2019MNRAS.482.4874S

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 (C_grav) 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 proper-motion data sets. The projected density profile in the investigated region is nicely reproduced by a single-mass King model, with core (r_c) and tidal (r_t) 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 ∼ r_c 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.