2017MNRAS.465.1046T

The Hubble Space Telescope UV Legacy survey of Galactic globular clusters (GC) is characterizing many different aspects of their multiple stellar populations. The 'Grundahl-jump' (G-jump) is a discontinuity in ultraviolet brightness of blue horizontal branch (HB) stars, signalling the onset of radiative metal levitation. The HB Legacy data confirmed that the G-jump is located at the same T_eff (≃11 500 K) in nearly all clusters. The only exceptions are the metal-rich clusters NGC 6388 and NGC 6441, where the G-jump occurs at T_eff ≃ 13-14 000 K. We compute synthetic HB models based on new evolutionary tracks including the effect of helium diffusion, and approximately accounting for the effect of metal levitation in a stable atmosphere. Our models show that the G-jump location depends on the interplay between the time-scale of diffusion and the time-scale of the evolution in the T_eff range 11 500 K<=T_eff<=14 000 K. The G-jump becomes hotter than 11 500 K only for stars that have, in this T_eff range, a helium mass fraction Y >= 0.35. Similarly high Y values are also consistent with the modelling of the HB in NGC 6388 and NGC 6441. In these clusters, we predict that a significant fraction of HB stars show helium in their spectra above 11 500 K, and full helium settling should only be found beyond the hotter G-jump.