|Figure 2: Sample two-color diagrams for three intervals of successively fainter apparent magnitudes. The superposed grid of iso-abundance lines for dwarf stars is based on theoretical model atmosphere calculations (Buser & Fenkart 1990) and has been used for initially classifying the stars into coarse metallicity classes: population I main sequence; intermediate population II main sequence; extreme population II main sequence; white objects; population I late-type red giants; (+) unclassified|
If a discontinuity is apparent in the above two-color distributions near , the stars redder than this gap are considered normal red giants (crosses) (Becker 1962, Becker & Fang 1982). Similarly, stars outside the theoretical grid for dwarfs (U-G > 3.0), but whose colors match the predicted synthetic loci for normal population I red giants (Buser 1978a), are classified as such. In either case, these stars are assigned absolute magnitudes (Buser 1981).
Objects far outside the theoretical calibrations in the two-color diagrams are either classified as "white objects" (open circles), or are left unclassified (plus signs); both these categories are excluded from the subsequent analysis of the density and luminosity functions.
Comparison with the local luminosity function based on the trigonometric parallax catalog of nearby stars by Gliese (1969) then provides the crucial consistency check: if the two disagree substantially and systematically, an iteration cycle is started by reassigning permissible alternative absolute magnitudes to an appropriately chosen number of suitable stars in such a way as to assimilate the resulting local luminosity function to the Gliese standard.
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