New Delhi: UBC 63, a loosely gravitationally bound group of stars located approximately 4,892 light-years from Earth, has been found to comprise two distinct star clusters rather than a single cluster, as previously believed.
A team led by Indian astronomers discovered that the poorly populated Galactic open cluster, earlier identified as one stellar system, actually consists of two separate clusters, UBC 63A and UBC 63B.
The chance discovery of the two clusters in UBC63 has provided a rare opportunity to study the aftermath of a close encounter between two stellar systems.
The findings also offer new insights into the dynamical evolution of star clusters in the Milky Way.
Star clusters are important building blocks of galaxies.
They comprise groups of stars that form within the same large reservoirs of gas and dust and subsequently evolve under the influence of their own gravity and the broader Galactic environment.
Historically, UBC 63 has remained largely overlooked, with limited scientific literature providing only preliminary parameter estimates that presumed it to be a single star cluster.
A team led by astronomers from India, including a Department of Science and Technology (DST) INSPIRE Senior Research Fellow (SRF), researchers from Gauhati University and researchers from the Indian Institute of Astrophysics (IIA), a DST autonomous institution, set out to investigate star formation dynamics within UBC 63.
Using data from the European Space Agency’s (ESA) Gaia mission, the researchers made an unexpected discovery: UBC 63 actually consists of two distinct clusters, UBC 63A and UBC 63B, which recently passed remarkably close to each other.
Encounters between star clusters are relatively uncommon. Therefore, detecting a system soon after such an encounter provides valuable clues about how gravitational interactions influence the structure, motion and long-term evolution of star clusters.
Gaia Mission Data Reveals Two Distinct Star Clusters
The Gaia mission, which provides exceptionally precise measurements of the positions and motions of stars across the Milky Way, revealed that the two clusters associated with UBC63 belong to significantly different age groups.
To investigate the stellar populations, the researchers studied the brightness and colours of the member stars.
They found that the stars formed two distinct sequences in the colour-magnitude diagram, an important astronomical tool used to investigate the evolutionary stages of star clusters.
The two separate sequences indicated that UBC63 might actually comprise two independent stellar systems rather than one cluster.
A detailed analysis of stellar motions and other properties, supported by robust machine learning techniques and advanced dynamical simulations powered by high-performance computing, subsequently confirmed the presence of two distinct stellar populations.
These two populations have now been designated UBC 63A and UBC 63B.
UBC 63A and UBC 63B Have a Huge Age Difference
One of the most significant findings of the research is the substantial age difference between the two clusters identified within UBC63.
UBC 63A is a young star cluster, approximately 21 million years old. In comparison, UBC 63B is considerably older, with an estimated age of approximately 562 million years.
This substantial difference in their ages demonstrates that the two stellar systems did not originate as a single star cluster.
The researchers further investigated how two clusters with such different formation histories came to be located so close to each other.
How Close Did the Two Star Clusters Come?
To understand the interaction between UBC 63A and UBC 63B, the researchers simulated their movements through the Milky Way.
The simulations revealed that, at the time when the younger cluster formed, the two systems were separated by approximately 60 parsecs, equivalent to around 196 light-years.
However, the two star clusters subsequently came remarkably close to each other approximately 6 million years ago.
During this close encounter, the separation between them reduced to only about 7 parsecs, or approximately 23 light-years.
The simulations helped reconstruct the history of this unusual encounter and establish how the two stellar systems moved relative to each other.
UBC 63A and UBC 63B Are Moving Apart Today
Although the two clusters came remarkably close to each other around 6 million years ago, they are no longer at such a close distance.
Today, UBC 63A and UBC 63B are approximately 26 parsecs, equivalent to around 85 light-years, apart.
Their present-day motion indicates that the two clusters are not gravitationally bound to each other. Instead, they are moving apart following their close encounter.
The discovery demonstrates how gravitational interactions between stellar systems can influence their positions and movements over time.
Gaia Mission Helps Uncover Hidden Structures in the Milky Way
The discovery of two separate clusters within UBC63 highlights the importance of modern astronomical surveys such as the European Space Agency’s Gaia mission.
By providing precise information about stellar positions and motions, Gaia enables astronomers to identify stellar populations that might otherwise remain unnoticed when stars are observed only in projection on the sky.
The research also demonstrates how machine learning techniques, dynamical simulations and high-performance computing can be combined with astronomical survey data to investigate the formation and evolution of star clusters.
The identification of UBC 63A and UBC 63B offers researchers a rare opportunity to examine the consequences of a close encounter between two stellar systems with substantially different ages and formation histories.
Indian Astronomers Behind the UBC 63 Discovery
The research on UBC63 was published in the Astronomical Journal.
The study was carried out by
- Samrat Biswas, INSPIRE Senior Research Fellow (SRF), Gauhati University
- Biman J Medhi, Gauhati University
- Sergio Messina, INAF – Catania Astrophysical Observatory
- Zhanpeng Zhu, Shanghai Normal University
- Songmei Qin, Shanghai Astronomical Observatory
- Sukanta Deb, Cotton University
- A. H. Sheikh, INSPIRE SRF, Gauhati University
- H. S. Das, Assam University
- Motohide Tamura, The University of Tokyo
- Maheswar Gopinathan, Indian Institute of Astrophysics
- Ram Sagar, Indian Institute of Astrophysics
Research Publication: https://iopscience.iop.org/article/10.3847/1538-3881/ae8c38
