2016MNRAS.455..785J

The radial density and the surface number density profiles are used to estimate the clusters' extent of NGC 2099 and NGC 6866. The field stars are decontaminated by the combined effort of dynamical and statistical cleaning approaches. The red branch stars remain on statistical cleaned colour-magnitude diagrams (CMDs), which are distinguished from main sequence (MS) through new proposed colour-magnitude ratio diagram (CMRD) approach. The fraction of red stars is high for NGC 6866 than that of NGC 2099. This approach gives 1599 and 1357 MS stars in NGC 2099 and NGC 6866, respectively. To them, membership probability is assigned through the spatiokinematic probability. For which, the mean proper motion of each cluster is calculated through Gaussian-fit technique on the data of proper motion values of MS stars. The values of membership probability are used to classify them into unlikely, likely, probable and most probable members (MPMs). The basic parameters (distance, age, etc.) and various colour-ratios are estimated by utilizing the UBVRIJHKW1W2 photometric magnitudes of MPMs and tested the extinction law through two-colour diagrams (TCDs). The photometric broadening on CMDs produced by stars having abnormal colours. Such stars are not considered for the dynamical study due to scattering. The nature of member evaporation from the core is understandable through the study of half-mass radius and dissociation time. The stellar evaporation rate in NGC 6866 is five times that of NGC 2099. Average ejection time for members of NGC 2099 is more than its age, while in the case of NGC 6866, it is found less than its age.