B. Twarog | Searcharxiv

arXiv2011

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

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.

astro-ph.SR↗

arXiv2011

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

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.

astro-ph.SR↗