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 (HBs), 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 red giant branch 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 main sequence (MS) of 0.84 M⊙ reaches the HB in about 12.5 Gyr, 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 pieces of evidence were found in favour of M14 being of extragalactic origin.