Yu-Qin Chen

Chemical composition of 90 F and G disk dwarfs

High resolution, high S/N spectra have been obtained for a sample of 90 F and G main-sequence disk stars covering the metallicity range 1:0 < (Fe/H) < +0:1, and have been analysed in a parallel way to the work of Edvardsson et al. (1993a) in order to re-inspect their re- sults and to reveal new information on the chemical evo- lution of the Galactic disk. Compared to Edvardsson et al. the present study in- cludes several improvements. Eective temperatures are based on the Alonso et al. (1996) calibration of color in- dices by the infrared flux method and surface gravities are calculated from Hipparcos parallaxes, which also allow more accurate ages to be calculated from a comparison of MV and Te with isochrones. In addition, more reli- able kinematical parameters are derived from Hipparcos distances and proper motions in combination with accu- rate radial velocities. Finally, a larger spectral coverage, 5600 8800 A, makes it possible to improve the abundance accuracy by studying more lines and to discuss several el- ements not included in the work of Edvardsson et al. The present paper provides the data and discusses some general results of the abundance survey. A group of stars in the metallicity range of 1:0 < (Fe/H) < 0:6 having a small mean Galactocentric distance in the stellar orbits, Rm < 7 kpc, are shown to be older than the other disk stars and probably belong to the thick disk. Excluding these stars, a slight decreasing trend of (Fe/H) with increasing Rm and age is found, but a large scatter in (Fe/H) (up to 0.5 dex) is present at a given age andRm. Abundance ratios with respect to Fe show, on the other hand, no signicant scatter at a given (Fe/H). The derived

Sulphur abundances in disk stars: A correlation with silicon

We have performed new determinations of sulphur and silicon abundances for a sample of 26 disk stars based on high-resolution, high signal-to-noise spectra. The results indicate a solar (S/Fe) for (Fe/H) > 0:3, below which (S/Fe) increases to0.25 dex at (Fe/H) = 1:0. We find that there is a good correlation between (S/H) and (Si/H), indicating the same nucleosynthetic origin of the two elements. It seems that the ratio of sulphur to silicon does not depend on metallicity for (Fe/H)> 1:0. The implications of these results on models for the nucleosynthesis of-capture elements and the chemical evolution of the Galaxy are discussed.

A SUBSTELLAR COMPANION TO THE INTERMEDIATE-MASS GIANT 11 COMAE

We report the detection of a substellar companion orbiting the intermediate-mass giant star 11 Com (G8 III). Precise Doppler measurements of the star from Xinglong Station and Okayama Astrophysical Observatory (OAO) reveal Keplerian velocity variations with an orbital period of 326.03 ± 0.32 days, a semiamplitude of 302.8 ± 2.6 m s−1, and an eccentricity of 0.231 ± 0.005. Adopting a stellar mass of 2.7 ± 0.3 M☉, the minimum mass of the companion is 19.4 ± 1.5 MJ, well above the deuterium-burning limit, and the semimajor axis is 1.29 ± 0.05 AU. This is the first result from a joint planet-search program between China and Japan aimed at revealing the statistics of substellar companions around intermediate-mass giants. 11 Com b emerged from 300 targets of the planet-search program at OAO. The programs current detection rate of brown dwarf candidates seems to be comparable to the rate of such detections around solar-type stars with orbital separations of 3 AU.

Chemical abundances of old metal-rich stars in the solar neighborhood

We report on chemical abundances for 15 old metal-rich stars in the solar neighborhood based on high-resolution, high signal-to-noise ratio observations. It is found that [O/Fe], [S/Fe], and probably [Ba/Fe] decrease with increasing metallicity, while the remaining elements, C, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Co, and Ni, generally have solar [X/Fe] ratios irrespective of metallicity. The kinematic data indicate a slight lag in the Galactic rotation for most stars. In combination with their low maximum distance perpendicular to the Galactic plane, Zmax, we suggest that most of the sample stars originate from the inner thin disk. This suggestion is supported by the similar abundance pattern of these stars as that of thin-disk stars. The connection of the old metal-rich stars with the old population of the inner disk suggests an inside-out formation of the disk. One exceptional case in our sample is HD 190360. It is suspected to be a thick-disk star based on the enhanced O, S, Mg, and Si abundances as well as the special kinematics, VLSR = -40 km s-1 and Zmax = 1.0 kpc. It shows that stars from a population other than the thin disk exist among old metal-rich stars in the solar neighborhood. The location of these stars, presently in the solar neighborhood, may indicate that orbit diffusion effects of old stars are quite significant.

