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

TL;DR

This paper introduces a new theoretical calibration of the Stroemgren hk metallicity index using alpha-enhanced models, demonstrating improved photometric metallicity estimates for red giants, with some limitations due to photometric accuracy and Ca-band effects.

Contribution

The paper presents a novel calibration of the hk index with alpha-enhanced models and applies it to field and cluster red giants, improving photometric metallicity estimates.

Findings

Photometric estimates agree with spectroscopic measurements within 0.1 dex.

Calibration works well for field RGs and NGC6522 cluster RGs.

Ca-band calibration may be affected by Ca abundance and chromospheric activity.

Abstract

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.