Long-term investigation of an open cluster Berkeley 65

Tarak Chand^1*Saurabh Sharma^1*Koshvendra Singh²Jeewan Pandey¹Aayushi Verma¹Harmeen Kaur¹Mamta Mamta¹Manojit Chakraborty¹Devendra K Ojha²Ajay Kumar Singh³

• ¹Aryabhatta Research Institute of Observational Sciences, Nainital, India
• ²Tata Institute of Fundamental Research, Mumbai, Maharashtra, India
• ³Bareilly College, Bareilly, Uttar Pradesh, India

We present a decade-long investigation of a poorly studied cluster, Berkeley 65 (Be 65), using deep optical data from the telescopes of ARIES, Nainital Observatory. We estimate its radius (R cluster = 1.6′ , aspect ratio of ∼1.1), distance (2.0 ± 0.1 kpc) and age (∼160 Myrs). A clear turn-off point at ∼1.7 M ⊙ in the mass function suggests the escape of low-mass stars, and the lower photometric mass compared to the dynamical mass indicates ongoing disruption due to external forces. Our long-baseline optical photometric data also identifies 64 periodic and 16 non-periodic stars in this region. We have presented the light curves and the classification of those variables. The periodic stars have periods ranging from ∼0.05 days to ∼3.00 days and amplitude ranges from ∼8 mmag to ∼700 mmag. The nonperiodic stars show variation from ∼30 mmag to ∼500 mmag. The periodic stars include main-sequence pulsating variables such as Slow Pulsating B-type, δ Scuti, RR Lyrae, and γ Doradus. We report a detached binary system and rotating variables similar to BY Draconis-type stars exhibiting variable brightness caused by starspots, chromospheric activity, and magnetic field-related phenomena.