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

TL;DR

This study measures the luminosity and mass functions of three distinct stellar populations in the globular cluster NGC 2808, providing new insights into its stellar composition and formation history.

Contribution

It presents the first direct measurement of the luminosity and mass functions for multiple stellar populations within a globular cluster.

Findings

Similar luminosity functions across the three main sequences.

Mass function slopes are approximately -1.2, -0.9, and -0.9 for the red, middle, and blue sequences.

Marginal evidence of mass function flattening for low-mass stars in the primordial population.

Abstract

High-precision HST photometry has recently 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 estimated 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 alpha=-1.2+/-0.3 for the red MS, alpha =-0.9+/-0.3 for the middle MS, and alpha = -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<~0.6 M_SUN 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.