M13 multiple stellar populations seen with the eyes of Str\"omgren photometry

TL;DR

This study uses Str"omgren photometry and Hubble data to analyze the spatial distribution of multiple stellar populations in M13, finding no significant segregation, indicating a well-mixed state.

Contribution

It demonstrates the effectiveness of the $c_{y}$ index in identifying multiple populations and combines wide-field and core data to assess their spatial distribution in M13.

Findings

Multiple populations are spatially mixed in M13.

The $c_{y}$ index reliably separates populations.

No significant radial segregation observed.

Abstract

We present a photometric study of M13 multiple stellar populations over a wide field of view, covering approximately 6.5 half-light radii, using archival Isaac Newton Telescope observations to build an accurate multi-band Str\"omgren catalogue. The use of the Str\"omgren index $c_{y}$ permits us to separate the multiple populations of M13 on the basis of their position on the red giant branch. The comparison with medium and high resolution spectroscopic analysis confirms the robustness of our selection criterion. To determine the radial distribution of stars in M13, we complemented our dataset with Hubble Space Telescope observations of the cluster core, to compensate for the effect of incompleteness affecting the most crowded regions. From the analysis of the radial distributions we do not find any significant evidence of spatial segregation. Some residuals may be present in the external regions where we observe only a small number of stars. This finding is compatible with the short dynamical timescale of M13 and represents, to date, one of the few examples of fully spatially mixed multiple populations in a massive globular cluster.