Physical Process Of Tidal Synchronization And Orbital Circularization In Rotating Binaries

We have used the Geneva stellar evolution code and the latest stellar evolution code MESA-r10398(Modules for Experiments in Stellar Astrophysics)to construct stellar models,and investigate the evolution of rotating massive single stars and binaries.It is shown that the dynamical effects and rotational mixing which caused by rotation have an important impact on stellar structure and evolution.The theoretical model of rotating stars is introduced in this work.We list the structural equations for rotating stars and discuss various instabilities and equations.In addition,tides are also a very important physical factor which can significantly affect the structure and evolution of stars.Based on the equation for orbital evolution,we explore all kinds of physical factors which can affect tidal synchronization and orbital circularization in this paper.For stars with radiative envelopes,radiative damping mechanism is required to explain the observed synchronization and circularization of close binaries.Several physical factors can have an influence on the process of radiative damping which is scaled with thermal timescale.These physical factors include stellar mass,initial velocity,orbital period,metallicity,overshooting,etc.According to the equations for angular momentum transfer and chemical elements diffusion,we can obtain how these physical factors affect the evolution of rotating binaries and the mixing of chemical elements in two rotating components.The results indicate that the binaries with massive stars,smaller initial spin velocities,smaller overshooting parameters,and shorter orbital periods can attain the equilibrium speed and orbital circularization early.The system with smaller initial spin velocities reaches the equilibrium speed and orbital circularization early because angular momentum transformation between spin and the orbit can shorten the orbital distance and increase the tidal torques.Nitrogen enrichment in binaries is weaker than the one in single stars due to tidal braking.The system with massive components,higher metallicities,larger overshooting parameters,and shorter orbital periods can display high nitrogen enrichment.Stellar radius is small in the star with lower mass,lower metallicities,slower spin speeds and larger overshooting parameters whereas the star with lower metallicities have higher surface effective temperature.Rapid rotating stars evolve towards low temperature and luminosity in the HR diagram.Spin angular momentum can attain a high value due to a strong tidal torque(the equilibrium tide).The tidal effect causes the nitrogen and helium enrichment occurs early in the binary system.At the early stage of the evolution,the single star model with higher initial velocities has larger stellar radius,but efficient rotational mixing can inhibit stellar expanding at the subsequent evolution.Central compactness is increased by the centrifugal force at the early stage of evolution but is reduced by rotational mixing induced by strong tides.The binary models with weak tides,the dynamical tide,have high values of central temperature and stellar radius.Rotational mixing in single stars can slow down the shrinkage of convective cores,while convective cores can be expanded by strong tides in the binary system.Efficient rotational mixing induced by tides can cause the star to evolve towards high temperature and luminosity in the HR diagram.