A Stromgren view of the multiple populations in globular clusters

TL;DR

This paper evaluates various Stromgren photometric indices to identify the most effective ones for studying multiple stellar populations in globular clusters, introducing a new index that is robust across different metallicities.

Contribution

It introduces a new photometric index delta_4 that effectively separates stellar generations in globular clusters regardless of metallicity.

Findings

Delta_4 index effectively distinguishes stellar populations across metallicities.

Photometric indices reveal multiple populations through color distribution differences.

Synthetic spectra show how chemical compositions influence Stromgren indices.

Abstract

We discuss a variety of photometric indices assembled from the uvby Stromgren system. Our aim is to examine the pros and cons of the various indices to find the most suitable one(s) to study the properties of multiple populations in globular clusters (GCs) discovered by spectroscopy. We explore in particular the capabilities of indices like m_1 and c_y at different metallicities. We define a new index delta_4=(u-v)-(b-y) to separate first and second stellar generations in GCs of any metal abundance, since it keeps the sensitivity to multiple stellar populations over all the metallicity range and at the same time minimizes the sensitivity to photometric errors. We detecte clear differences in the red giant branches of the GCs examined, like skewness or bi/multi-modality in color distribution. We connect the photometric information with the spectroscopic results on O, Na abundances we obtained in our survey of GCs. Finally, we compute the effects of different chemical composition on the Stromgren filters and indices using synthetic spectra.