We present a new calibration of the Stroemgren metallicity index m _ { 1 } using red giant ( RG ) stars in a sample of Galactic globular clusters ( GGCs : M92 , M13 , NGC 1851 , 47 Tuc ) that cover a broad range in metallicity ( -2.2 \leq \hbox { [ Fe / H ] } \leq - 0.7 ) , are marginally affected by reddening uncertainties ( E ( \hbox { \it B - - V\ / } ) \leq 0.04 ) and for which accurate u,v,b,y Stroemgren photometry is available to well below the turnoff region .
The main difference between the new empirical 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 the b–y color .
These colors present a stronger sensitivity to effective temperature , and the MIC relations show a linear and well-defined slope .
The net difference between photometric estimates and spectroscopic measurements , for RG stars in five GGCs : M71 , NGC 288 , NGC 362 , NGC 6397 , NGC 6752 , is 0.04 \pm 0.03 dex with \sigma = 0.11 dex .
We also apply the new MIC relations to a sample of field stars for which spectroscopic metallicity ( -2.4 \leq \hbox { [ Fe / H ] } \leq - 0.5 ) , accurate Strömgren photometry , and reddening estimates ( Anthony-Twarog & Twarog 1994 , 1998 ) are all available .
We find that the difference between photometric estimates and spectroscopic measurements is on average -0.14 \pm 0.01 dex , with \sigma = 0.17 dex . We also provide two independent sets of MIC relations based on evolutionary models that have been transformed into the observational plane by adopting either semi-empirical or theoretical color-temperature relations ( CTRs ) .
We apply the semi-empirical \alpha - enhanced MIC relations to the nine GCs and find that the difference between photometric estimates and spectroscopic measurements is 0.04 \pm 0.03 dex , with \sigma = 0.10 dex .
A similar agreement is also found for the sample of field stars , and indeed the difference is -0.09 \pm 0.03 dex , with \sigma = 0.19 dex .
The difference between metallicity estimates based on theoretical scaled-solar and spectroscopic measurements -0.11 \pm 0.03 dex , with \sigma = 0.14 dex for the nine GGCs and -0.24 \pm 0.03 dex , with \sigma = 0.15 dex for the field stars .
On the whole , current findings support the evidence that new Strömgren MIC relations provide metallicity estimates with an intrinsic accuracy better than 0.2 dex .