Astron. Astrophys. 355, 994-1002 (2000)

6. Summary

Red giants in the globular clusters Centauri, M 55, and M 22 together with field giants from Anthony-Twarog & Twarog (1998) have been used to revise the Grebel & Richtler (1992) metallicity calibration of the Strömgren diagram. For all giants in Cen and M 22, accurate and homogeneous iron abundances from high resolution spectroscopy are available in the literature. M 55 has a well determined average iron abundance value. In total, 62 CN-weak giants have been used. CN-rich stars have been excluded, since their value mimics a too high iron abundance in the diagram. In order to cover a wide metallicity range, [Fe/H] dex, our new calibration is connected to a previous calibration by Grebel & Richtler (1992) around solar metallicities. In the color range mag, for which our calibration is valid, the loci of equal iron abundances lie on straight lines.

We emphasize that it was possible to find a uniform metallicity calibration in the indicated parameter range that seems to have no obvious dependencies on luminosity within the errors. In particular, the new calibration seems to be valid for globular cluster as well as for field giants. No variation of metallicity sensitivity with metallicity in the diagram has been found.

On average, the precision of a metallicity determination with our new calibration for a single giant is in the order of 0.11 dex. Average abundances of giants within a cluster and relative abundances between clusters can be determined with much higher precision, depending on the number of red giants per cluster sample. The application of the new calibration to independent samples of red giants with published Strömgren photometry agrees very well with otherwise determined abundances.

For the red giants in Cen and M 22, the influence of CN-strong stars on the metallicity calibration has been studied. For Strömgren metallicities higher than -1.0 dex, CN-weak stars cannot be distinguished in the diagram from stars with lower iron abundances but higher CN band strengths. However, the difference between the Strömgren metallicity of CN-rich stars and their spectroscopically determined iron abundance is correlated to the CN band strengths. This might be used to determine the iron abundance of a red giant if its Strömgren colors and one of the CN indices are known, or alternatively to detect CN-rich stars when their iron abundances are known.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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