The young Galactic cluster NGC 225: binary stars’ content and total mass estimate

ABSTRACT Galactic star clusters are known to harbour a significant amount of binary stars, yet their role in the dynamical evolution of the cluster as a whole is not comprehensively understood. We investigated the influence of binary stars on the total mass estimate for the case of the moderately populated Galactic star cluster NGC 225. The analysis of multi-epoch radial velocities of the 29 brightest cluster members, obtained over two observational campaigns, in 1990–1991 and in 2019–2020, yields a value of binary fraction of α = 0.52 (15 stars out of 29). Using theoretical isochrones and Monte Carlo simulations we found that the cluster mass increases at least 1.23 times when binaries are properly taken into account. By combining Gaia Early Data Release 3 (EDR3) photometric data with our spectroscopic observations, we derived estimates of NGC 225 fundamental parameters as follows: mean radial velocity $\langle$Vr$\rangle$ = −9.8 ± 0.7 km s−1; log (τ) = 8.0–8.2 dex; distance D = 676 ± 22 pc; and colour excess E(B − V) = 0.29 ± 0.01 mag.