Stellar Abundance and Galactic Chemical Evolution through LAMOST Spectroscopic Survey

A project of a spectroscopic surveyof Galactic structure and evolutionwith a Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) is presented. The spectro- scopic 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 detailedstudies of the selected stars with differentchemicalcomposition,kinematics and dynamics. The universe consists of millions of galaxies, in which there is a special one - the Milky Way - where all we stay. The Milky Way is also the subject most concerned by astronomers since it links our knowledge of stellar evolution and some important problems of formation of the universe. It can be used as an ideal laboratory for the studies of large scale structure of the Universe and gas distribution. We can observe more easily the various objects in the Milky Way than those in other galaxies. The modern study of the Milky Way by the method of star counting was started in late 19th century. Other galaxies were identified to be similar stellar systems as the Milky Way at beginning of 20th century. Considering that the Milky Way is a typical spiral galaxy in which our solar system is located, it is the best sample with which various theoretical models can be checked when we study the formation and evo- lution of the galaxies. The structure and chemical evolution of a galaxy can be represented by the different distribution and motion of the stars with different metallicities. There was no exciting progress in the field of Galactic study before 1980s due to the lack of sufficient observational data. Thanks to the rapid development of high resolution spectrographs and some large scale

A Catalog of Moving Group Candidates in the Solar Neighborhood

Based on the kernel estimator and wavelet technique, we have identified 22 moving group candidates in the solar neighborhood from a sample which includes around 14,000 dwarfs and 6000 giants. Six of them were previously known as the Hercules stream, the Sirus-UMa stream, the Hyades stream, the Caster group, the Pleiades stream, and the IC 2391; five of them have also been reported by other authors. 11 moving group candidates, not previously reported in the literature, show prominent structures in dwarf or giant samples. A catalog of moving group candidates in the solar neighborhood is presented in this work.

The [Zn/Fe]-[Fe/H] trend for disk and halo stars

Zn abundances, derived from a model atmosphere analysis of the λ6362.35 A Zn  line, are presented for 44 thin disk, 10 thick disk and 8 halo dwarf stars in the metallicity range −1.0 < (Fe/H) < +0.2. It is found that (Zn/ Fe) in thin disk stars shows a slight increasing trend with decreasing metallicity reaching a value (Zn/Fe) � +0. 1a t (Fe/H) = −0.6. The thick disk stars in the metallicity range −0.9 < (Fe/H) < −0.6 have an average (Zn/Fe) � +0.15 dex, whereas five alpha-poor and Ni-poor halo stars in the same metallicity range have (Zn/Fe) � 0.0 dex. These results indicate that Zn is not an exact tracer of Fe as often assumed in abundance studies of damped Lyman-alpha systems (DLAs). A better understanding of the nucleosynthesis of Zn is needed in order to obtain more detailed information on the past history of star formation in DLAs from e.g. the observed sulphur/zinc ratio.

The abundance patterns of Sirius and Vega

On the basis of high-resolution echelle spectra obtained with the Coude Echelle Spectrograph attached to the 2.16 m telescope at the Beijing Astronomical Observatory (Xinglong, China) for Sirius and Vega, we performed an elemental abundance analysis of 29 ions, in which 23 elements are included, with Teff = 9880, log g = 4.40, [Fe/H] = 0.50, and ξt = 1.85 km s-1 for Sirius and Teff = 9430, log g = 3.95, [Fe/H] = -0.57, and ξt = 1.50 km s-1 for Vega. The results show different abundance patterns for these two stars. The iron peak elements seem to be enhanced by about 1.0 dex in Sirius relative to Vega. Comparison with previous investigations is present.

Chemical Abundances of 15 Extrasolar Planet Host Stars

Based on high-resolution, high signal-to-noise ratio spectra, we have derived accurate metallicities for 15 extrasolar planet host stars, as well as differential abundances for 16 other elements. The companions of three stars, HD 29587, HD 98230, and possibly HD 190228, have been shown to have masses in the brown dwarf regime and give an average value of the metallicities [Fe/H] = -0.44 ± 0.17. If we exclude these three stars, the rest of the sample stars are metal-rich relative to the Sun, covering the range from -0.12 to 0.41, with an average [Fe/H] value of 0.16 ± 0.17. The stars with planets show a slight overabundance pattern for [Na/Fe], [Mg/Fe], [Al/Fe], and [Sc/Fe]. On the other hand, [C/Fe], [O/Fe], [Si/Fe], [Ca/Fe], [Ti/Fe], [V/Fe], [Cr/Fe], [Ni/Fe], and [Ba/Fe] are approximately solar in the sample. The stars with planets also show underabundances for [K/Fe] and slight overabundances for [Mn/Fe], but stars with brown dwarfs show a contrary trend for these two elements. An interesting result is that sulfur displays enhanced values of [S/Fe], ranging from 0.04 to 0.40 through the sample. These results are used to investigate the connection between giant planets and high metallicity and to probe the influence of this process on the other elements.

The C and N abundances in disk stars

Abundance analysis of carbon and nitrogen has been performed for a sample of 90 F and G type mainsequence disk stars with a metallicity range of -1.0 < [Fe/H] < +0.2 using the Cl and N I lines. We confirm a moderate carbon excess in the most metal-poor disk dwarfs found in previous investigations. Our results suggest that carbon is enriched by superwinds of metal-rich massive stars at the beginning of the disk evolution, while a significant amount of carbon is contributed by low-mass stars in the late stage. The observed behavior of [N/Fe] is about solar in the disk stars, irrespective of the metallicity. This result suggests that nitrogen is produced mostly by intermediate-mass stars.