Frequency distribution of metal abundance for faint stars in the direction of the north galactic pole

Using the published three-color photometric data of faint stars in the direction of the north galactic pole, we separated G dwarfs from subgiants by means of a statistical method which utilizes the fact that the star counts are related to both the luminosity function and the density function. Based on the sample of G dwarfs thus separated, we derived the cumulative metallicity distributions for three intervals of height above the galactic plane, 1.0 kpc< or =z<1.5 kpc, 0.2 kpc< or =z<1.5 kpc, and 0.0 kpc< or =z<1.0 kpc, and found that the gradient in the mean metallicity exists at least up to zapprox.1.5--2.0 kpc. The derived results are interpreted by a two-zone model of chemical evolution which describes the collapse of gas toward the galactic plane. To reproduce the vertical distributions of both stellar density and metallicity simultaneously, a slight flattening of the initial stellar mass function (IMF) outside the disk region is necessary as compared with the IMF in the disk. It is shown that the disk component within Vertical BarzVertical Bar< or approx. =1.0 kpc is formed by the infall of processed gaseous matter from outside the disk region, whereas the disk component within Vertical BarzVertical Bar< or approx. =0.2 kpc by further concentration of the gas onto the galactic plane