A new visit to the variable stars in M56 and its colour magnitude diagram structure

Abstract

We present a $VI$ CCD photometric study and a membership analysis of the globular cluster M56 (NGC 6779). This produced a decontaminated CMD from field stars, which enabled a better confrontation with theoretical isochrones; Zero-Age Horizontal Branches (ZAHB) and post-ZAHB evolutionary tracks. Post He-flash evolutionary models with a He-core mass of 0.5 M${}_{\odot }$ and envelopes of 0.04 - 0.18 M${}_{\odot }$, cover the complete Horizontal Branch. Models with total mass $\sim$ 0.68 M${}_{\odot }$ explain the RR Lyrae, while those with a mass $\sim$ 0.56 M${}_{\odot }$ and a very subtle envelope, explain the Pop II cepheids with a progenitor in the blue tail of the HB. %Using the mean $(V-I)$ at minimum color curve for the RRab stars, we derived a value for the reddening of $E(B-V)$ = 0.26 $\pm$ 0.02. Based on the Fourier decomposition of the $V$ light curve of a single cluster member RRc star, we determined a metallicity of $[Fe/H{]}_{\mathrm{ZW}}$ = -1.96 $\pm$ 0.09. Several independent distance determination approaches lead to a mean distance to M56 of $⟨d⟩$ = 9.4 $\pm$ 0.4 kpc. Finally, we report 5 new variables: 1 SX Phe, 3 EB, and 1 RRc.

13 pages, 8 figures, To be published by Revista Mexicana de Astronomia y Astrofisica

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• A vindication of the RR Lyrae Fourier light curve decomposition for the calculation of metallicity and distance in globular clusters
• Zwicky Transient Facility and Globular Clusters: The Period-Luminosity and Period-Wesenheit Relations for SX Phoenicis Variables in the gri-Band
• RR Lyrae stars in the globular cluster Palomar 2