转动双星同步和轨道圆化的物理过程研究

Physical process of tidal synchronization and orbital circularization in rotating binaries

潮汐效应是影响恒星结构和演化非常重要的物理因素.本文研究了影响潮汐同步和轨道圆化的物理因素,如恒星质量、初始转速、轨道周期、金属丰度、对流超射等,并根据转动恒星的角动量转移和元素扩散方程,给出了这些因素对转动双星演化和元素混合的影响.结果表明:具有大质量子星、初始转速慢、对流超射小、轨道周期短的双星系统,能更早地达到平衡速度和轨道圆化;初始转速快的恒星,由于潮汐同步过程减速,双星系统中氮元素增丰没有单星的氮元素增丰显著;大质量星、高金属丰度、超射大和短周期的双星系统,氮增丰相对显著;质量小、金属丰度低、转速慢、超射大的恒星具有较小的恒星半径,而低金属丰度恒星表面却具有较高的有效温度,快速转动单星向低温和低光度端演化.

The tide is a very important physical factor which can significantly affect the structure and evolution of stars.The physical factors which can affect tidal synchronization and orbital circularization are explored in this paper. For stars with radiative envelopes, radiative damping mechanism is required to explain the observed synchronization and circularization of close binaries. A star can experience a range of oscillations that arise from, and are driven by, the tidal field： the dynamical tides. The dynamical tide is the dynamical response to the tidal force exerted by the companion; it takes into account the elastic properties of the star, and the possibilities of resonances with its free modes of oscillation.The dissipation mechanism acting on this kind of tide is the deviation from adiabaticity of the forced oscillation, due to the radiative damping. 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 indicates 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.At synchronous states, the tidal torque is zero and stellar winds continue to brake the star. Therefore, two components cannot keep the synchronous state for a long time. At the equilibrium state, the tidal torque is counteracted by wind torques. Therefore, the equilibrium speed is less than the synchronous one. 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 results reveal that 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.