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Astronomers from the Aryabhatta Research Institute of Observational Sciences (ARIES) in India and elsewhere have employed the Devasthal Fast Optical Telescope (DFOT) to conduct long-term photometric observations of an open cluster known as Berkeley 65. Results of the observations, May 30 on the arXiv preprint server, shed more light on the properties of this cluster.

Open clusters (OCs) are groups of stars loosely gravitationally bound to each other, formed from the same giant molecular cloud. To date, more than 1,000 OCs have been discovered in the Milky Way, and astronomers are interested in studying them in detail as it could be crucial for improving our understanding of the formation and evolution of our galaxy.

Berkeley 65 (Be 65 for short) is a galactic open cluster at a distance of some 7,400 light years. It is a poorly studied OC in the galactic disk, with an estimated age of around 160 million years.

A team of astronomers led by Tarak Chand of ARIES decided to take a closer look at Berkeley 65, hoping to get more insights into its properties and nature. For this purpose, they analyzed DFOT deep optical photometric data covering a period of about 10 years.

"The long-term photometric monitoring of Be 65 was done using the 1.3 m f/4 Devasthal Fast Optical Telescope at the Aryabhatta Research Institute of Observational Sciences, located at Devasthal, India," the researchers wrote in the paper.

The study found that Berkeley 65 has a more or less circular shape and its radius is approximately 3.1 light years. The distance to the cluster was measured to be about 6,500 light years.

The astronomers found evidence of mass segregation in Berkeley 65 and the dynamical age of the cluster was estimated to be around 7.5 million years. These findings suggest that the mass segregation can be due to the internal dynamical evolution of the cluster.

According to the paper, Berkeley 65 has a photometric mass of 164 solar masses, while its dynamical mass is 5,581 solar masses. This indicates that Berkeley 65 has lost stellar mass, and thus is in the process of disruption.

"The Be 65 cluster, located within the galactic disk in an environment of gas and dust, with an age of approximately 160 Myr, seems to have lost stars much earlier than its τevap (∼0.75 Gyr) time scale, suggesting dominant roles of the external forces in its dynamical evolution," the authors of the paper concluded.

As part of the study, the researchers identified 64 periodic and 16 non-periodic stars, with periods ranging from 0.05 days to 3 days and amplitudes between 8 mmag and 742 mmag. Out of them, 35 were confirmed to be members of Berkeley 65, while the rest belong to the field population. The periodic stars include main-sequence pulsating variables such as slow-pulsating B-type, Delta Scuti, RR Lyrae, and Gamma Doradus.

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