Tailo, M.M.TailoD'Antona, F.F.D'AntonaMilone, A. P.A. P.MiloneBellini, A.A.BelliniVENTURA, PaoloPaoloVENTURADI CRISCIENZO, MarcellaMarcellaDI CRISCIENZOCASSISI, SantiSantiCASSISIPiotto, G.G.PiottoSalaris, M.M.SalarisBrown, T. M.T. M.BrownVesperini, E.E.VesperiniBEDIN, LuigiLuigiBEDINMarino, A. F.A. F.MarinoNardiello, D.D.NardielloAnderson, J.J.Anderson2021-02-022021-02-0220170035-8711http://hdl.handle.net/20.500.12386/30161The 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<SUB>eff</SUB> (≃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<SUB>eff</SUB> ≃ 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<SUB>eff</SUB> range 11 500 K≲T<SUB>eff</SUB>≲14 000 K. The G-jump becomes hotter than 11 500 K only for stars that have, in this T<SUB>eff</SUB> 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.STAMPAenArticle10.1093/mnras/stw27902-s2.0-85014743707000393782000073http://arxiv.org/abs/1610.08264v1https://academic.oup.com/mnras/article/465/1/1046/24547402017MNRAS.465.1046TFIS/05 - ASTRONOMIA E ASTROFISICA