The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters - XI. The horizontal branch in NGC 6388 and NGC 6441
Date Issued
2017
Author(s)
Tailo, M.
•
D'Antona, F.
•
Milone, A. P.
•
Bellini, A.
•
•
•
•
Piotto, G.
•
Salaris, M.
•
Brown, T. M.
•
Vesperini, E.
•
•
•
Nardiello, D.
•
Anderson, J.
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.
Volume
465
Issue
1
Start page
1046
Issn Identifier
0035-8711
Ads BibCode
2017MNRAS.465.1046T
Rights
open.access
File(s)![Thumbnail Image]()
Loading...
Name
stw2790.pdf
Description
Pdf editoriale
Size
928.38 KB
Format
Adobe PDF
Checksum (MD5)
a4da86b9bfb25f9e40a241ae5ec3c7b3