A new study of the variable star population in the Hercules globular cluster (M13; NGC 6205)

Abstract

We present the results from $VI$ CCD time-series photometry of the globular cluster M13 (NGC 6205). From the Fourier decomposition of the light curves of RRab and RRc stars we found an average metallicity of [Fe/H]zw = -1.58 $\pm$ 0.09. The distance to the cluster was estimated as 7.1 $\pm$ 0.1 kpc from independent methods related to the variable star families RR Lyrae, SX Phe and W Virginis, from the luminosity of the theoretical ZAHB and from the orbit solution of a newly discovered contact binary star. The RR Lyrae pulsation modes are segregated by the red edge of the first overtone instability strip in this OoII type cluster. A membership analysis of 52,800 stars in the field of the cluster is presented based on \textit{Gaia}-DR2 proper motions which enabled the recognition of 23,070 likely cluster members, for 7,630 of which we possess $VI$ photometry. The identification of member stars allowed the construction of a clean CMD and a proper ZAHB and isochrone fitting, consistent with a reddening, age and distance of 0.02 mag, 12.6 Gyrs and 7.1 kpc respectively. We report seven new variables; one RRc, two SX Phe stars, three SR and one contact binary. V31 presents double-mode nature and we confirm V36 as RRd. Fifteen variable star candidates are also reported. The analysis of eighteen stars in the field of the cluster, reported as RR Lyrae from the $Gaia$-DR2 data base reveals that at least seven are not variable. We noted the presence of a high velocity star in the field of the cluster.

18 pages, 21 figures

References in corpus (6)

• A New Algorithm For Difference Image Analysis
• A 1.4-GHz Arecibo Survey for Pulsars in Globular Clusters
• Isochrones for Old (> 5 Gyr) Stars and Stellar Populations. I. Models for $-2.4\le$ [Fe/H] $\le +0.6$, $0.25\le Y\le 0.33$, and $-0.4\le$ [$\alpha$\Fe] $\le +0.4$
• Variable stars in the Fornax dSph Galaxy. II. Pulsating stars below the horizontal branch
• Difference image analysis: The interplay between the photometric scale factor and systematic photometric errors
• CCD time-series photometry of variable stars in globular clusters and the metallicity dependence of the horizontal branch luminosity

Cited by in corpus (2)

• Isochrone fitting of Galactic globular clusters -- II. NGC6205 (M13)
• Dwarfs or giants? Stellar metallicities and distances in the Canada-France-Imaging-Survey from $ugrizG$ multi-band photometry