A REVISED DDO ABUNDANCE CALIBRATION FOR POPULATION-I RED GIANTS

Several arguments that justify establishing a revised abundance calibr ation for DDO photometry of population I red giants are presented. The components of the blanketing vector in the DDO C(45-48) vs C(42-45) d iagram are determined for late-type dwarfs and giants. We have redefin ed the DDO cyanogen anomaly and calibrated it against metallicity. The sample of field giants now available with abundances derived from hig h dispersion spectroscopy is substantially larger than previously avai lable, leading to a more accurate abundance calibration. Iso-abundance lines in the C(41-42) vs C(42-45) diagram have been determined for po pulation I G and K giants and an iterative method for deriving abundan ces of these stars is described. We show that the new DDO abundances a re in very good agreement with those derived from high dispersion spec troscopy. The new method improves by about 0.1 dex the DDO abundances derived for early G and/or late K giants, with respect to the deltaCN method of Janes (1975).