|Annu. Rev. Astron. Astrophys. 1997. 35:
Copyright © 1997 by . All rights reserved
In this section, I discuss some implications and uses of the most basic chemical composition information, namely metallicity. The word metallicity has more than one meaning: The precise definition is that metallicity is the mass fraction of all elements heavier than helium, denoted by the symbol Z; this is not always practical for observers because information usually does not exist for all elements. For observational stellar astronomy, metallicity is more often used to refer to the iron abundance. Unless explicitly stated the word metallicity used here refers to [Fe/H], the logarithmic iron abundance relative to the solar value.
4.1. The Disk
Because the main-sequence lifetimes of G and F dwarfs are comparable to the age of the Galaxy, all the G dwarfs ever born are assumed to still exist (although see discussion of metallicity-dependent lifetimes by Bazan & Mathews 1990), and so these stars can provide a complete picture of Galactic chemical evolution. Early studies of the metallicity distribution of G dwarfs, within about 25 pc of the sun (van den Bergh 1962, Schmidt 1963, Pagel & Patchett 1975), showed that there is a deficit of metal-poor stars relative to the prediction of the Simple model; this is the well-known G-dwarf problem. The metallicities of these early studies were based on UV excesses (see Wallerstein & Carlson 1960, Sandage 1969), which are accurate to approximately 1 = 0.25 dex (Pagel & Patchett 1975); although Norris & Ryan (1989) claim uncertainties of ± 0.45 dex. The observed metallicity distributions contain biases that must be taken into account in order to obtain the true metallicity function (e.g. see Sommer-Larsen 1991, Pagel 1989).
Many possible explanations were presented to account for the G-dwarf problem (e.g. Audouze & Tinsley 1976), but infall of metal-poor gas onto the disk was the most favored solution. To fit the observed metallicity function by this scheme, the original disk was at most 5% of the present disk mass (Pagel 1989), with mass infall occurring over several billion years. Variants of the Simple model exist that include gas infall in various ways (e.g. Larson 1974, 1976, Lynden-Bell 1975, Clayton 1985, 1988, Pagel 1989). All of these models predict a strict age-metallicity relation (AMR) with no abundance dispersion.
In these models, the halo could not have been responsible for the bulk of the gas infall because the present-day luminous halo mass is only a few percent of the disk (Sandage 1987, Pagel 1989); a metallicity function of the disk+halo still suffers a paucity of metal-poor stars relative to the simple model (e.g. Worthey 1996). (Tosi 1988) showed that infall of gas with metallicity 0.1 Z provides as effective an explanation of the observed disk metallicity distribution function as infall of zero metallicity gas; however, infalling gas with Z = 0.4 Z is excluded by observations.
A number of studies over the last decade and a half have combined star count and kinematic information with metallicities estimated from UV excesses (e.g. Sandage & Fouts 1987), ubvy photometry (e.g. Nissen & Schuster 1991), and low S/N spectra (e.g. Carney et al 1987, Jones et al 1995). The assembled databases have been used to imply the existence of various Galactic populations. For example, the thick disk of Gilmore & Reid (1983) is characterized by scale height of ~ 1.3 pc, mean [Fe/H] ~ -0.6 dex, and dispersion 0.3 dex (Gilmore & Reid 1983, Gilmore 1984, Gilmore & Wyse 1985, Wyse & Gilmore 1986), with no apparent metallicity gradient. (Wyse & Gilmore 1995) conclude that the data are best fit by overlapping thick and thin disks; the thick disk has a mean metallicity of [Fe/H] ~ -0.7 dex, ranging from -0.2 to -1.4 dex. A low metallicity tail, extending down to [Fe/H] ~ -2 to -3, was claimed by (Norris & Ryan (1991), Beers & Sommer-Larsen (1995), Pagel & Tautvaisiene (1995). Typical star count models yield thick disk to thin disk ratios of a few percent (e.g. Majewski 1993). The thin disk metallicity peaks near [Fe/H] = -0.25 dex, ranging from +0.2 to -0.8 dex (Wyse & Gilmore 1995).