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The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters - XI. The horizontal branch in NGC 6388 and NGC 6441

Tailo, M and D'Antona, F and Milone, AP and Bellini, A and Ventura, P and Di Criscienzo, M and Cassisi, S and Piotto, G and Salaris, M and Brown, TM and Vesperini, E and Bedin, LR and Marino, AF and Nardiello, D and Anderson, J (2017) The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters - XI. The horizontal branch in NGC 6388 and NGC 6441. Monthly Notices of the Royal Astronomical Society, 465 (1). pp. 1046-1056. ISSN 0035-8711

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Abstract

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 Teff (≃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 Teff ≃ 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 Teff range 11 500 K≲Teff≲14 000 K. The G-jump becomes hotter than 11 500 K only for stars that have, in this Teff 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. © 2016 The Authors.

Item Type: Article
Additional Information: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Uncontrolled Keywords: 0201 Astronomical And Space Sciences
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Publisher: Oxford University Press
Date Deposited: 11 May 2017 09:45
Last Modified: 09 Sep 2017 11:16
DOI or Identification number: 10.1093/mnras/stw2790
URI: http://researchonline.ljmu.ac.uk/id/eprint/6403

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