Photometric Abundances for G Dwarfs: A Cautionary Tale

Analysis of cluster and field star uvby data demonstrates the existence of apreviously undetected discrepancy in a widely used photometric metallicitycalibration for G dwarfs. The discrepancy is systematic and stronglycolor-dependent, reducing the estimated [Fe/H] for stars above [Fe/H] ~ -0.2 bybetween +0.1 and +0.4 dex, and creating a deficit of metal-rich stars amongdwarfs of mid-G and later spectral type. The source of the problem, triggeredfor stars with b-y greater than about 0.47, appears to be an enhancedmetallicity dependence for the c1 index that increases as temperature declines.The link between c1, normally a surface gravity indicator, and metallicityproduces two secondary effects. The deficit in the photometric abundance for acool dwarf is partially compensated by some degree of evolution off the mainsequence and cool dwarfs with metallicities significantly above the Hyades arefound to have c1 indices that classify them as giants. The potential impact ofthe problem on stellar population studies is discussed.Comment: 12 columns, preprint format; 9 figures, gzipped compressed postscript; accepted for May 2002 Astronomical Journa