Chemical Abundances Of 102 Stars In The Galactic Disks

The long-lived F and G dwarfs in solar neighbourhood can be used as the living fossils for the evolution of the Galaxy,the elemental abundances in their stellar atmospheres reflect directly the chemical composition in different phases of the Galactic evolution,and provide a powerful probe to explore the structure and chemical evolution of the Galaxy.Therefore, these stars are always the main objects in astronomy observation.High resolution and high signal-to-noise ratio spectra of 102 F and G dwarfs were observed with the Coud(?) Echelle Spectrograph attached to the 2.16 m telescope at the National Astronomical Observatories(Xinglong,China).The data reduction was performed using the ESO-MIDAS package,including the extraction of one-dimensional spectrum and the measurements of equivalent widths.The local thermodynamic equilibrium model atmospheres of Kurucz was adopted,the effective temperature was determined from the b-y, V-B and V-K colour indices using the IRFM calibrations,and the surface gravity was calculated from Hipparcos parallax.Elemental abundances were derived from the code of ABONTEST8 by requiring the calculated equivalent widths from the model should match the observed ones.The abundances of 14 elements(O,Na,Mg,Al,Si,Ca,Fe,So,Ti,V,Cr,Mn, Ni,Ba) in 102 program stars were present,and the calculating errors of abundances were estimated.Stellar ages were determined from Yonsei-Yale isochrones.Stellar space velocities U,V and W were calculated using the stellar data of Hipparcos parallax,proper motion,radial velocity,et al.The population membership of the programm stars was identified using pure kinematic criteria,70 stars belong to thin disk,30 stars belong to thick disk,and 2 stars are halo.Based on the results,the trends of elemental abundances with the metallicity in thin and thick disks were discussed,as well as the the age-metallicity relation.The main results are summarized as follows:(1) The average age of the thick disk stars is older than the thin disk stars,and the age-metallicity relation for the thick disk stars is more obvious than that for the thin disk stars.(2) The abundances ofα-elements(Mg,Si,Ca,Ti) in the thin and thick disk stars show no obvious distinction.In the metallicity range of -0.7≤[Fe/H]＜0.1,there are not only thin disk stars with typical abundances for thick disk stars but also thin disk stars with typical abundances for thick disk stars.But on the average,the[α/Fe]ratios for stars of the thick disk are higher than those of the thin disk stars. (3) The separation for[O/Fe]of the thin and the thick disk stars is also not clear.In general,[O/Fe]show a decreasing trend with the increase of[Fe/H],but there is a "knee" cut-down at[Fe/H]≈-0.4 for the thin disk stars.(4) For abundances[Na/Fe],there is no distinct offset between the thin and thick disk stars.[Na/Fe]shows a decreasing trend with the increase of[Fe/H]for metal-poor stars,while an increasing trend for metal-rich stars.For the other odd-Z element,the trend[Al/Fe]is similar to[α/Fe],and[Al/Fe]in thick disk stars is more overabundant than that in thin disk stars.(5) The abundance trends of the iron-group elements Sc,V,Cr,Mn,and Ni show different behavior.[Sc/Fe]shows different trends for thin and thick disk stars.For thin disk stars,[Sc/Fe]trends is similarly as[α/Fe].For thick disk stars,[Sc/Fe]is obviously overabundant,and there is a peak for[Sc/Fe]trends at about[Fe/H]=-0.6.In the whole metallicity range of the sample stars,the trends of[V/Fe],[Cr/Fe],[Ni/Fe]are nearly flat around their values in solar system,and no clear separations between the thin and thick disk stars.On the average,[V/Fe]shows a slight enhancement for the thick disk stars compared with the thin disk stars,and contrary to[Cr/Fe].[Mn/Fe]is underabundant to the solar ratio in metal-poor stars,and show an increasing trend with the increase of[Fe/H],appearing as a mirror of[α/Fe],but there is no clear distinction between the thin and thick disk stars.(6) In the overlap range of metallicity between the thin and thick disk stars,[Ba/Fe]can not be clearly separated into two different trends,but on the average,[Ba/Fe]for the thick disk stars is lower about 0.1dex than that for the thin disk stars.While for thick disk stars with [Fe/H]＜-0.7,[Ba/Fe]shows a decreasing trend with the decrease of[Fe/H].It seems that,for our sample,there is no obvious differences in abundance ratios between the thin and thick disk stars,even for theα-elements which are generally thought as typical elements with different abundances between the thin and thick disk stars.This result maybe partly resulted from the membership of the sample stars,because there are some transition stars or Hercules stream stars which kinematic properties just located in the middle of the thin and the thick disk stars.So that more observations are required to confirm whether or not the elemental abundances are distinct different between the thin and the thick disk stars.