Density Distribution of Faint Stars in the Direction of the North Galactic Pole

Abstract A method is proposed to separate density functions along the line of sight for both disk- and halo-population stars with three-color photometry. The observed star counts as a function of visual magnitude and color are synthesized by an integral equation, where the luminosity function is assumed to be that of van Rhijn for the disk stars and that of globular clusters M92 and M3 for the field halo stars, respectively. Adopting the conventional exponential density law for both disk and halo stars, we derive the density gradeint d log D(z)/dz (z in kpc) toward the north galactic pole. The density gradient is almost uniquely determined to be —1.4 for the disk stars and —0.2 for the halo stars with the halo density outnumbering the disk beyond z≥2 kpc. The halo mass density is estimated to be (3–5) × 10−4 M◉ pc−3 in the solar neighborhood.