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]similar to -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 c(1) index that increases as temperature declines. The link between c(1), 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 c(1) indices that classify them as giants. The potential impact of the problem on stellar population studies is discussed.