On the non-synchronous rotation of binary systems

During the evolution of the binary system,many physical processes occur,which can influence the orbital angular velocity and the spin angular velocities of the two components,and influence the non-synchronous or synchronous rotation of the system.These processes include the transfer of masses and angular momentums between the component stars,the loss of mass and angular momentum via stellar winds,and the deformation of the structure of component stars.A study of these processes indicates that they are closely related to the combined effects of tide and rotation.This means,to study the synchronous or non-synchronous rotation of binary systems,one has to consider the contributions of different physical processes simultaneously,instead of the tidal effect alone.A way to know whether the rotation of a binary system is synchronous or non-synchronous is to calculate the orbital angular velocity and the spin angular velocities of the component stars.If all of these angular velocities are equal,the rotation of the system is synchronous.If not,the rotation of the system is non-synchronous.For this aim,a series of equations are developed to calculate the orbital and spin angular velocities.The evolutionary calculation of a binary system with masses of 10M⊙+6M⊙shows that the transfer of masses and angular momentums between the two components,and the deformation of the components structure in the semidetached or in the contact phase can change the rotation of the system from synchronous into non-synchronous rotation.     

Contact binaries: I. An inspection of the HSB contact binary model by comparison of relationships obtained from theoretical light curves with that from astronomical observations

The light curve is one of the most important photometric characteristics of variable stars, which can supply physical information about many stars. So, light curves are the best candidate to inspect a theoretical model of binaries. One important feature of the light curve is the difference of two light minima of the light curve, namely the difference between the primary eclipse depth and the secondary eclipse depth (DED). In this paper, the secondary eclipse depths of theoretical and observational light curves are studied. Firstly, a method to calculate the theoretical light curves of an eclipsing binary with non-spherical components is proposed, which can be put into the HSB contact binary model [Huang R Q, et al. Chin J Astron Astrophys, 2007, 7: 235–244; Song H F, et al. Chin J Astron Astrophys, 2007, 7: 539–550]. Theoretical light curves and the DED of the binary can be obtained at every evolutionary phase. The relationships of DED with mass and luminosity are presented and show special features for the contact binaries. Secondly, a large amount of observational data is collected, from which 11 massive, intermediate-mass contact binaries and 9 low-mass contact binaries are chosen and the two relationships are obtained using theoretical light curves. Finally, in order to check whether the HSB contact binary model can be used in contact binary systems with massive, intermediate-mass and low-mass components, a comparison is performed for the above mentioned relationships obtained from theoretical light curves with those from the astronomical observations. The results show a good agreement for contact binary systems with all different masses.