2009ApJS..182..509D

We present a multiwavelength photometric analysis of the globular cluster M2. The data set has been obtained by combining high-resolution (Hubble Space Telescope/WFPC2 and ACS) and wide-field (Galaxy Evolution Explorer) space observations and ground-based (MEGACAM-CFHT, EMMI-NTT) images. The photometric sample covers the entire cluster extension from the very central regions up to the tidal radius and beyond. It allows an accurate determination of the cluster center of gravity and other structural parameters derived from the star count density profile. Moreover, we study the Blue Straggler Star (BSS) population and its radial distribution. A total of 123 BSSs have been selected, and their radial distribution has been found to be bimodal (highly peaked in the center, decreasing at intermediate radii, and rising outward), as already found in a number of other clusters. The radial position of the minimum of the BSS distribution is consistent with the radius of avoidance caused by the dynamical friction of massive (1.2 M_☉) objects over the cluster age. We also searched for gradients in the red giant branch (RGB) and the asymptotic giant branch (AGB) populations. At the 2σ level, we found an overabundance of AGB stars within the core radius and confirmed the result of Sohn et al. that the central region of M2 is bluer than the outer part. We show that the latter is due to a deficit of very luminous RGB stars in the central region.