Stromgren Photometry of Galactic Globular Clusters. I. New Calibrations of the Metallicity Index

Abstract

We present a new calibration of the Strömgren metallicity index m 1 using red giant (RG) stars in four globular clusters (GCs: M92, M13, NGC 1851, 47 Tuc) with metallicity ranging from -2.2 to -0.7, marginally affected by reddening [E(B - V) ≤ 0.04] and with accurate (u, v, b, y) photometry. The main difference between the new metallicity-index-color (MIC) relations and similar relations available in the literature is that we have adopted the u - y and v - y colors instead of b - y. These colors present a stronger sensitivity to effective temperature, and the MIC relations show a linear slope. The difference between photometric estimates and spectroscopic measurements for RGs in M71, NGC 288, NGC 362, NGC 6397, andNGC 6752 is 0.04 ± 0.03 dex (σ = 0.11 dex). We also apply the new MIC relations to 85 field RGs with metallicity ranging from -2.4 to -0.5 and accurate reddening estimates. We find that the difference between photometric estimates and spectroscopic measurements is -0.14 ± 0.01 dex (σ = 0.17 dex). We also provide two sets of MIC relations based on evolutionary models that have been transformed into the observational plane by adopting either semiempirical or theoretical color-temperature relations. We apply the semiempirical relations to the nine GCs and find that the difference between photometric and spectroscopic metallicities is 0.04 ± 0.03 dex (σ = 0.10 dex). A similar agreement is found for the sample of field RGs, with a difference of -0.09 ± 0.03 dex (with σ = 0.19 dex). The difference between metallicity estimates based on theoretical relations and spectroscopic measurements is -0.11 ± 0.03 dex (σ = 0.14 dex) for the nine GCs and -0.24 ± 0.03 dex (σ = 0.15 dex) for the field RGs. Current evidence indicates that new MIC relations provide metallicities with an intrinsic accuracy better than 0.2 dex. © 2007. The American Astronomical Society. All rights reserved.