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沃尔夫-拉叶(Wolf-Rayet stars, WR)星是一类非常特殊的恒星,具有强烈的星风损失,造成氢包层丢失,仅具有裸露的氦核. WR星被认为是Ib/Ic型超新星的前身星,研究WR星的形成及内部核合成具有重要意义.根据转动恒星的角动量转移和元素扩散方程,研究了影响WR星结构与演化的各种物理因素.如恒星质量、初始转速、轨道周期、金属丰度等.大质量、初始转速快和金属丰度高的单星模型,星风物质损失率大,易于形成WR星.金属丰度低的恒星由于星风弱,不容易丢失氢包层,不容易形成WR星.然而,快速转动使低金属丰度恒星产生化学成分均匀的演化,极大增加了对流核的质量,相应减小了氢包层厚度,也可以产生WR星.双星系统中发生洛希瓣物质交换,将主星大量的氢包层物质转移到次星上,也可使低金属丰度恒星产生WR星.另外,洛希瓣物质交换,减少了氢包层的厚度以及对流核的温度和核反应速率,主星表面的4He,12C, 19F, 22Ne, 23Na, 25Mg等元素的质量丰度高于相同初始条件的单星模型,而1H, 14N, 16O, 20Ne和26Al等元素的质量丰度却低于单星模型.总之,大质量星、初始转速快、金属丰度高、短轨道周期双星系统是形成WR星的有利条件.

Wolf-Rayet stars (WR stars) were discovered by French astronomers Charles Wolf and Georges Rayet in 1867. The Wolf-Rayet (WR) stars are the evolved descents of the most massive, extremely hot (temperatures up to 200000 K) and very luminous (105－106 ) O stars, with 25－30 solar mass for solar metallicity. The WR stars possess very strong stellar winds, which have velocities up to 3000 km/s and wind mass loss rate a year. These winds are observed in the broad emission line profiles (sometimes, even P-Cygni profiles) of WR spectra in the optical and UV range. Actually, these winds are so strong that they can peel the star and convert it into a nude nucleus without envelope. It has been found that three bright galactic stars located at Cygnus region have broad strong emission bands, rather than absorptions lines, superposed on the typical continuum of hot stars. In 1930 Beals correctly identified these features as emission lines produced by high ionized elements such as helium, carbon, nitrogen and oxygen. The physical factors which can affect the evolution of WR stars are explored in this paper. These physical factors include stellar mass, initial velocities, orbital periods, metallicities, etc. According to the equations for angular momentum transfer and chemical element diffusion, we can ascertain how these physical factors influence the evolution of WR stars and the mixing of chemical elements in WR stars. The result indicates that massive stars with high initial velocities and metallicities have strong stellar winds and be prone to producing WR stars. In contrast with the counterpart with high metallicities, it is hard for the single star with low metallicity to generate WR star due to weak wind. However, the star with very high initial velocity and low metallicity can form chemical homogenious evolution. The star has an enlarged convective core and a very thin hydrogen envelope and it can also generate WR star. The component in the binary system with short orbital period can transfer mass to the companion star through Roche lobe overflow, and this physical process can produce WR star under the condition of low metallicity. Furthermore, mass removal due to Roche lobe overflow reduces the temperature of stellar convective core and rate of nuclear reaction. It is shown that mass metallicities of chemical elements including 4He, 12C, 19F, 22Ne, 23Na, 25Mg in the primary star are higher than those in the single stars, whereas mass metallicities of chemical elements including 1H, 14N, 16O, 20Ne, and 26Al are lower than those in the single counterparts. In a word, the conditions for massive stars with high initial velocities and metallicities in the binary system with short orbital period favor the formation of WR stars.