The variable stars in the field of the bulge cluster NGC 6558

TL;DR

This study surveyed 78 variable stars in the field of the Galactic bulge cluster NGC 6558, identifying cluster members, classifying variables, and estimating the cluster's properties using Gaia and other data sources.

Contribution

It provides the first membership analysis of variable stars in NGC 6558 and identifies two new cluster member variables, refining the cluster's characteristics.

Findings

Only 9 variables are likely cluster members.

Two new member variables discovered: an eclipsing binary and a semi-regular variable.

Estimated the cluster's mean metallicity and distance.

Abstract

We made a survey of the variable stars in a $13.2 \times 13.2$ arcmin$^2$ centered on the field of the Galactic bulge cluster NGC 6558. A total of 78 variables was found in the field of the cluster. Many of these variables are included in the Catalogue of Variable Stars in Galactic Globular Clusters (Clement et al. 2001), OGLE or Gaia-DR3 data releases. A membership analysis based on the proper motions of Gaia-DR3 revealed that many of these variables do not belong to the cluster. We employed the data from the aforementioned surveys and our own data in the VI photometric system to estimate the periods, which along with the light curves morphology and position in a deferentially dereddened colour-magnitude diagram(CMD), help classifying the variable types. Two new member variables were found; an eclipsing binary (V18) and a semi-regular SR/L (V19). In the end we conclude that only 9 variables are likely cluster members. Member variables were used to discuss the mean metallicity and distance of the parental cluster and find the average values.