PHOTOMETRIC DISCRIMINATE FOR GK DWARFS OF DISK POPULATIONS

The young and old disk populations, with ages respectively less than and greater than about 2×109 yr, are separated in the (R-I,42-45) plane, where the 42-45 index of the DDO system is based on bandpasses of 75 Å half-width and effective wavelengths of 4257 and 4571 Å. A similar, but smaller separation is seen in the (g,B2-VI) plane where g is the Geneva system alternative to 42-45 but with bandpasses of 175 Å half-width and effective wavelengths of 4015 and 4476 Å. Although models, based on the assumption that this separation results from a veil of weak metal lines (Magain 1983), may explain these phenomena in the (g,B2-VI) plane for the hotter stars, more models are required to test it for the DDO system and for cooler objects. The available evidence suggests that an additional causation, probably connected with the CN abundance, also exists.