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

TL;DR

This study identifies a large binary star population in the open cluster UBC 274, revealing its extended tidal tails and signs of ongoing disruption, with binaries sharing similar spatial and kinematic properties as single stars.

Contribution

It provides the first detailed analysis of binary stars in UBC 274, showing their distribution and kinematics, and highlights the cluster's disruption process affecting both single and binary stars.

Findings

Binary stars extend to the cluster's tidal tails.

Binary and single stars share similar spatial distributions.

Cluster shows signs of being tidally disrupted.

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 once the cluster color-magnitude diagram is corrected by differential reddening and spans mass ratios (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's center reveal that it extends out to the observed boundaries of the cluster's tidal tails (~ 6 times the cluster's radius) following a spatial distribution indistinguishable from that of cluster Main Sequence (MS) stars. Furthermore, binary stars with q values smaller or larger 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's center and mean cluster space velocity, respectively. We found that, cluster members located all along the tidal tails, irrespective of being a single or binary star, move relatively fast. The projection of their motions on the Galactic plane resembles that of a rotating solid body, while those 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 like mass segregation driven by two-body relaxation.