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|Title:||The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. III. A Quintuple Stellar Population in NGC 2808||Authors:||Milone, A. P.
Marino, A. F.
|Issue Date:||2015||Journal:||THE ASTROPHYSICAL JOURNAL||Number:||808||Issue:||1||First Page:||51||Abstract:||In this study we present the first results from multi-wavelength Hubble Space Telescope (HST) observations of the Galactic globular cluster (GC) NGC 2808 as an extension of the Hubble Space Telescope UV Legacy Survey of Galactic GCs (GO-13297 and previous proprietary and HST archive data). Our analysis allowed us to disclose a multiple-stellar-population phenomenon in NGC 2808 even more complex than previously thought. We have separated at least five different populations along the main sequence and the red giant branch (RGB), which we name A, B, C, D, and E (though an even finer subdivision may be suggested by the data). We identified the RGB bump in four out of the five RGBs. To explore the origin of this complex color-magnitude diagram, we have combined our multi-wavelength HST photometry with synthetic spectra, generated by assuming different chemical compositions. The comparison of observed colors with synthetic spectra suggests that the five stellar populations have different contents of light elements and helium. Specifically, if we assume that NGC 2808 is homogeneous in [Fe/H] (as suggested by spectroscopy for Populations B, C, D, E, but lacking for Population A) and that population A has a primordial helium abundance, we find that populations B, C, D, E are enhanced in helium by ∆Y ̃ 0.03, 0.03, 0.08, 0.13, respectively. We obtain similar results by comparing the magnitude of the RGB bumps with models. Planned spectroscopic observations will test whether Population A also has the same metallicity, or whether its photometric differences with Population B can be ascribed to small [Fe/H] and [O/H] differences rather than to helium. <P />Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.||Acknowledgments:||We warmly thank David Yong, who has performed the statistical analysis with the Mcluster CRAN package, described in Section 3.1 and Aaron Dotter and Bob Sharp for useful discussions on the statistical significance of the stellar populations. The anonymous referee and the Statistical Editor of this journal, Prof. Eric Feigelson, have provided several suggestions that have improved the quality of the paper. A. P. M. and H. J. acknowledge support by the Australian Research Council through Discovery Early Career Researcher Award DE150101816 and Discovery Project grant DP150100862. S. C. and G. P. acknowledge partial support by PRIN-INAF 2014. G. P. acknowledges partial support by Progetto di Ateneo (Universita’ di Padova) 2014.||URI:||http://hdl.handle.net/20.500.12386/23790||URL:||https://iopscience.iop.org/article/10.1088/0004-637X/808/1/51||ISSN:||0004-637X||DOI:||10.1088/0004-637X/808/1/51||Bibcode ADS:||2015ApJ...808...51M||Fulltext:||open|
|Appears in Collections:||1.01 Articoli in rivista|
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checked on Sep 25, 2020
checked on Sep 25, 2020
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