Searching for variable stars in the cores of five metal rich globular clusters using EMCCD observations

TL;DR

This study used EMCCD observations and difference image analysis to identify and classify known and new variable stars in the cores of five metal-rich globular clusters, including the first double mode RR Lyrae in NGC 6441.

Contribution

It provides a comprehensive census of variable stars in the cluster cores using high-precision light curves, revealing new variables and detailed classifications, especially in NGC 6441.

Findings

Identified 48-49 new variables in the clusters.

Detected the first double mode RR Lyrae in NGC 6441.

Presented detailed classifications of variable stars.

Abstract

In this paper, we present the analysis of time-series observations from 2013 and 2014 of five metal rich ([Fe/H] $>$ -1) globular clusters: NGC~6388, NGC~6441, NGC~6528, NGC~6638, and NGC~6652. The data have been used to perform a census of the variable stars in the central parts of these clusters. The observations were made with the electron multiplying CCD (EMCCD) camera at the Danish 1.54m Telescope at La Silla, Chile, and they were analysed using difference image analysis (DIA) to obtain high-precision light curves of the variable stars. It was possible to identify and classify all of the previously known or suspected variable stars in the central regions of the five clusters. Furthermore, we were able to identify, and in most cases classify 48, 49, 7, 8, and 2 previously unknown variables in NGC~6388, NGC~6441, NGC~6528, NGC~6638, and NGC~6652, respectively. Especially interesting is the case of NGC~6441, for which the variable star population of about 150 stars has been thoroughly examined by previous studies, including a Hubble Space Telescope study. In this paper we are able to present 49 new variable stars for this cluster, of which one (possibly two) are RR Lyrae stars, two are W Virginis stars, and the rest are long period semi-regular/irregular variables on the red giant branch. We have also detected the first double mode RR Lyrae in the cluster.