Volume 537, January 2012
|Number of page(s)||15|
|Section||Galactic structure, stellar clusters and populations|
|Published online||11 January 2012|
Instituto de Astrofìsica de Canarias, 38200 La Laguna, Tenerife, Canary Islands,
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2 Department of Astrophysics, University of La Laguna, 38200 La Laguna, Tenerife, Canary Islands, Spain
3 Dipartimento di Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, Padova, 35122, Italy
4 INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova 35122, Italy
5 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
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6 INAF – Osservatorio Astronomico di Collurania, via Mentore Maggini, 64100 Teramo, Italy
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7 Max Plank Institute for Astrophysics, Postfach 1317, 85741 Garching, Germany
Received: 18 January 2011
Accepted: 22 July 2011
High-precision HST photometry has revealed that the globular cluster (GC) NGC 2808 hosts a triple main sequence (MS) corresponding to three stellar populations with different helium abundances. We carried out photometry on ACS/WFC HST images of NGC 2808 with the main purpose of measuring the luminosity function (LF) of stars in the three different MSs, and the binary fraction in the cluster. We used isochrones to transform the observed LFs into mass functions (MFs). We estimate that the fraction of binary systems in NGC 2808 is fbin ≃ 0.05, and find that the three MSs have very similar LFs. The slopes of the corresponding MFs are α = −1.2 ± 0.3 for the red MS, α = −0.9 ± 0.3 for the middle MS, and α = −0.9 ± 0.4 for the blue one, the same, to within the errors. There is marginal evidence of a MF flattening for masses ℳ ≤ 0.6ℳ⊙ for the the reddest (primordial) MS. These results represent the first direct measurement of the present-day MF and LF in distinct stellar populations of a GC, and provide constraints on models of the formation and evolution of multiple generations of stars in these objects.
Key words: stars: luminosity function, mass function / Hertzsprung-Russell and C-M diagrams / binaries: general / stars: Population II / globular clusters: general / globular clusters: individual: NGC 2808
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, under the programs GO-9899 and GO-10922.
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2012
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