2019MNRAS.485.3076N

In this paper, we present a multiwavelength analysis of the complex stellar populations within the massive globular cluster Mayall II (G1), a satellite of the nearby Andromeda galaxy projected at a distance of 40 kpc. We used images collected with the Hubble Space Telescope in UV, blue and optical filters to explore the multiple stellar populations hosted by G1. The m_F438W versus m_F438W - m_F606W colour-magnitude diagram shows a significant spread of the red giant branch, that divides 1 mag brighter than the red clump. A possible explanation is the presence of two populations with different iron abundance or different C+N+O content, or different helium content, or a combination of the three causes. A similar red giant branch split is observed also for the Galactic globular cluster NGC 6388. Our multiwavelength analysis gives also the definitive proof that G1 hosts stars located on an extended blue horizontal branch. The horizontal branch of G1 exhibits similar morphology as those of NGC 6388 and NGC 6441, which host stellar populations with extreme helium abundance (Y > 0.33). As a consequence, we suggest that G1 may also exhibit large star-to-star helium variations.