Formation and internal nucleosynthesis in massive rotating Wolf-Rayet stars

Abstract

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<inline-formula><tex-math id="Z-20191028034712-1">\begin{document}$ L_{\odot} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_Z-20191028034712-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_Z-20191028034712-1.png"/></alternatives></inline-formula>－10⁶<inline-formula><tex-math id="M1">\begin{document}$L_{\odot}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_M1.png"/></alternatives></inline-formula>) O stars, with 25<inline-formula><tex-math id="Z-20191028034831-1">\begin{document}$ M_{\odot} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_Z-20191028034831-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_Z-20191028034831-1.png"/></alternatives></inline-formula>－30<inline-formula><tex-math id="M2">\begin{document}$M_{\odot}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_M2.png"/></alternatives></inline-formula> 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 <inline-formula><tex-math id="M200">\begin{document}$10^{-5} M_{\odot}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_M200.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20191040_M200.png"/></alternatives></inline-formula> 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 ascertainhow 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. Thestar 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.