K. F. Tan

Systematic Non-LTE study of the −2.6 ≤ [Fe/H] ≤ 0.2 F and G dwarfs in the solar neighborhood - I. Stellar atmosphere parameters

We present atmospheric parameters for 51 nearby FG dwarfs uniformly distributed over the -2.60 60000) Shane/Hamilton and CFHT/ESPaDOnS observed spectra and non-local thermodynamic equilibrium (NLTE) line formation for Fe I and Fe II in the classical 1D model atmospheres. The spectroscopic method was tested with the 20 benchmark stars, for which there are multiple measurements of the infrared flux method (IRFM) Teff and their Hipparcos parallax error is -0.75, or Teff 4.20. NLTE analysis is crucial for the VMP turn-off and subgiant stars, for which the shift in log g between NLTE and LTE can be up to 0.5 dex. The obtained atmospheric parameters will be used in the forthcoming papers to determine NLTE abundances of important astrophysical elements from lithium to europium and to improve observational constraints on the chemo-dynamical models of the Galaxy evolution.

SILICON ABUNDANCES IN NEARBY STARS FROM THE Si I INFRARED LINES

We have used high-resolution, high signal-to-noise ratio infrared spectra from the Subaru Telescope atop Mauna Kea. Line formation calculations of Si I infrared lines in the atmospheres of nearby stars are presented. All abundance results of [Si/Fe] are derived from local thermodynamic equilibrium (LTE) and NLTE statistical equilibrium calculations and spectrum synthesis methods. We found that NLTE effects for Si I infrared lines are important even for metal-rich stars (>0.1 dex), and the NLTE effects may depend on the surface gravities. A good agreement of silicon abundances between the optical and infrared lines is obtained when the NLTE effects are included, while a large difference is found for the LTE results. The derived silicon abundances are overabundant for metal-poor stars.

A Possible Signature of Non-uniform Be-α Relationships for the Galaxy

Most of the previous studies on beryllium abundances in metal-poor stars have taken different Galactic populations as a whole when investigating the production and evolution of Be. In this Letter, we report on the detection of systematic differences in [α/H]-A(Be) relationships between the low- and high-α stars which were identified by previous works. We remind that one should be more careful in investigating the Galactic evolution of Be with a sample comprising different Galactic populations, because such a mixed sample may lead to inaccurate Be-Fe/Be-O relationships.

RED GIANT STARS FROM THE SLOAN DIGITAL SKY SURVEY. II. DISTANCES

We present distance determinations for a large and clean sample of red giant branch stars selected from the ninth data release of the Sloan Digital Sky Survey. The distances are calculated based on both observational cluster fiducials and theoretical isochrones. Distributions of distances from the two methods are very similar with peaks at about 10 kpc and tails extending to more than 70 kpc. We find that distances from the two methods agree well for the majority of the sample stars; though, on average, distances based on isochrones are 10% higher than those based on fiducials. We test the accuracy of our distance determinations using 332 stars from 10 Galactic globular and open clusters. The average relative deviation from the literature cluster distances is 4% for the fiducial-based distances and 8% for the isochrone-based distances, both of which are within the uncertainties. We find that the effective temperature and surface gravity derived from low-resolution spectra are not accurate enough to essentially improve the performance of distance determinations. However, for stars with significant extinction, effective temperature may help to better constrain their distances to some extent. We make our sample stars and their distances available from an online catalog. The catalog comprises 17,941 stars with reasonable distance estimations reaching to more than 70 kpc, which is suitable for the investigation of the formation and evolution of the Galaxy, especially the Galactic halo.

A Non-local Thermodynamic Equilibrium Analysis of Boron Abundances in Metal-poor Stars

