Recently, the International Astronomy Journal "The Astrophysical Journal Supplement Series" published the research result of Dr. LI Zhi and Researcher LI Yan from the Yunnan Astronomical Observatories of the Chinese Academy of Sciences (CAS). This work used the k-omega model to handle convective overshooting in the interiors of massive stars, while also considering the effect of stellar rotation, enabling the evolutionary paths of Wolf-Rayet (WR) stars to better match most observational data. The theoretical model concluded that the mass range for WNC stars is 15-36 M8, and the final mass of WR stars is approximately 9-23 M8, which is similar to the mass range of WNC, WC, and WO stars in the Milky Way.

WR stars are highly luminous and exhibit a spectacular appearance. They have very high mas- loss rates, and their spectral characteristics displaying strong and broad emission lines. WR stars are mainly divided into WN (with strong emission lines of He and N) and WC (with strong emission lines of He, C, and O) subtypes. The surface atmospheric composition of WN and WC stars respectively reveals the products of the CNO-cycle and the 3α reaction. The powerful stellar winds continuously strip away surface material from the stars, gradually revealing the internal structure of the stars on their surfaces. Analyzing the evolutionary characteristics of different subtypes of WR stars is of significant importance for studying the internal structure of massive stars.

This study utilized the k-omega model to treat convective overshooting in the interiors of massive stars and calculated evolutionary grids for WR stars with different masses (ranging from 25 to 120 M8). The results indicate that using the k-omega model can yield larger convective core masses, leading to larger partial mixing zones in the overshooting region, thereby resulting in broader chemical element transition zones. When employing the classical mixing-length theory (MLT) to handle convective overshooting, the setting of the convective overshooting mixing coefficient (f_ov) increases with increasing stellar mass. For massive stars capable of forming WR stars (greater than 30 M8), an f_ov value of 0.027 can be chosen when using the MLT model to obtain results similar to those of the k-omega model.

Stellar winds gradually strip away hydrogen elements from the surface of stars, leading to the formation of hydrogen-deficient WR stars. These stars are primarily classified into WNC, WC, and WO subtypes, with WNC stars representing transitional stars between the WN and WC subtypes. After considering the effects of rotational mixing in the stellar interiors, the mass range for WNC stars is found to be 15-36 M8, which can effectively explain the presence of higher-mass WNC stars in the Milky Way. The WNC/WR ratio in the rotational model and non-rotational model is 0.059 and 0.004, respectively. It's only after considering the rotational model that we can obtain results that are close to the proportion of WNC stars in the Milky Way (0.021, 14 out of 667).

Contact:
LI Zhi
Yunnan Observatories, CAS
Email: lizhi@ynao.ac.cn