A binary star sequence in the outskirts of the disrupting Galactic open cluster UBC 274

Abstract

We report the identification of a numerous binary star population in the recently discovered ∼3 Gyr old open cluster UBC 274. It becomes visible when the cluster color-magnitude diagram is corrected by differential reddening and spans mass ratio (q) values from 0.5 up to 1.0. Its stellar density radial profile and cumulative distribution as a function of the distance from the cluster center reveal that it extends out to the observed boundaries of the tidal tails of the cluster (about six times the cluster radius) following a spatial distribution indistinguishable from that of cluster main-sequence (MS) stars. Furthermore, binary stars with q values lower or higher than 0.75 do not show any spatial distribution difference either. From Gaia DR2 astrometric and kinematics data we computed Galactic coordinates and space velocities with respect to the cluster center and mean cluster space velocity, respectively. We found that cluster members located throughout the tidal tails move relatively fast, regardless of whether they are a single or binary star. The projection of their motions onto the Galactic plane resembles that of a rotating solid body, while the motions along the radial direction from the Galactic center and perpendicular to the Galactic plane suggest that the cluster is being disrupted. The similarity of the spatial distributions and kinematic patterns of cluster MS and binary stars reveals that UBC 274 is facing an intense process of disruption that has apparently swept out any signature of internal dynamic evolution, such as mass segregation driven by two-body relaxation.