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

1. Introduction

Strömgren photometry has been proven to be a reliable metallicity indicator for globular cluster giants and subgiants (e.g. Richtler 1989, Grebel & Richtler 1992 and references therein). The location of late type (G and K) giants in the Strömgren diagram is correlated with their metallicities, especially with their iron and CN abundances. Whereas the color is not sensitive to metallicity, the Strömgren v filter includes several iron absorption lines as well as the CN band at 4215Å, and therefore is a metallicity sensitive index (e.g. Bell & Gustafsson 1978). Within a certain color range, mag, the loci of constant iron abundance of giants and supergiants can be approximated by straight lines. This is valid for CN-"normal" ( CN-weak) stars. CN-strong stars, due to their higher absorption in the v filter, scatter to higher values and therefore mimic a higher Strömgren metallicity than their actual iron abundance would correspond to. If a cluster is known to have only small star-to-star variations in its iron abundance, the scatter to higher values can be used to uncover CN-rich member stars. For giants redder than mag the calibration breaks down due to TiO and MgH absorption in the y band.

Until now the calibration for Strömgren metallicities is based on the spectroscopically determined iron abundance of relatively metal-rich stars, and only few metal poorer stars (Grebel & Richtler 1992). This calibration worked well for the determination of cluster abundances in the Magellanic Clouds (Hilker et al. 1995a, 1995b; Dirsch et al. 2000), but failed to reproduce the slope of constant metallicity lines in the diagram for the new data of metal poor globular clusters. Other previous metallicity calibrations in the Strömgren plane have been published for the regime of F and G dwarfs (Schuster & Nissen 1989) and F dwarfs and giants in the small color range of (Malyuto 1994).

In the meantime, large samples of homogeneous spectroscopic measurements of cluster as well as field giants have become available. Furthermore, CCD Strömgren photometry, in particular in globular clusters, is becoming popular (e.g. Grundahl et al. 1999). Therefore, a revised, more accurate calibration of the metallicity sensitive Strömgren indices is needed. In this paper, the Strömgren metallicity calibration is extended to more metal-poor stars ([Fe/H] dex). The sample of stars used for the new calibration contains red giants of Centauri, M 55, and M 22, and field giants from a homogenized sample of Anthony-Twarog & Twarog (1998). Cen is known for its star-to-star variations in CN as well as iron abundances (e.g. Vanture et al. 1994). For 40 giants in Cen, Norris & Da Costa (1995) measured accurate abundances from high resolution spectroscopy. M 22 also shows CN abundance variations (e.g. Norris & Freeman 1982, 1983). Both clusters have been used to study the influence of the CN band strengths on the Strömgren metallicity.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
helpdesk.link@springer.de