Stroemgren photometry of Galactic Globular Clusters (I)

M92, M13, NGC1851, 47Tuc :: new preprint

[astro-ph/0707.1019]
Stroemgren photometry of Galactic Globular Clusters. I. New Calibrations of the metallicity index:

Authors: A. Calamida (1 and 2), G. Bono (1), P.B. Stetson (3), L. M. Freyhammer (4), S. Cassisi (5), F. Grundahl (6), A. Pietrinferni (5), M. Hilker (7), F. Primas (7), T. Richtler (8), M. Romaniello (7), R. Buonanno (2), F. Caputo (1), M. Castellani (1), C. E. Corsi (1), I. Ferraro (1), G. Iannicola (1), L. Pulone (1) ((1) OAR/INAF, (2) Univ. Roma Tor Vergata, (3) HIA/NRC, (4) Univ. Lancashire, (5) OACT/INAF, (6) Aarhus Univ., (7) ESO, (8) Univ. de Concepcion) (Submitted on 6 Jul 2007)

Abstract: We present a new calibration of the Stroemgren metallicity index m1 using red giant (RG) stars in 4 globular clusters (GCs:M92,M13,NGC1851,47Tuc) with metallicity ranging from [Fe/H]=-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 adopted the u-y/v-y colors instead of the 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,NGC288,NGC362,NGC6397, and NGC6752 is 0.04+/-0.03dex (sigma=0.11dex). We also apply the MIC relations to 85 field RGs with metallicity raning from [Fe/H]=-2.4 to -0.5 and accurate reddening estimates. We find that the difference between photometric estimates and spectroscopic measurements is-0.14+/-0.01dex (sig=0.17dex). We also provide two 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. We apply the semi-empirical relations to the 9 GCs and find that the difference between photometric and spectroscopic metallicities is 0.04+/-0.03dex (sig=0.10dex).A similar agreement is found for the sample of field RGs, with a difference of -0.09+/-0.03dex (sig=0.19dex).The difference between metallicity estimates based on theoretical relations and spectroscopic measurements is -0.11+/-0.03dex (sig=0.14dex) for the 9 GGCs and -0.24+/-0.03dex (sig=0.15dex) for the field RGs. Current evidence indicates that new MIC relations provide metallicities with an intrinsic accuracy better than 0.2dex.

51 pages, 19 figures, accepted for submission to ApJ
http://babbage.sissa.it/abs/0707.1019

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