Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 437, Issue 2, Pages 1609-1627Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stt1993
Keywords
techniques: spectroscopic; stars: abundances; stars: Population II; globular clusters: general; globular clusters: individual: NGC 2808
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Funding
- Australian Research Council [DP120100475]
- Ministry of Science and Technology of the Kingdom of Spain [AYA 2010-16717]
- Instituto de Astrofisica de Canarias [P3-94]
- Proyecto Fondecyt [1110326, 1110303]
- BASAL Center for Astrophysics and Associated Technologies [PFB-06]
- FONDAP Center for Astrophysics [15010003]
- Proyecto Anillo [ACT-86]
- Chilean Ministry for the Economy, Development and Tourism's Programa Iniciativa Cientifica Milenio [P07-021-F]
- CAPES
- Proyecto GEMINI-CONICYT [32100022]
- School of Engineering at Pontificia Universidad Catolica de Chile
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We present an abundance analysis of 96 horizontal branch (HB) stars in NGC 2808, a globular cluster exhibiting a complex multiple stellar population pattern. These stars are distributed in different portions of the HB and cover a wide range of temperature. By studying the chemical abundances of this sample, we explore the connection between HB morphology and the chemical enrichment history of multiple stellar populations. For stars lying on the red HB, we use GIRAFFE and UVES spectra to determine Na, Mg, Si, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn, Y, Ba and Nd abundances. For colder, blue HB stars, we derive abundances for Na, primarily from GIRAFFE spectra. We were also able to measure direct non-local thermodynamic equilibrium He abundances for a subset of these blue HB stars with temperature higher than similar to 9000 K. Our results show that: (i) HB stars in NGC 2808 show different content in Na depending on their position in the colour-magnitude diagram, with blue HB stars having higher Na than red HB stars; (ii) the red HB is not consistent with a uniform chemical abundance, with slightly warmer stars exhibiting a statistically significant higher Na content; and (iii) our subsample of blue HB stars with He abundances shows evidence of enhancement with respect to the predicted primordial He content by Delta Y = +0.09 +/- 0.01 +/- 0.05 (internal plus systematic uncertainty). Our results strongly support theoretical models that predict He enhancement among second-generation(s) stars in globular clusters and provide observational constraints on the second-parameter governing HB morphology.
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