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 CMD decontaminated 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 Mʘ and envelopes of 0.04 - 0.18 Mʘ, cover the complete horizontal branch. Models with total mass ≈ 0.68 Mʘ explain the RR Lyrae, while those with a mass ≈ 0.56 Mʘ and a very subtle envelope, explain the Pop II cepheids with a progenitor in the blue tail of the HB. Based on the Fourier decomposition of the V light curve of a single cluster member RRc star, we determined a metallicity of [Fe/H]ZW = −1.96 ± 0.09. Several independent distance determination approaches lead to a mean distance to M56 of ⟨d⟩ = 9.4 ± 0.4 kpc. Finally, we report 5 new variables: one SX Phe, three EB, and one Rrc.