Indian Astronomers Redefine UBC 63 as Binary Star Cluster

A team of Indian astronomers has redefined the star cluster named 'UBC 63' as a 'binary star cluster', meaning a double star cluster. This significant identification occurred as the cluster passed very closely. Located approximately 4892 light years away from Earth, this cluster was previously considered a single star cluster, but new research has revealed that it actually comprises two separate star clusters.

According to a PIB report, UBC 63 is a cluster with a low number of stars that are loosely bound by gravity, known as a 'galactic open cluster'. This sudden discovery provides a rare opportunity to study the state of two stellar systems after they have passed very close to each other. This has yielded new information regarding the evolution and dynamics of star clusters within our galaxy, the 'Milky Way'.

Star clusters are critical units of galaxy formation, forming within vast reservoirs of gas and dust. Subsequently, they evolve under their own gravity and the influence of the surrounding galactic environment. Until now, very little attention had been paid to UBC 63, and based on limited available information, preliminary assumptions had treated it as a single star cluster.

This research work involved 'INSPIRE' Senior Research Fellows (SRF) of the Department of Science and Technology (DST), researchers from Guwahati University, and researchers from the DST's autonomous institution, the 'Indian Institute of Astrophysics' (IIA). The team utilised data obtained through the European Space Agency's (ESA) 'Gaia' mission. During the investigation, it was found that UBC 63 actually consists of two clusters named UBC 63A and UBC 63B, which recently passed very close to one another.

Such an encounter between star clusters is a very rare event. Therefore, discovering a system immediately after such an event can provide important clues on how gravitational interactions, cluster structure, their motion, and long-term evolution are affected. Data from the 'Gaia' mission showed that the ages of these two clusters are entirely different, as Gaia provides extremely accurate measurements of the positions and motions of stars in the Milky Way.

Researchers studied the brightness and colours of the stars in UBC 63. They observed that these stars form two distinct sequences in a 'colour-magnitude diagram'. In astronomy, this diagram is used to investigate the evolutionary stages of star clusters, which confirmed that UBC 63 could indeed be a mixture of two different stellar systems.

Detailed analysis of stellar motion and other characteristics was conducted using robust machine learning techniques and advanced dynamical simulations based on high-performance computing. This confirmed the presence of two separate clusters. UBC 63A is a young cluster aged approximately 21 million years, while UBC 63B is much older, with an estimated age of approximately 562 million years. This vast difference in age indicates that these two clusters did not originate as a single group.

Researchers further investigated how these two clusters with different histories came so close to each other. Simulations of their motion in the Milky Way revealed that when the young cluster was formed, these two clusters were approximately 60 parsecs (approximately 196 light years) apart. About 6 million years ago, they came very close to each other, and the distance between them reduced to just 7 parsecs (approximately 23 light years).

At present, these two clusters are approximately 26 parsecs (approximately 85 light years) apart. Their motion makes it clear that they are not gravitationally bound to each other, but are now moving away after having come close. This study demonstrates the capability of modern astronomical surveys like 'Gaia' to reveal hidden structures that are not visible when stars in the sky are viewed merely as a flat projection.

This research has been published in an astronomical journal, with contributions from Samrat Biswas (INSPIRE SRF, Guwahati University), Biman J. Medhi, Sergio Messina (INAF - Catania Astrophysical Observatory), Zhanpeng Zhu (Shanghai Normal University), Songmei Qin (Shanghai Astronomical Observatory), Sukanta Deb (Kapas University), A.H. Sheikh (Guwahati University), H.S. Das (Assam University), Motohide Tamura (University of Tokyo), Maheshwar Gopinathan (Indian Institute of Astrophysics), and Ram Sagar (Indian Institute of Astrophysics).