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http://hdl.handle.net/20.500.12386/30161
Titolo: | The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters - XI. The horizontal branch in NGC 6388 and NGC 6441 | Autori: | Tailo, M. D'Antona, F. Milone, A. P. Bellini, A. VENTURA, Paolo DI CRISCIENZO, Marcella CASSISI, Santi Piotto, G. Salaris, M. Brown, T. M. Vesperini, E. BEDIN, Luigi Marino, A. F. Nardiello, D. Anderson, J. |
Data pubblicazione: | 2017 | Rivista: | MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY | Numero: | 465 | Fascicolo: | 1 | Da pagina:: | 1046 | 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 T<SUB>eff</SUB> (≃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 T<SUB>eff</SUB> ≃ 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 T<SUB>eff</SUB> range 11 500 K≲T<SUB>eff</SUB>≲14 000 K. The G-jump becomes hotter than 11 500 K only for stars that have, in this T<SUB>eff</SUB> 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. | URI: | http://hdl.handle.net/20.500.12386/30161 | URL: | http://arxiv.org/abs/1610.08264v1 https://academic.oup.com/mnras/article/465/1/1046/2454740 |
ISSN: | 0035-8711 | DOI: | 10.1093/mnras/stw2790 | Bibcode ADS: | 2017MNRAS.465.1046T | Fulltext: | open |
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