The Na-O anticorrelation in horizontal branch stars. I. NGC 2808

Abstract

Globular clusters have been recognized to host multiple stellar populations. A spectacular example of this is the massive cluster NGC2808, where multiple populations have been found along the horizontal branch (HB) and the main sequence (MS). Studies of red giants showed that this cluster appears homogeneous insofar Fe abundance is concerned, but it shows an extended anticorrelation between Na and O abundances. The Na-poor, O-rich population can be identified with the red MS, and the Na-rich, O-poor one with the blue one. This may be understood in terms of different He content, He being correlated with Na. A prediction of this scenario is that He-rich, Na-rich He-core burning stars, because they are less massive, will end up on the bluer part of the HB, while He-poor, Na-poor stars will reside on the red HB. The aim of this paper is to verify this prediction. To this purpose, we acquired high-resolution spectra of regions including strong O and Na lines in several tens of HB stars of NGC2808, sampling both the red and blue parts of the HB. We limited our analysis to those blue HB stars cooler than the gap at 11,500 K, because diffusion and radiative pressure are known to strongly modify the atmospheric composition of warmer stars. We indeed found a strict correspondence between the colour of the HB stars and their Na and O abundances: all blue HB stars are very O-poor and Na-rich. In addition, we found that while all the red HB stars are more O-rich and Na-poor than the blue ones, there is a moderate Na-O anticorrelation among them as well. This anticorrelation is in turn related to the colour of the red HB stars. These results reinforce the connection between Na and O abundances and the second parameter phenomenon, and show that there are more than three stellar populations in NGC 2808 because only a fraction of the red HB stars belong to the primordial population of this cluster.

Astronomy and Astrophysics, in press. 9 pages, 4 figures

References in corpus (22)

• The r-process of stellar nucleosynthesis: Astrophysics and nuclear physics achievements and mysteries
• Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters
• Fast rotating massive stars and the origin of the abundance patterns in galactic globular clusters
• The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters Without Previous HST Photometry
• The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851
• A non-LTE study of neutral and singly-ionized calcium in late-type stars
• The double Subgiant Branch of NGC 1851: the role of the CNO abundance
• An abundance analysis of bright giants in the globular cluster NGC 1851
• Multiple stellar populations in Magellanic Clouds clusters. I. An ordinary feature for intermediate age globulars in the LMC?
• The fraction of binary systems in the core of thirteen low-density Galactic globular clusters
• A large C+N+O abundance spread in giant stars of the globular cluster NGC 1851
• The Peculiar Horizontal Branch Morphology of the Galactic Globular Clusters NGC 6388 and NGC 6441: new insights from UV observations
• The Na-O anticorrelation in horizontal branch stars. III. 47 Tuc and M 5
• Nucleosynthesis in Magnetically Driven Jets from Collapsars
• Stroemgren photometry of Galactic Globular Clusters. I. New Calibrations of the metallicity index
• On the origin of the stellar halo and multiple stellar populations in the globular cluster NGC 1851
• Rotation and Macroturbulence in Metal-poor Field Red Giant and Red Horizontal Branch Stars
• The self-enrichment scenario in intermediate metallicity globular clusters
• The Na-O anticorrelation in horizontal branch stars. II. NGC1851
• Chemical and kinematical properties of BSSs and HB stars in NGC 6397
• Constraints on Helium Enhancement in the Globular Cluster M3 (NGC 5272): The Horizontal Branch Test
• The link between chemical anomalies along the red giant branch and the horizontal branch extension in globular clusters

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• Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters
• The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. I. Overview of the Project and Detection of Multiple Stellar Populations
• What is a globular cluster? An observational perspective
• N-body modeling of globular clusters: Masses, mass-to-light ratios and intermediate-mass black holes
• The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XVI. The helium abundance of multiple populations
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• The Horizontal Branch in the UV Colour Magnitude Diagrams. II. The case of M3, M13 and M79
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• Searching for multiple populations in star clusters using the China Space Station Telescope
• Chemistry of multiple stellar populations in the mono-metallic, in situ, bulge globular cluster NGC 6388
• A stellar population synthesis approach to the Oosterhoff dichotomy
• Investigation of the Globular Cluster NGC 2808 with the Ultra-Violet Imaging Telescope
• The chemical compositions of multiple stellar populations in the globular cluster NGC 2808
• New sub-grouping of multiple stellar populations in NGC 2808 based on low-resolution spectroscopy
• On the Use of the Number Count of Blue Horizontal--Branch Stars to Infer the Dominant Building Blocks of the Milky Way Halo
• Exploring the nature and synchronicity of early cluster formation in the Large Magellanic Cloud: III. Horizontal Branch Morphology
• A Wide View of the Galactic Globular Cluster NGC 2808: Red Giant and Horizontal Branch Star Spatial Distributions
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