A novel approach to determining the age of stellar clusters: application to NGC 188

ABSTRACT We present a new independent determination for the age of one of the oldest open clusters, NGC 188: $6.41 \pm 0.33$ Gyr. We utilize a novel approach where we jointly fit the radial velocities and spectral energy distributions (SED) of six binary star systems in the cluster. The joint fit has 21 free parameters: 12 stellar masses, six orbital inclination angles, as well as a common cluster age, distance, and extinction, $A_V$. The fit to the SEDs utilizes Mesa Isochrones and Stellar Tracks (MIST) stellar isochrones, and the fitted stellar parameters are presented in HR diagrams (R–$T_{\rm eff}$, R–M, and M–$T_{\rm eff}$) showing the stars’ different states of evolution. These isochrones are compared with PARSEC and Y2 isochrones to obtain an estimate of the uncertainties introduced by different stellar models. Of the 3017 stars taken from the NGC 188 field, 333 possible member stars were selected using Gaia data and analysed separately for their mean distances and proper motions. We find a distance to NGC 188 of $1850 \pm 12$ pc compared with the independent photometric distance found from the SED fitting of $1897 \pm 58$ pc.