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

Abstract

We present dynamical status of the Galactic globular cluster NGC 6656 using spatial distribution of Blue Straggler Stars (BSSs). A combination of multi-wavelength highresolution 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 HST and 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 ′′ .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 ′′ . An average mass of the BSSs is determined as 1.06 ± 0.09 M ⊙ 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 ∼ 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 ± 0.016. Based on the radial distribution and A + rh parameter, we conclude that NGC 6656 is an intermediate dynamical age cluster.

GlobULeS. IV. UVIT/AstroSat Detection of Extremely Low Mass White Dwarf Companions to Blue Straggler Stars in NGC 362

“…The photometry on the WFI images was performed following the approach described in A06 (Anderson et al 2006). The PSF's shape varies greatly with position; hence, nine PSFs per CCD chip (3 × 3) were developed to limit spatial fluctuation to less than 1% (Singh & Yadav 2018). The PSFs were developed using an empirical grid with a quarter-pixel resolution.…”

Peculiarities in the horizontal branch stars of globular cluster NGC 1851: Discovery of a Blue straggler companion to an EHB star

“…We have used Gaia DR2 proper motion data to obtain the membership probability of stars in the outer region. Thus, we have the membership probability for stars in the entire cluster region (r ∼ r t ), which is used for the selection of genuine cluster members (Singh & Yadav 2018).…”

Understanding the dynamical status of Galactic Globular Cluster NGC 6656

“…For the completeness of the sample we used only F 606W brighter than 21 mag. We used the value of C grav and structural parameters obtained from Singh & Yadav (2019) and fitted the king profile in the outer region (10 < r < 90 ). The PDP shown in Fig.…”

“…Based on the shape of observed BSS radial distribution, Ferraro et al (2012) grouped BSS into three families corresponding to different dymical ages. We studied the BSS radial distribution of the GGC NGC 6656 and found it to be an intermediate dynamical status (Singh & Yadav (2019)). However, it has been found that, the process of BSS segregation can be delayed by the presence of dark remnants, in particular, by a significant population of stellar mass black holes (Lanzoni et al 2016).…”