The ubiquitous nature of the horizontal branch second U-jump. A link with the Blue Hook scenario?

In a previous paper we reported on a discontinuity in the extreme horizontal branch (EHB) of the Galactic globular cluster NGC 6752, which we called the second U-jump. This feature was attributed to a combination of post zero-age horizontal branch evolution and diffusion effects. In this follow-up study we analyze other EHB clusters and show that the second U-jump is a common feature among EHB clusters reaching Teff≥23 000 K, and that its onset in different clusters converges around Teff˜ 21 000 ± 3000 K. We also present near-ultraviolet diagrams of ωCen and NGC 2808, the only two objects with spectroscopically confirmed ``blue hook'' stars (Teff≥35 000 K). We confirm predictions of a photometric discontinuity separating late from early-helium flashers. Moreover, we present empirical evidence that the second U-jump population might be mainly composed of early-helium flashers. Lastly, we revisit the discussion on the ubiquitous nature of the gaps and jumps so far identified in the blue HB tails, suggesting a possible discrete nature of the distribution in temperature of the HB stars.

Based on observations with the ESO/MPI 2.2 m and ESO/NTT telescopes, located at La Silla Observatory (Chile) and on observations with the NASA/ESA Hubble Space Telescope.