The [?/Fe] Distribution And The Galaxy Structure On The Bases Of LAMOST Giants

The structure and evolution of the Milky Way is an important hot topic in the field of astrophysics.The Galactic disk is the main structural component of the Milky Way,but whether there is a sub-structure of the so-called thin-and thick-disks has always been a controversial issue.Using the stellar spectra from the LAMOST Spectroscopic Survey of the Galactic Anti-center(LSS-GAC),we obtained the distributions of [a/Fe] in the R-z space of a large sample of disk stars,connecting with the star kinematics characteristics,we want to investigate the stellar chemical evolution of the Galactic disk and to reveal the structure of the Galactic disk.First,we selected about 770,000 giant stars from the DR3 of the LAMOST spectroscopic survey and obtained the [a/Fe] abundance ratios.The distributions of [a/Fe] in R-z space shows that the Galactic disk is truly consist of two stellar components with high and low[a/Fe] ratios,respectively.Then,we analyze the gradients of [a/Fe] in vertical direction and radial direction of the Galactic plane,and find there is a positive vertical gradient of [a/Fe]along with the increasing of the distance(z)away from the Galactic plane,and a negative radial gradient of [a/Fe] along with the increasing of the distance(R)from the Galactic center.Second,based on the distributions of [a/Fe] along with [Fe/H] of our giant sample stars,we divided the sample disk stars into two components with a artificial line between the high-athick disk and low-a thin disk substructures.We analyze the relative counts of stars belong to the thick and thin disk(NTD/ND)and find a significant negative gradient of NTD/ND in radial direction with the increasing of the Galactocentric distance R,while NTD/ND shows an obvious positive gradient in vertical direction with the increasing z,similar to the gradient of[a/Fe],the gradient of NTD/ND increases more sharply in the inner disk than that in the outer disk.Thirdly,we selected 51,042 giants in solar neighborhood(distance away from the Sun less than 1 kpc)from the DR4 of LAMOST survey,and classified their membership belong to the thin or thick disk by the kinematic criterion and the chemical standard(the abundance ratios of [a/Fe]),respectively.We find about 6%-8.7% thin disk stars in kinematics infiltratethe thick disk stars in chemical standard.Similarly,there are about 4.2%-20.6% thick stars classified using kinematic criterion infiltrate the thin disk stars in chemical standard.In summary,we analyzed the abundance distributions of [a/Fe] in about 820,000 giant stars from the LAMOST spectroscopic survey,which is the largest sample of giant stars with[a/Fe] abundances.We studied the distributions of [a/Fe] along with [Fe/H] in R-z space in detail.Based on our results,we present our conclusions as follows:(1)The Galactic disk is truly consist of two substructures with different [ a /Fe]abundances.The thin disk with lower [a/Fe] ratios is mainly located within 0.4 kpc away from the Galactic plane but extends to the edge of the whole disk.While the thick disk with higher [a/Fe] ratios is mainly located more than 1 kpc outside the Galactic plane and mainly in inner disk.The thick disk stars have relative higher [a/Fe] ratios and larger scatter of space velocities.(2)Although the Galactic disk has two components of thin and thick disks,there is no a distinct separation between the thin and thick disk ether in metallicity or the [a/Fe] ratios,as well as in the space position.(3)Even in solar neighborhood,we cannot separated the thin and thick disk using a simple single criterion,there are heavy blended between the thin and thick disk stars ether using the kinematic criterion or the chemical standard of [a/Fe] ratios.(4)There is an apparent “warp” in the edge of the Galactic disk.