The Abundance Trends Of α-elements In The Galactic Halo And Disk Stars

The elemental abundances of the long-lived F and G-dwarfs in the solar neighbourhood provide a unique record of the progressive chemical enrichment of the instellar medium where the stars formed. So that these dwarf stars can be used as living fossils of the Galactic evolution to probe the Galactic structure and evolution history. Therefore this kind of stars is always the major objects of the observational astronomy.Our study presents a statistic analysis on the abundance pattern ofα-element in a large sample of dwarf stars. The observational data is consists of two parts: the first is the 102 F and G-dwarfs observed with the 2.16 m telescope and its attached coude spectrograph in Xinglong Observatory of the National Astronomical Observatories; the second is compiled from 17 observational works provided the abundances ofα-elements (O, Mg, Si, Ca, Ti). Eliminating the duplicate stars among these observational works, 1161 F and G-dwarf stars were remained including of 672 thin disk stars, 281 thick disk stars, 60 halo stars, and 48 transition stars between the thin and thick disks, identified just by the criteria of pure kinematics.The aim of our study is to investigate the abundance trends ofαelements (Mg, Si, Ca, Ti) and O element along with the metallicity [Fe/H] based on a large sample of dwarf stars, and thus provide reliable observational constraints on the model of formation and chemical evolution of the Galaxy. First, the selection criteria of the sample stars are introduced. Second, the stellar kinematics and the kinematic criteria of identifying the membership of stellar population are present. Third, the abundance trends of [α/Fe] and [O/Fe] along with [Fe/H] are investigate in detail for different stellar populations such as the thin disk, the thick disk and the halo stars. The gradient of [α/Fe] with the stellar average orbital radius (the galactocentric distance R_m) and the maximum distance from the Galactic plane Z_max are also discussed for different stellar populations.Based on the statistical analysis ofα-element abundances of the large sample of 1113 dwarf stars, we concluded some important results as follows:(1) The metallicity ranges of individual stellar populations: thin disk stars with -1.0 <[Fe/H] <+0.5; thick disk stars with -3.0 <[Fe/H] <+0.5; halo stars with -3.0 <[Fe/H] <-0.7. Therefore, the three populations overlap greatly in metallicity, and can not be identified only by stellar metallicity.(2) The stellar average orbital radius distribution: thin disk stars lie in R_m = 5.5 10.0 kpc with the main range of 7.0 9.0 kpc; thick disk stars lie in R_m = 4.0 11.0 kpc with the main range of 5.5 7.5 kpc; Halo stars lie in R_m = 4.0 10.0 kpc with the main range of 4.5 6.0kpc. That means there is overlap in the average orbital radius R_m among the thin disk, thick disk, and halo stars.(3) The distribution of the maximum distance from the Galactic plane Z_max: thin disk stars with Z_max <1 kpc; thick disk stars with Z_max = 0.1 4 kpc; halo stars Z_max> 1 kpc. Therefore, there are some overlap in Z_max between the thin and thick disk stars, as well as between the thick disk and halo stars.(4) The fourα-elements (Mg, Si, Ca, Ti) have the same chemical distribution and are all overabundance in metal-poor stars, but [α/Fe] show different trends with metallicity [Fe/H] for different stellar populations. For thin disk stars, the abundance trend decreases linearly from about [α/Fe] = 0.3 at [Fe/H] = -1.0 to solar values at [Fe/H] = 0. While for the thick disk stars, the trend of [α/Fe] shows a"knee"downward, starting with an overabundance of [α/Fe] = 0.4 at about [Fe/H] = -2.5, and remaining flat until [Fe/H] = -0.7, then it declines toward the solar values. The Mg element show different trends with the otherα-elements. For the Mg element, the abundance trend is nearly a horizontal line at the range of -2.0<[Fe/H]<-0.7,but for the otherα-elements, the trend of [α/Fe] decreases slowly and linearly.(5) The thin and the thick disk stars overlap in metallicity and have no serious difference with [α/Fe] vs. [Fe/H] in the range -1.0 < [Fe/H] < +0.5, and we can not separate the thin disk stars from the thick one based on the the abundance ofα-elements. The medial value of [α/Fe] of thick disk stars is higher than that of thin disk stars seriously.(6) For the metal-rich stars ([Fe/H]> 0) of the thin disk, the relative abundance of [α/Fe] of the fourα-elements Mg, Si, Ca, Ti show different evolution trends with metallicity [Fe/H]: [Mg/Fe] decreases continue to below that of the solar system with the increasing of [Fe/H], [Si/Fe] and [Ca/Fe] keep on the solar abundances when the increasing of [Fe/H], while [Ti/Fe] increases with the increasing of [Fe/H].(7) [O/Fe] has the same evolution slope with [Mg/Fe], but for the thin disk stars, the trend of [O/Fe] shows a"knee"downward at [Fe/H]≈-0.4. The [α/O] decreases linearly with the increasing of [O/H], but the [α/O] of thin disk and thick disk stars is also mixed and can not be separated only by this abundance value.(8) There is noα-elements abundance gradient. The thin and thick disk could barely be able to be separated at Rm = 7kpc, but still part of the thick disk stars mixed with thin disk stars; on vertical direction of silver plate, when the Z_max = 1kpc,αelement abundance [α/Fe] in the thin disk and thick disk can be separated, but some of thick disk stars still mix with the thin disk. At Z_max = 4kpc, the [α/Fe] of thick disk and halo stars [α/Fe] can be clearly separated.(9) The kinematics of thin disk, thick disk and halo stars are significantly different, specially for the speed VLSR. The thin disk stars with VLSR>-60km/s and Vtotal <90km/m; The thick disk stars with -180km/sVtotal> 90km/s; The halo stars with VLSR>-180km/s and Vtotal> 90km/s. In addition, the three components of the velocity dispersions are different too, the velocity dispersion of stars increases from the thin disk to thick disk and then to turn halo. The abundances of [α/Fe] have no correlation with the kinematic velocities.(10) The abundances of [α/Fe] in halo stars also shows a decreasing trend with the increasing of [Fe/H] in whole, but the abundance dispersions are much larger than the analytical errors. The abundances of [α/Fe] are seriously mixed between the thick disk and the halo stars. There are some low-αhalo stars in which the [α/Fe] is lower than that of the thin disk stars.Our results show that, for most elements, the elements abundance in the thin disk and thick disk stars have no significant differences, even theα-element abundances are also mixed and can not present one definite value of [α/Fe] to classify different stellar populations. This may be due to the selection effect of the different observations. Therefore, more observations of a large sample stars are necessary to investigate whether there is a clearly separation in the [α/Fe] abundances between the thin disk and thick disk stars.