Theoretische Zweifarbendiagramme für hohe galaktische Breiten

It is the aim of the present paper to give two‐colour diagrams, which are valid for different distances z from the galactic plane. The diagrams are computed on the basis of theoretical considerations and take into account the differing part of members of the halo population. For several reasons the considerations are limited to the range 0.30 mag ≦ ( B — V ) ≦ 0.73 mag. The method proceeds from the relative frequency function of uv‐excesses in the solar vicinity (different for stars with colour indices ( B — V ) smaller and larger than 0.45 mag) and the relative frequency function of velocity components perpendicular to the galactic plane. The latter function was derived with the uv‐excess as a second parameter. In general the position of a star in the two‐colour diagram depends on its state of evolution. For this reason it is necessary to know the fraction of differently evolved stars in a given sample. With these foundations it is possible to calculate mean uv‐excesses and therefore mean two‐colour diagrams for given distances above the galactic plane. It seems to be probable that these mean values keep constant with z for ( B — V ) ≦ 0.45 mag. For ( B — V ) larger than 0.45 mag the mean uv‐excess rises to about 0.20 mag. This maximum value is nearly attained at z = 1000 parsecs already. Theoretical two‐colour diagrams are also presented for some fixed intervals in apparent magnitude. As usual they are given as mean curves but also in form of lines of equal frequency of points in the diagram. The accuracy of the mean values is about 0.02 mag (m. e.). From the diagrams some general conclusions concerning the determination of interstellar extinction at high galactic latitudes are drawn.