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Photometric Abundances for G Dwarfs: A Cautionary Tale
Analysis of cluster and field star uvby data demonstrates the existenceof a previously undetected discrepancy in a widely used photometricmetallicity calibration for G dwarfs. The discrepancy is systematic andstrongly 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 ofmetal-rich stars among dwarfs of mid G and later spectral type. Thesource of the problem, triggered for stars with b-y greater than about0.47, appears to be an enhanced metallicity dependence for thec1 index that increases as temperature declines. The linkbetween c1, normally a surface gravity indicator, andmetallicity produces two secondary effects. The deficit in thephotometric abundance for a cool dwarf is partially compensated by somedegree of evolution off the main sequence, and cool dwarfs withmetallicities significantly above the Hyades are found to havec1 indices that classify them as giants. The potential impactof the problem on stellar population studies is discussed.
| A large, complete, volume-limited sample of G-type dwarfs. I. Completion of Stroemgren UVBY photometry
Four-colour photometry of potential dwarf stars of types G0 to K2,selected from the Michigan Spectral Catalogues (Vol. 1-3), has beencarried out. The results are presented in a catalogue containing 4247uvby observations of 3900 stars, all south of δ = -26deg. Theoverall internal rms errors of one observation (transformed to thestandard system) of a program star in the interval 8.5 < V < 10.5are 0.0044, 0.0021, 0.0039, and 0.0059, respectively, in V, b-y, m_1_ ,and c_1_. The purpose of the catalogue, combined with earliercatalogues, is to allow selection of a large, complete, volume-limitedsample of G- and K-type dwarfs, investigate their metallicitydistribution, and compare it to predictions of various models ofgalactic chemical evolution. Future papers in this series will discussthese subjects.
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