The Study Of The Orbital Period Changes Of The Low-mass-ratio And Deep-contact Binaries

Low-mass-ratio and deep-contact binaries are a group of W UMa-type binaries with a mass-ratio q<0.25 and a degree of contact f>50%.They are located in the end of the evolutionary chain of binary,which are in the intermediate stage between contact binary and single,rapid-rotation star.Exploring this type of binaries is particularly important for understanding the dynamics of coalescence of binary system and the formation of the blue straggler stars.In this thesis,the orbital period variation of two low-mass-ratio and deep-contact binaries have been investigated,and the statistically study of 53 samples has been performed.The results are summarized as follows:1.By investigating the orbital period variations of two low-mass-ratio and deep-contact binaries,FN Cam and KN Per,we find that their orbital periods display secular increase at the rates of P = 4.41×10-7 d·yr-1 and P = 4.18×10-7 d·yr-1,respectively.The secular period increases are caused by the continuous mass transfer from the less massive secondary component to the more massive primary component.With mass continuously transferring,the orbital angular momentum of FN Cam and KN Per would gradually decrease while their spin angular momentum increases.When the total spin angular momentum increases to one third of the orbital angular momentum,they would encounter the so-called Darwin instability and evolve into the single,rapid-rotation star.2.From a statistical analysis of 53 low-mass-ratio and deep-contact binaries,we found that(1)all of them should contain at least a continuous period change(secular increase/decrease or periodic variation);(2)the most of samples show a secular period variation at a rate of P～10-7 d·yr-1;(3)no significant correlation can be found between the orbital period variations and the physical parameters(mass ratio and degree of contact);(4)the periodic variation of orbital period should be not popular for all low-mass-ratio and deep-contact binaries.