Galactic Disk Chemical Evolution And Star Formation History

The metallicity distribution of G dwarfs in the solar neighborhood is one of theimportant constraints on models of the chemical evolution of the Galactic disk. Themain goal of this paper is to study the star formation and chemical evolution of theGalactic disk based on the observed metallicity distribution function. At the same time,more efforts have been made to discuss the formation scenario of the thick disk.Primarily, based on a new and enlarged sample of G dwarfs within 25pc from theSun, we derived a new G-dwarf metallicity distribution function in the solar neighbor-hood. Since older stars generally have lower metallicities and larger scale heightsrelative to the Galactic plane than that of the younger stars, their relative number shouldbe artificially underestimated by the limitation of our sample. To solve this problem, wehave adopted the correction procedure introduced by Sommer-Larsen (1991). We alsohave corrected the final distribution for observational errors. The resulted metalli-citydistribution differs from that of Rocha-Pinto & Maciel (1996) (which is widely adopted)by having a larger width and smaller amplitude of the single peak. Moreover, the metal-poor tail of the new distribution extends to [Fe/H]=-1.5, which is consistent with theobservations of the lowest metallicity of the thick disk stars..Secondly, we compared four analytical models of the chemical evolution of theGalactic disk based on the new G-dwarf metallicity distribution. A chemically inhomo-geneous ISM is considered by assuming that at any time stars are born with a spread intheir metallicities, the spread being a Gaussian in the logarithm of the metallicity aroundthe mean metallicity of that epoch, It is showed that the simple and the collapse modelsdeviate greatly from the observed metallicity distribution even though the inhomoge-neity of the ISM is considered. The PIE and PPY models are in agreement with themetal-poor tail of the observed metallicity distribution. This suggests that PIE and PPYmodels can be used to reasonably describe the chemical evolution of the Galactic disk atthe early epoch. However, disagreements between model predictions and observationsare presented for the distribution of the relatively metal-rich G dwarfs in the solarneighborhood. This indicates that the closed-box model for the Galactic disk does notwork.Finally, we introduce a two-component model for the chemical evolution of theGalactic disk, which assumes that the formations of the thick and thin disk occur in twomain accretion episodes. The infall rate is assumed to be Gaussian. Both the pre-thinand post-thin scenarios for the formation of the Galactic disk are considered. Comparingmodel predictions with the new G-dwarf metallicity distribution in the solar neighbor-hood, we use the X~2-test to derive the best-fit and compare the reasonableness of fourdifferent models, which are closed-box, one-component, pre-thin and post-thin models.Our results show that, the predictions of both the pre-thin and post-thin model are ingood agreement with the new observed G-dwat metallicity distribution, i.e., it isdifficult to conclude which one is better based on the current observational constraints.However, the local present density of the thick disk required for the pre-thin model toget the best-fit is larger than most of results from star counts, yet this disagreement doesnot exist fOr the post-thin model. From this point of view, it seems that the post-thinmodel should be more reasonable.Moreover, the predictions of our best-fit model agree well not only with theobserved data in the solar neighborhood, but also with the main observational featuresof the Galactic disk. However, contrary to the observations in the HII regions, our modelpredicts the oxygen abundance gradient in the outer region is steeper than that of theinner region.