NGC1605a and b: an old binary open cluster in the Galaxy

TL;DR

This paper reports the discovery of a binary open cluster in the Galaxy, NGC 1605a and b, which are likely forming through tidal capture and merging, providing insights into cluster formation and disruption processes.

Contribution

It presents the first detailed observational evidence of a binary open cluster formed by tidal capture and merging, with analysis of their properties and dynamics.

Findings

NGC 1605a and b are at the same distance and share similar kinematics.

The clusters have ages of 2 Gyr and 600 Myr, respectively, with a 1.4 Gyr age difference.

Tidal debris suggests ongoing merging and possible formation of smaller bound structures.

Abstract

This work communicates the discovery of a binary open cluster within the Galaxy. NGC 1605 presents an unusual morphology with a sparse stellar distribution and a double core in close angular proximity. The 2MASS and Gaia-EDR3 field-star decontaminated colour-magnitude diagrams (CMDs) show two distinct stellar populations located at the same heliocentric distance of $\sim2.6$ kpc suggesting that there are two clusters in the region, NGC 1605a and NGC 1605b, with ages of $2$ Gyr and $600$ Myr, respectively. Both Gaia parallax and PM distributions are compact and very similar indicating that they are open clusters (OCs) and share the same kinematics. The large age difference, 1.4 Gyr, points to a formation by tidal capture during a close encounter and the close spatial proximity and similar kinematics suggest an ongoing merger event. There are some prominent tidal debris that appear to trace the clusters' orbits during the close encounter and, unexpectedly, some of them appear to be bound structures, which may suggest that additionaly to the evaporation the merging clusters are being broken apart into smaller structures by the combination of Galactic disk, Perseus arm, and mutual tidal interactions. In this sense, the newly found binary cluster may be a key object on the observational validation of theoretical studies on binary cluster pairs formation by tidal capture as well as in the formation of massive clusters by merging, and tidal disruption of stellar systems.