The VMC Survey. XI. Radial Stellar Population Gradients in the Galactic Globular Cluster 47 Tucanae

TL;DR

This study uses deep near-infrared observations to reveal radial variations in stellar populations within 47 Tucanae, indicating multiple generations with differing ages, helium, and metallicity distributions across the cluster.

Contribution

It provides detailed evidence of radial stellar population gradients and age spread in 47 Tucanae using near-infrared color-magnitude diagrams.

Findings

Radial gradients in helium abundance and metallicity from core to periphery.

Presence of second-generation stars concentrated towards the core.

Outer regions are consistent with a simple stellar population.

Abstract

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 ~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.