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.

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 ARELLANO FERRO have a large range of He abundance causing the observed HB structure, particularly the color 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.…”