Study On The Distributions Of [?/Fe] Ratios In Dwarf Stars From The LAMOST Survey And The Structure Of The Galactic Disk

The structure and chemical evolution of the Galaxy is one of the important fundamental topic in astrophysics.As the main structure and component of the Galaxy,the structure and evolution of the Galactic disk itself is very important to understand the structure and evolution of the Galaxy.While it is still an open question about the disk structure at present,such as the existence of the thick disk is true or not,as well as the thin and thick disks can be separated distinctly in spatial position and chemical abundance distributions,etc,are all in debate.In this work,we will use the new wide field and large aperture optical telescope(LAMOST)spectrum survey to obtain spectra of large sample of stars,and analyze the distributions of[?/Fe] ratios in the disk stars to investigate the structure and evolution of the Galactic disk.The research content of the thesis consists of four parts.First,1.28 million samples of dwarfs were obtained from LAMOST DR4 data according to certain sample selection criteria.Secondly,using the space position,space velocity and relative abundance information of the stars presented by the Add Value Catalogue V2 of the LAMOST Spectroscopic Survey of the Galactic Anti-center(LSS-GAC),we compared the consistency between the [?/Fe] ratios obtained from the LAMOST low resolution spectra of the stars and the [?/Fe] ratios presented by the analysis of the high resolution higher SNR spectra in literatures,and give a detailed analysis of the distributions for the [?/Fe] ratios of the sample stars in different R-Z spaces.Thirdly,we divide the large sample to two subsamples,the thick disk stars with high [?/Fe]ratios and the thin disk stars with low [?/Fe] ratios,according to the characteristics of [?/Fe]distributions in R-Z space.Taking into consideration of the correction of sample selection,we obtained the sale height for the thin-and thick-disk using the counting stars analysis method respectively.Finally,the methods of classifying the thin and thick disks are discussed,and the limitations of differentiating thin and thick disks by the [?/Fe] ratios are also discussed.The innovation of our work is that we first studied the structure of the galacticdisk using the spatial distribution analysis of millions of dwarfs,which is the largest and most complete stellar abundance sample to date.Our main findings are as follows:(1)The [?/Fe] ratios presented by the Add Value Catalogue V2 of LSS-GAC obtained by template matching method is reliable,with a system difference of 0.02 dex between its values and the [?/Fe] ratios given by high resolution spectra,and a standard deviation 0.09 dex.(2)The distributions of the [?/Fe] ratios alone with the metallicity and space position of the large sample dwarfs show that the Galactic disk is truly consist of two structural components as thin and thick disks:: thick disk stars have higher [?/Fe] ratios than the thin disk in general,with an average over abundance of 0.19 dex.The metallicity of the thick disk star is generally lower than that of thin disk star,but there is no strict boundary and the two overlap in a relatively large metallicity range(-1.0 < [Fe/H] <0.0).The thick disk is further away from the Galactic plane than the thin disk.(3)Taking into account the sample selection effect,the star counting analysis shows that the scale height of the thick disk is higher than that of the thin disk,and with the increase of radial galactic center distance R,the scale height of the thick disk increases significantly faster than that of the thin disk.(4)Edge thickening occurs in both the thin and thick disks,and the distribution structure of stars on and below the Galactic plane is asymmetric.