The radial distribution of blue stragglers in Galactic globular cluster NGC 6656 − clues to the dynamical status.

Abstract

We present the dynamical status of the Galactic globular cluster NGC 6656 using the spatial distribution of blue straggler stars (BSSs). A combination of multi-wavelength high-resolution space- and ground-based data are used to cover a large cluster region. We determine the centre of gravity (Cgrav) and construct the projected density profile of the cluster using the probable cluster members selected from the Hubble Space Telescope and Gaia Data Release 2 propermotion data sets. The projected density profile in the investigated region is nicely reproduced by a single-mass King model, with core (rc) and tidal (rt) radii of 75.2 ± 3.1 and 35 .6 ± 1 .1 respectively. In total, 90 BSSs are identified on the basis of proper-motion data in the region of radius 625. The average mass of the BSSs is determined as 1.06 ± 0.09 M and with an age range of 0.5 to 7 Gyr. The BSS radial distribution shows a bimodal trend, with a peak in the centre, a minimum at r ∼ rc and a rising tendency in the outer region. The BSS radial distribution shows a flat behaviour in the outermost region of the cluster. We also estimate the A+ rh parameter as an alternative indicator of the dynamical status of the cluster; this is found to be 0.038 ± 0.016. Based on the radial distribution and A+ rh parameter, we conclude that NGC 6656 is an intermediate dynamical age cluster