Abstract

DERAS, D.; ARELLANO FERRO, A.; BUSTOS FIERRO, I.  and  YEPEZ, M. A.. A New Visit to the Variable Stars in M56 and its Colour-Magnitude Diagram Structure. Rev. mex. astron. astrofis [online]. 2022, vol.58, n.1, pp.121-132.  Epub Jan 23, 2023. ISSN 0185-1101.  https://doi.org/10.22201/ia.01851101p.2022.58.01.10.

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, zeroage horizontal branches (ZAHB) and post-ZAHB evolutionary tracks. Post He-flash evolutionary models with a He-core mass of 0.5 M⨀ and envelopes of 0.04 - 0.18 M⨀, cover the complete horizontal branch. Models with total mass ≈ 0:68 M⨀ explain the RR Lyrae, while those with a mass ≈ 0:56 M⨀ 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.