A new photometric study of M14 (NGC 6402): an interpretation of the horizontal branch and beyond

Abstract

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.

Zwicky Transient Facility and Globular Clusters: The Period–Luminosity and Period–Wesenheit Relations for Type II Cepheids

“…We have also searched the literature for new TIICs, and updated equatorial coordinates, periods, and classifications of TIICs in our preliminary list. We identified five new, or reclassified, TIICs: V24 in M10 (Rozyczka et al 2018), V167 in M14 (Yepez et al 2022), V34 and ZK3 in M15 (Bhardwaj et al 2021), and V24 in M22 (Rozyczka et al 2017. Similarly, we rejected the TIICs that were reclassified as other types of variable stars in recent work, they included V1 in M10 (identified as a semiregular variable in Rozyczka et al 2018), V72 and V142 in M15 (identified as an RR Lyrae and an anomalous Cepheid, respectively, in Bhardwaj et al 2021), V21 and V22 in M28 (identified as a long-period variable and an RR Lyrae, respectively, in Prieto et al 2012), V8 in M79 (identified as a semiregular variable in Bond et al 2016), and V7 in M92 (identified as an anomalous Cepheid in Osborn et al 2012).…”

A vindication of the RR Lyrae Fourier light curve decomposition for the calculation of metallicity and distance in globular clusters

“…As a result of different evolutionary sequences, the objects on the HB have a large range in He abundance causing the observed HB structure, particularly the colour range, and the breadth; the higher the value of Y, the more luminous the corresponding ZAHB would be. Recently it has been shown that small variations in the He-burning core mass would also contribute to the observed breadth of the HB (Yepez et al 2022). Hence, the values of the L parameter employed in the present investigation may be responding to these effects, which in turn may be responsible, at least partially, of the scatter observed in the correlations.…”

Rr Lyrae Stars in the Globular Cluster Palomar 2

“…each filter, respectively. The interested reader can find the details of this transformation approach in Yepez et al (2022).…”

“…In the right panel CMD of Figure 2, all variable stars have been marked with a red circle if they are cluster members or a black circle otherwise. As a reference we included two isochrones from the models of VandenBerg et al ( 2014) for [Fe/H]=−1.6 and −2.0 and a theoretical horizontal branch built by Yepez et al (2022). Isochrones and HB were placed at a distance of 26.1 kpc Bonatto & Chies-Santos (2020).…”

“…We adopted from these authors an average foreground reddening of E(B − V ) = 0.93. As a reference we included two isochrones from the models of VandenBerg et al (2014) for [Fe/H]=−1.6 and −2.0 and a theoretical horizontal branch built byYepez et al (2022). Isochrones and HB have been placed at a distance of 26.1 kpc(Bonatto & Chies-Santos 2020).…”