The Luminosity Function and Density Distribution of Disk Population Stars

Abstract

The density distribution and luminosity function of the disk population are investigated, using star counts as a function of V and B-V for several thousand stars near the North Galactic Pole. Having estimated the contributions to the counts due to Population II stars and disk giants and subgiants, the remaining stars are assumed to constitute a pure disk-dwarf population. Density distributions for stars in successive (B-V)_0 intervals are calculated and combined to form a composite disk density distribution. The agreement with Oort’s K-giant distribution is satisfactory. There is evidence that dwarfs redder than (B-V)_0=1.4 are concentrated in a narrow layer in the plane, similar to the interstellar gas. Determination of the luminosity function for the reddest disk dwarfs indicates these stars are five to ten times as numerous as was previously thought, thus accounting for a significant fraction of the missing mass in the solar neighborhood. The luminosity function is extrapolated to estimate the contribution to the missing mass of main sequence stars fainter than those considered in the analysis.