The radial distribution of Blue stragglers in Galactic Globular Cluster NGC 6656 $-$ Clues on the dynamical status

Gaurav Singh, R. K. S. Yadav

Published 2018-10-31Version 1

We present dynamical status of the Galactic globular cluster NGC 6656 using 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 \textit{HST} and \textit{Gaia} DR2 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 ($r_{t}$) radius as $75\arcsec.2$ $\pm$ $3\arcsec.1$ and $35\arcmin.6$ $\pm$ $1\arcmin.1$ respectively. In total, 90 BSSs are identified on the basis of proper motion data in the region of radius $625\arcsec$. An average mass of the BSSs is determined as $1.06$ $\pm$ $0.09$ $M_{\sun}$ and with an age range of 0.5 to 7 Gyrs. The BSS radial distribution shows a bimodal trend, with a peak in the centre, a minimum at $r \sim r_c$ 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 $A^{+}_{rh}$ parameter as an alternative indicator of the dynamical status of the cluster and is found to be $0.038$ $\pm$ $0.016$. Based on the radial distribution and $A^{+}_{rh}$ parameter, we conclude that NGC 6656 is an intermediate dynamical age cluster.