We present a CCD VI photometric study of the globular cluster M14. Particular attention is given to the variable stars. This allowed new classifications and cluster membership considerations. New variables are reported; 3 RRc, 18 SR and 1 SX Phe. The Fourier decomposition of RR Lyrae light curves lead to the mean cluster metallicity of [Fe/H]ZW = −1.3 ± 0.2. Several independent methods yield a mean distance of 9.36 ± 0.16 kpc. A Colour-Magnitude diagram outlined by the cluster members enabled a matching with theoretical predictions of isochrones and zero-age horizontal branches, whose fitting to the observations is in good agreement with the above distance and metallicity. The Oosterhoff type of M14 is confirmed as Oo-int, and the pulsating mode distribution of RR Lyrae stars on the HB shows that the bimodal region of the instability strip is shared by RRab and RRc stars. By modelling the mass loss at the RGB after the He flash events, we were able to represent the blue tail of the HB, using a core mass of 0.48 M⊙ and total masses of 0.52-0.55 M⊙. A progenitor star on the MS of 0.84 M⊙ reaches the HB in about 12.5 Gyrs, consonant with previous age determinations of the cluster. Type II Cepheids of M14 may be interpreted as products of post-HB evolution, driven by the complex processes involving the burning of the very thin low mass hydrogen and helium shells of these stars and their minuscule envelopes. No evidences were found in favor of M14 being of extragalactic origin.
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.
We present a CCD VI time-series analysis of the globular cluster NGC 6139 and its variable star population. Using the astrometric data available in Gaia-DR3 we performed a membership analysis that enabled the construction of a clean Colour-Magnitude Diagram (CMD). Variable stars in the field of the cluster reported by Gaia-DR3 and newly discovered ones in this paper are classified and their membership is critically evaluated. We report two cluster member RRc (V12, V15) and four SR (V13, V14, V17, V18) not previously detected and assign variable names to V11 and V16 detected by Gaia as they proof to be cluster members. Light curves and periods for non-member Gaia eclipsing binaries, semi regular variables and newly detected RR Lyrae stars are provided. Fourier decomposition of the light curves of the cluster member RRab and RRc stars leads to the values [Fe/H]=−1.63 dex, and distance of 9.63±0.68 kpc. The Oosterhoff type II nature of the cluster is confirmed. We adopted the mean reddening E(B − V)=0.786 mag and performed a differential reddening analysis based on the dispersion of the red giant branch. The differential map allowed a mild correction of the CMD.
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