Comprehensive photometric study of an extremely low mass ratio deep contact binary in the globular cluster M 4

Abstract

Abstract A comprehensive photometric study and an investigation of the orbital period variation of V53 in the globular cluster M 4 are presented. The photometric study reveals that the mass ratio and the contact degree of V53 are q ∼ 0.078 and f ∼ 69%, respectively. The observed variation in the light curve can be explained by adjusting the spot parameters. V53 belongs to extreme mass ratio (q ≤ 0.25), deep contact (f ≥ 50%) binaries, and its mass ratio is close to the minimum mass ratio predicted by theoretical studies, making it a potential object for studying the evolution of binaries and the formation of blue stragglers and FK Com-type stars. The orbital period of V53 shows a long-term decrease at a rate of dp/dt = 5.89(±0.02) × 10−8 d yr−1. This secular period decrease may be caused by the combination of mass transfer from the more massive component to the less massive component and an angular momentum loss via magnetic braking. As this mass transfer and angular momentum loss continues, V53 will ultimately evolve into a single fast-rotation star. By studying the statistics of all the contact binaries in globular clusters that have been analyzed, we found a possible correlation between the contact degree and whether or not a contact binary is a blue straggler. A contact binary is likely to become a blue straggler when its fill-out factor is more than 46.25(±2.05)%. More samples should be introduced to confirm this preliminary result in the future.