AN ABUNDANCE CALIBRATION FOR DDO PHOTOMETRY OF POPULATION-II G AND K GIANTS

New photometry with the four primary filters of the DDO system is presented for a sample of metal-poor giants with abundances determined from high-dispersion spectroscopy. In addition, DDO photometry of very metal-deficient stars with photometric abundances is also given. These data, combined with published DDO photometry of field and globular cluster stars, are used to establish an abundance calibration for Population II giants. A new DDO index delta4548 is presented, which correlates very well with [Fe/H], even for extremely metal-poor red giants ([Fe/H] = -3.0). The present abundance calibration represents an improvement over previous ones, because it exhibits less scatter and a more extended range towards lower abundances. Iso-abundance lines have been determined in the C0(45-48) versus C0(42-45) diagram and a new procedure for determining abundances of Population II giants is described. Finally, the DDO abundance scale established here for globular clusters is in good agreement with that of Zinn.