Chemically peculiar candidates in 18 Milky Way globular clusters

¹ Department of Theoretical Physics and Astrophysics, Masaryk University, Kotlářská 2, 60200 Brno, Czech Republic
² Astronomical Institute of Czech Academy of Sciences, Fričova 298, 251 65 Ondřejov, Czech Republic
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 1 December 2025
Accepted: 31 March 2026

Abstract

Context. Recent discoveries show that chemically peculiar (CP) stars also reside in globular clusters (GCs). The channels leading to chemical peculiarity are, however, still under discussion.

Aims. The main goals of our investigation are to identify the CP stars in GCs and search for systematic relations among different stellar groups commonly observed in GCs and chemical peculiarity. Additionally, we search for possible correlations of the occurrence rate of CP stars with the cluster properties.

Methods. We obtained BVg2 multicolour photometry for 18 southern GCs with the 1.54 m Danish telescope located at La Silla observatory in Chile. The data were reduced and processed with standard IRAF tools. To avoid blending, we omitted central parts of the clusters. We determined the membership of the stars to a particular GC based on the proper motion and parallax of the stars from the Gaia DR3 catalogue. Colour-magnitude diagrams were constructed, and only stars on horizontal branch and in the region of blue stragglers were considered for further analysis. Construction of the Δa diagram allowed us to identify CP candidate stars with high probability. To test the reliability of our approach, we used spectroscopic observations of M53.

Results. Except for four GCs, CP candidates were found in all other GCs with the average fraction of 4.2%. We found that a significant portion of the candidates are blue stragglers. These constitute a full sample of identified CP candidates in four GCs and represent more than 60% of the candidates in ten GCs.

Conclusions. The a photometry was found as a powerful tool for the identification of CP stars in GCs. We discovered a possible connection between chemical peculiarity and blue straggler stars.