The non-local thermodynamic equilibrium (NLTE) line formation of neutral boron in the atmospheres of cool stars are investigated. Our results confirm that NLTE effects for the B I resonance lines, which are due to a combination of overionization and optical pumping effects, are most important for hot, metalpoor, and low-gravity stars; however, the amplitude of departures from LTE found by this work are smaller than that of previous studies. In addition, our calculation shows that the line formation of B I will get closer to LTE if the strength of collisions with neutral hydrogen increases, which is contrary to the result of previous studies. The NLTE line formation results are applied to the determination of boron abundances for a sample of 16 metal-poor stars with the method of spectrum synthesis of the B I 2497 Å resonance lines using the archived HST/GHRS spectra. Beryllium and oxygen abundances are also determined for these stars with the published equivalent widths of the Be II 3131 Å resonance and O I 7774 Å triplet lines, respectively. The abundances of the nine stars which are not depleted in Be or B show that, no matter the strength of collisions with neutral hydrogen may be, both Be and B increase with O quasi-linearly in the logarithmic plane, which confirms the conclusions that Be and B are mainly produced by primary process in the early Galaxy. The most noteworthy result of this work is that B increases with Fe or O at a very similar speed as, or a bit faster than Be does, which is in accord with the theoretical models. The B/Be ratios remain almost constant over the metallicity range investigated here. Our average B/Be ratio falls in the interval [13±4, 17±4], which is consistent with the predictions of spallation process. The contribution of B from the ν-process may be required if the B/B isotopic ratios in metal-poor stars are the same as the ⋆Based on observations made with ESO telescopes and NASA/ESA Hubble Space Telescope obtained from the ESO/ST-ECF Science Archive Facility; based on spectral data retrieved from the ELODIE archive at Observatoire de Haute-Provence (OHP). Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China; [tan, sjr, gzhao]@bao.ac.cn Graduate University of Chinese Academy of Sciences, Beijing 100049, ChinaThe non-local thermodynamic equilibrium (NLTE) line formation of neutral boron in the atmospheres of cool stars are investigated. Our results confirm that NLTE effects for the B I resonance lines, which are due to a combination of overionization and optical pumping effects, are most important for hot, metal-poor, and low-gravity stars; however, the amplitude of departures from local thermodynamic equilibrium (LTE) found by this work is smaller than that of previous studies. In addition, our calculation shows that the line formation of B I will get closer to LTE if the strength of collisions with neutral hydrogen increases, which is contrary to the result of previous studies. The NLTE line formation results are applied to the determination of boron abundances for a sample of 16 metal-poor stars with the method of spectrum synthesis of the B I 2497 A resonance lines using the archived HST/GHRS spectra. Beryllium and oxygen abundances are also determined for these stars with the published equivalent widths of the Be II 3131 A resonance and O I 7774 A triplet lines, respectively. The abundances of the nine stars which are not depleted in Be or B show that, no matter what the strength of collisions with neutral hydrogen may be, both Be and B increase with O quasilinearly in the logarithmic plane, which confirms the conclusions that Be and B are mainly produced by the primary process in the early Galaxy. The most noteworthy result of this work is that B increases with Fe or O at a very similar speed as, or a bit faster than, Be does, which is in accord with the theoretical models. The B/Be ratios remain almost constant over the metallicity range investigated here. Our average B/Be ratio falls in the interval [13 ± 4, 17 ± 4], which is consistent with the predictions of the spallation process. The contribution of B from the ν-process may be required if the 11B/10B isotopic ratios in metal-poor stars are the same as the meteoric value. An accurate measurement of the 11B/10B ratios in metal-poor stars is crucial to understanding the production history of boron.

A NON-LTE STUDY OF SILICON ABUNDANCES IN GIANT STARS FROM THE Si i INFRARED LINES IN THE zJ-BAND*

We investigate the feasibility of the Si I infrared (IR) lines as Si abundance indicators for giant stars. We find that Si abundances obtained from the Si I IR lines based on the local thermodynamic equilibrium (LTE) analysis show large line-to-line scatter (mean value of 0.13dex), and are higher than those from the optical lines. However, when the non-LTE effects are taken into account, the line-to-line scatter reduces significantly (mean value of 0.06dex), and the Si abundances are consistent with those from the optical lines. The typical average non-LTE correction of [Si/Fe] for our sample stars is about

THREE MOVING GROUPS DETECTED IN THE LAMOST DR1 ARCHIVE

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The CH(G) Index as a New Criterion for Selecting Red Giant Stars

0.35dex. Our results demonstrate that the Si I IR lines could be reliable abundance indicators provided that the non-LTE effects are properly taken into account.

CaII H&K emission distribution of ~ 120 000 F, G and K stars in LAMOST DR1

We analyze the kinematics of thick disk and halo stars observed by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope. We have constructed a sample of 7993 F, G, and K nearby main-sequence stars (d 3. Two of them (the Hyades-Pleiades stream and the Arcturus-AF06 stream) have been identified previously. We also find evidence for a new stream (centered at V ~ –180 km s–1) in the halo. The formation mechanisms of these three streams are analyzed. Our results support the hypothesis that the Arcturus-AF06 stream and the new stream originated from the debris of a disrupted satellite, while the Hyades-Pleiades stream has a dynamical origin.

Halo stream candidates in the LAMOST DR2

We have measured the CH G band (CH(G)) index for evolved stars in the globular cluster M3 based on the Sloan Digital Sky Survey (SDSS) spectroscopic survey. It is found that there is a useful way to select red giant branch (RGB) stars from the contamination of other evolved stars such as asymptotic giant branch (AGB) and red horizontal branch (RHB) stars by using the CH(G) index versus (g – r)0 diagram if the metallicity is known from the spectra. When this diagram is applied to field giant stars with similar metallicity, we establish a calibration of CH(G) = 1.625(g – r)0 – 1.174(g – r)2 0 – 0.934. This method is confirmed by stars with [Fe/H] ~ –2.3 where spectra of member stars in globular clusters M15 and M92 are available in the SDSS database. We thus extend this kind of calibration to every individual metallicity bin ranging from [Fe/H] ~ –3.0 to [Fe/H] ~ 0.0 by using field red giant stars with 0.4 ≤ (g – r)0 ≤ 1.0. The metallicity-dependent calibrations give CH(G) = 1.625(g – r)0 – 1.174(g – r)2 0 + 0.060[Fe/H] – 0.830 for –3.0 < [Fe/H] ≤ –1.2 and CH(G) = 0.953(g – r)0 – 0.655(g – r)2 0 + 0.060[Fe/H] – 0.650 for –1.2 < [Fe/H] < 0.0. The calibrations are valid for the SDSS spectroscopic data set, and they cannot be applied blindly to other data sets. With the two calibrations, a significant number of the contaminating stars (AGB and RHB stars) were excluded and thus a clear sample of red giant stars is obtained by selecting stars within ±0.05 mag of the calibration. The sample is published online and it is expected that this large and clean sample of RGB stars will provide new information on the formation and evolution of the Galaxy.