AAVSO: American Association of Variable Star Observers

A New Study of the Variable Star Population in the Hercules Globular Cluster (M13; NGC 6205) (Abstract)

Volume 48 number 1 (2020)

Dan J. Deras
Cerrada Tenayuca 228, A201, Ciudad de Mexico, CDMX 3310, Mexico; dderas@astro.unam.mx
A. Arellano Ferro
address correspondence to Dan J. Deras, Cerrada Tenayuca 228, A201, Ciudad de Mexico, CDMX 3310, Mexico; dderas@astro.unam.mx
C. Lazaro
address correspondence to Dan J. Deras, Cerrada Tenayuca 228, A201, Ciudad de Mexico, CDMX 3310, Mexico; dderas@astro.unam.mx
I. H. Bustos Fierro
address correspondence to Dan J. Deras, Cerrada Tenayuca 228, A201, Ciudad de Mexico, CDMX 3310, Mexico; dderas@astro.unam.mx
J. H. Calderon
address correspondence to Dan J. Deras, Cerrada Tenayuca 228, A201, Ciudad de Mexico, CDMX 3310, Mexico; dderas@astro.unam.mx
S. Muneer
address correspondence to Dan J. Deras, Cerrada Tenayuca 228, A201, Ciudad de Mexico, CDMX 3310, Mexico; dderas@astro.unam.mx
Sunetra Giridhar
address correspondence to Dan J. Deras, Cerrada Tenayuca 228, A201, Ciudad de Mexico, CDMX 3310, Mexico; dderas@astro.unam.mx

Abstract

(Abstract only) 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 ± 0.09. The distance to the cluster was estimated as 7.1 ± 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 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, 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 in the Gaia DR2 database reveals that at least seven are not variable. We noted the presence of a high velocity star in the field of the cluster.