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

TL;DR

This study examines the binary star content of the Galactic cluster NGC 225 and demonstrates that accounting for binaries increases the total mass estimate by at least 23%, using spectroscopic and photometric data.

Contribution

It provides the first detailed analysis of binary stars in NGC 225 and quantifies their impact on the cluster's mass estimation using combined observational and simulation methods.

Findings

Binary fraction of 52% among brightest members

Cluster mass increases by at least 1.23 times when binaries are included

Derived fundamental parameters: velocity, age, distance, and reddening

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 $\alpha =0.52$ (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 EDR3 photometric data with our spectroscopic observations, we derived estimates of NGC 225 fundamental parameters as follows: mean radial velocity $<V_r> = -9.8 \pm 0.7$ km s$^{-1}$, $\log(\tau)$=8.0-8.2 dex, distance $ D = 676 \pm 22$ pc, and colour excess $E(B-V) = 0.29 \pm 0.01$ mag.