The ubiquitous nature of the horizontal branch second U-jump^*

A link with the Blue Hook scenario?

¹ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy e-mail: [piotto,bedin,ortolani,recio,deangeli]@pd.astro.it
² INAF - Osservatorio Astronomico di Collurania, Via M. Maggini, 64100 Teramo, Italy e-mail: cassisi@te.astro.it
³ INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy e-mail: castelli@ts.astro.it

Corresponding author: Y. Momany, momany@pd.astro.it

Received: 17 November 2003
Accepted: 19 February 2004

Abstract

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  K, and that its onset in different clusters converges around  ± 3000 K. We also present near-ultraviolet diagrams of ωCen and NGC 2808, the only two objects with spectroscopically confirmed “blue hook” stars ( 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.