EDP Sciences
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Volume 420, Number 2, June III 2004
Page(s) 605 - 617
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20035699

A&A 420, 605-617 (2004)
DOI: 10.1051/0004-6361:20035699

The ubiquitous nature of the horizontal branch second U-jump

A link with the Blue Hook scenario?
Y. Momany1, L. R. Bedin1, S. Cassisi2, G. Piotto1, S. Ortolani1, A. Recio-Blanco1, F. De Angeli1 and F. Castelli3

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

(Received 17 November 2003 / Accepted 19 February 2004 )

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 $T_{\rm eff}\ge 23\,000$ K, and that its onset in different clusters converges around $T_{\rm eff}\sim 21\,000$  $\pm$ 3000 K. We also present near-ultraviolet diagrams of  $\omega$Cen and NGC 2808, the only two objects with spectroscopically confirmed "blue hook" stars ( $T_{\rm eff}\ge 35\,000$ 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.

Key words: stars: horizontal-branch -- stars: Hertzsprung-Russell (HR) and C-M diagrams -- stars: Population II -- Galaxy: globular clusters: general -- ultraviolet: stars

Offprint request: Y. Momany, momany@pd.astro.it

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