Luminosity and mass functions of the three main sequences of the globular cluster NGC 2808^⋆,⋆⋆

A. P. Milone^1,2,3, G. Piotto^3,4, L. R. Bedin^4,5, S. Cassisi⁶, J. Anderson⁵, A. F. Marino⁷, A. Pietrinferni⁶ and A. Aparicio^1,2

¹ Instituto de Astrofìsica de Canarias, 38200 La Laguna, Tenerife, Canary Islands, Spain
e-mail: milone@iac.es; aparicio@iac.es
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
e-mail: giampaolo.piotto@unipd.it
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
e-mail: bedin@stsci.edu; jander@stsci.edu
⁶ INAF – Osservatorio Astronomico di Collurania, via Mentore Maggini, 64100 Teramo, Italy
e-mail: cassisi@oa-teramo.inaf.it; pietrinferni@oa-teramo.inaf.it
⁷ Max Plank Institute for Astrophysics, Postfach 1317, 85741 Garching, Germany
e-mail: amarino@MPA-Garching.MPG.DE

Received: 18 January 2011
Accepted: 22 July 2011

Abstract

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 f_bin ≃ 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.