We present a deep near-infrared color–magnitude diagram of the Galactic globular cluster 47 Tucanae , obtained with the Visible and Infrared Survey Telescope for Astronomy ( VISTA ) as part of the VISTA near-infrared Y,J,K _ { s } survey of the Magellanic System ( VMC ) .
The cluster stars comprising both the subgiant and red-giant branches exhibit apparent , continuous variations in color–magnitude space as a function of radius .
Subgiant-branch stars at larger radii are systematically brighter than their counterparts closer to the cluster core ; similarly , red-giant-branch stars in the cluster ’ s periphery are bluer than their more centrally located cousins .
The observations can very well be described by adopting an age spread of \sim 0.5 Gyr as well as radial gradients in both the cluster ’ s helium abundance ( Y ) and metallicity ( Z ) , which change gradually from ( { Y } = 0.28 ,Z = 0.005 ) in the cluster core to ( { Y } = 0.25 ,Z = 0.003 ) in its periphery .
We conclude that the cluster ’ s inner regions host a significant fraction of second-generation stars , which decreases with increasing radius ; the stellar population in the 47 Tuc periphery is well approximated by a simple stellar population .