronomy National Astronomical ObservatoriesChinese Academy of Sciences Nanjing Institute of Astronomical Optics & Technology National Astronomical ObservatoriesChinese Academy of Sciences

We have compared stellar parameters, including temperature, gravity and metallicity, for common stars in the LAMOST DR2 and SDSS DR12/APOGEE datasets. It is found that the LAMOST dataset provides a more well-defined red clump feature than the APOGEE dataset in the Teff versus log g diagram. With this advantage, we have separated red clump stars from red giant stars, and attempt to establish calibrations between the two datasets for the two groups of stars. The results show that there is a good consistency in temperature with a calibration close to the one-to-one line, and we can establish a satisfactory metallicity calibration of[Fe/H]APOGEE= 1.18[Fe/H]LAMOST ＋ 0.11 with a scatter of ～ 0.08 dex for both the red clump and red giant branch samples. For gravity, there is no correlation for red clump stars between the two datasets, and scatters around the calibrations of red giant stars are substantial. We found two main sources of scatter in log g for red giant stars. One is a group of stars with 0.00253 × Teff- 8.67 〈 log g 〈 2.6 located in the forbidden region, and the other is the contaminated red clump stars, which could be picked out from the unmatched region where stellar metallicity is not consistent with position in the Teff versus log g diagram. After excluding stars in these two regions,we have established two calibrations for red giant stars, log g APOGEE = 0.000615 ×Teff,LAMOST＋ 0.697 × log g LAMOST- 2.208（σ = 0.150） for [Fe/H] 〉-1 and log gAPOGEE= 0.000874×Teff,LAMOST＋0.588×log g LAMOST-3.117（σ = 0.167）for [Fe/H] 〈-1. The calibrations are valid for stars with Teff = 3800- 5400 K and log g = 0- 3.8 dex, and are useful in work aiming to combine the LAMOST and APOGEE datasets in a future study. In addition, we find that an SVM method based on asteroseismic log g is a good way to greatly improve the accuracy of gravity for these two regions, at least in the LAMOST dataset.

关键词： stars： late type—stars： fundamental parameters—stars： atmospheres—stars： abundances

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CaII H&K emission distribution of ～120000 F,G and K stars in LAMOST DR1

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Research in Astronomy and Astrophysics 2015年第8期15卷 1282-1293页

作者： Jing-Kun Zhao Terry D.Oswalt yu-qin chen Jian-Rong Shi Ke-Feng Tan Xiang-Song Fang Gang Zhao Ming Yang Yong Zhang Yong-Hui Hou yue-Fei Wang Key Laboratory of Optical Astronomy National Astronomical ObservatoriesChinese Academy of Sciences Embry-Riddle Aeronautical University 600 S.Clyde Morris Blvd.Daytona Beach FL 32114USA Nanjing Institute of Astronomical Optics & Technology National Astronomical ObservatoriesChinese Academy of Sciences

We present chromospheric activity index SHK measurements for 119 995 F, G and K stars with high signal-to-noise ratio（S/N 〉 80） spectra, extracted from LAMOST DR1 in SDSS g band. The index δS for each of these stars is calculated by the difference between its SHK value and the baseline determined from very inactive stars. The effect of metallicity on measurement of δS varies with stellar Teff. No evident Vaughan-Preston gap appears in our sample. The relation between δS and vertical distance from the Galactic plane is determined for stars with Teff 〈 5500 K. Stars with higher δS tend to be closer to the Galactic plane. Two open clusters in the DR1 sample, M45 and M67, exhibit the expected general trend that δS decays with age.For stars with Teff 〉 5500 K, similar δS levels appear in both young and old cluster stars, which supports Pace＇s suggestion that caution should be exercised when deriving the age of a single star by using its chromospheric activity. Finally, we investigate the relation between δS and the kinematics of our sample.

关键词： stars： activity

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