Letters to the Editor

Strong radial segregation between sub-populations of evolutionary homogeneous stars in the Galactic globular cluster NGC 6752^⋆

¹ Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile
e-mail: vkravtsov@ucn.cl
² Sternberg Astronomical Institute, University Avenue 13, 119992 Moscow, Russia
³ Isaac Newton Institute of Chile, Ministerio de Educación de Chile, Casilla 8-9, Correo 9, Santiago, Chile
e-mail: inewton@terra.cl, falvarad@eso.org
⁴ ESO – European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
e-mail: gmarconi@eso.org

Received: 22 October 2010
Accepted: 17 January 2011

Abstract

Aims. We investigate the new and still poorly studied matter of so-called multiple stellar populations (MSPs) in Galactic globular clusters (GGCs). Studying MSPs and their accumulated data can shed more light on the formation and evolution of GGCs and other closely related fundamental problems. We focus on the strong relation between the radial distribution of evolutionary homogeneous stars and their U-based photometric characteristics in the nearby GGC NGC 6752 and compare this with a similar relation we found in NGC 3201 and NGC 1261.

Methods. We use our new multi-color photometry in a fairly wide field of NGC 6752, with particular emphasis on the U band and our recent and already published photometry made in NGC 3201 and NGC 1261.

Results. We found and report here for the first time a strong difference in the radial distribution between the sub-populations of red giant branch (RGB) stars that are bluer and redder in color (U − B), as well as between sub-giant branch (SGB) stars brighter and fainter in the U-magnitude in NGC 6752. Moreover, the fainter SGB and redder RGB stars are similarly much more centrally concentrated than their respective brighter and bluer counterparts. Virtually the same applies to NGC 3201. We find evidence in NGC 6752 as in NGC 3201 that a dramatic change in the proportion of the two sub-populations of SGB and RGB stars occurs at a radial distance close to the half-mass radius, R_h, of the cluster. These results are the first detections of the radial trend of the particular photometric properties of stellar populations in GGCs. They imply a radial dependence of the main characteristics of the stellar populations in these GGCs, primarily of the abundance, and (indirectly) presumably of the kinematics.