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Frank Grundahl

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

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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
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.
Original languageEnglish
JournalThe Astrophysical Journal Letters
ISSN2041-8205
Publication statusPublished - 19 Oct 2011

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