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arXiv · 1110.4314

On a new theoretical calibration of the Stroemgren hk metallicity index: NGC6522 as a first test case

A. Calamida·G. Bono·C. Corsi·G. Iannicola·V. Ripepi·B. Anthony-Twarog·B. Twarog·M. Zoccali·R. Buonanno·S. Cassisi·I. Ferraro·F. Grundahl·A. Pietrinferni·L. Pulone
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Abstract

We present a new theoretical calibration of the Stroemgren metallicity index hk by using alpha-enhanced evolutionary models transformed into the observational plane by using atmosphere models with the same chemical mixture. We apply the new Metallicity--Index--Color (MIC) relations to a sample of 85 field red giants (RGs) and find that the difference between photometric estimates and spectroscopic measurements is on average smaller than 0.1 dex with a dispersion of sigma = 0.19 dex. The outcome is the same if we apply the MIC relations to a sample of eight RGs in the bulge globular cluster NGC6522, but the standard deviation ranges from 0.26 (hk, v-y) to 0.49 (hk, u-y). The difference is mainly caused by a difference in photometric accuracy. The new MIC relations based on the (Ca-y) color provide metallicities systematically more metal-rich than the spectroscopic ones. We found that the Ca-band is affected by Ca abundance and possibly by chromospheric activity.

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A. Calamida, G. Bono, C. Corsi, G. Iannicola, V. Ripepi, B. Anthony-Twarog, B. Twarog, M. Zoccali, R. Buonanno, S. Cassisi, I. Ferraro, F. Grundahl, A. Pietrinferni, L. Pulone. 2011-10-19. On a new theoretical calibration of the Stroemgren hk metallicity index: NGC6522 as a first test case. https://doi.org/10.1088/2041-8205%2F742%2F2%2Fl28

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