Photometric Abundances for G Dwarfs: A Cautionary Tale

Analysis of cluster and field star uvby data demonstrates the existence of a previously undetected discrepancy in a widely used photometric metallicity calibration for G dwarfs. The discrepancy is systematic and strongly color dependent, reducing the estimated [Fe/H] for stars above [Fe/H] ∼ -0.2 by between +0.1 and +0.4 dex and creating a deficit of metal-rich stars among dwarfs of mid G and later spectral type. The source of the problem, triggered for stars with b-y greater than about 0.47, appears to be an enhanced metallicity dependence for the c₁ index that increases as temperature declines. The link between c₁, normally a surface gravity indicator, and metallicity produces two secondary effects. The deficit in the photometric abundance for a cool dwarf is partially compensated by some degree of evolution off the main sequence, and cool dwarfs with metallicities significantly above the Hyades are found to have c₁ indices that classify them as giants. The potential impact of the problem on stellar population studies is discussed.