Stromgren photometry of galactic globular clusters. I. New calibrations of the metallicity index

We present a new calibration of the Stromgren 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, and NGC 6752 is 0: 04 +/- 0: 03 dex (sigma = 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 (sigma = 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 (sigma = 0.10 dex). A similar agreement is found for the sample of field RGs, with a difference of -0.09 +/- 0.03 dex (with sigma = 0.19 dex). The difference between metallicity estimates based on theoretical relations and spectroscopic measurements is -0.11 +/- 0.03 dex (sigma = 0.14 dex) for the nine GCs and -0.24 +/- 0.03 dex (sigma = 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.