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.

Rr Lyrae Stars in the Globular Cluster Palomar 2

“…To this end, we employed the method of Bustos Fierro & Calderón (2019), which is based on the Balanced Iterative Reducing and Clustering using Hierarchies (BIRCH) algorithm developed by Zhang et al (1996). The method and our approach to it have been described in a recent paper by Deras et al (2022). We recall here that our method is based on a clustering algorithm at a first stage and a detailed analysis of the residual overdensity at a second stage; member stars extracted in the first stage are labeled M1, and those extracted in the second stage are labeled M2.…”