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 pres ented for a sample of metal-poor giants with abundances determined fro m high-dispersion spectroscopy. In addition, DDO photometry of very me tal-deficient stars with photometric abundances is also given. These d ata, combined with published DDO photometry of field and globular clus ter stars, are used to establish an abundance calibration for Populati on 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 improvemen t over previous ones, because it exhibits less scatter and a more exte nded range towards lower abundances. Iso-abundance lines have been det ermined in the C0(45-48) versus C0(42-45) diagram and a new procedure for determining abundances of Population II giants is described. Final ly, the DDO abundance scale established here for globular clusters is in good agreement with that of Zinn.