Stroemgren photometry of the Bulge: the Baade's Window and the globular cluster NGC 6522

TL;DR

This study uses Stroemgren photometry to analyze the metallicity of stars in Baade's Window and NGC 6522, revealing that the hk index may be influenced by factors beyond calcium abundance.

Contribution

It introduces a new theoretical calibration of the hk index for metallicity estimation and compares different color indices for accuracy.

Findings

hk index yields systematically higher metallicities.

hk index affected by calcium and other opacity sources.

Method distinguishes field and cluster stars effectively.

Abstract

We present Ca-uvby Stroemgren photometry of the Baade's Window, including the Galactic globular cluster NGC 6522. We separate field and cluster stars by adopting color-color planes and proper motions. We then estimate the global metallicity of red-giants (RGs) in NGC 6522 by using a new theoretical metallicity calibration of the Stroemgren index hk presented in Calamida et al. (2011). We find that metallicities estimated by adopting the hk index and the (Ca-y) color are systematically more metal-rich than metallicities estimated with hk and the (u-y), (v-y) and (b-y) colors. Current evidence indicate that the hk metallicity index is affected not only by the Calcium abundance, but also by another source of opacity.