Researchers at the Yunnan Observatories of the Chinese Academy of Sciences have conducted a simulation of the evolution of massive stars, with masses ranging from 50 to 150 solar masses, during the Nitrogen sequence Wolf-Rayet star (WNL) phase. The study utilized a newly-developed k−ω model to handle the convective overshooting processes within the stellar interior, offering a more precise representation of the complex convective overshooting phenomena that occur within massive stars. This work has been published in the Astrophysical Journal.

At the boundary between the convective and radiative zonesof stars, fluid retains inertia and ‘overshoots’ the convective zone, thereby bringing elements from the convective zone into the radiative zone. WNL stars typically form during the core hydrogen burning, with their surfaces enriched in nitrogen due to mixing and strong winds that strip away their outer envelopes. Convection and overshooting transport nucleosynthesis products to outer layers, causing anomalous surface enrichment and changing stellar fates. Studying WNL stars, therefore, provides valuable insights into the effects of convection and mass loss on stellar evolution.

The researchers compared the results from the k−ω model with those from the previously used exponential decay model for handling the overshooting zone. They found that the k−ω model predicts a broader range of mass and lifetime evolution for the WNL stars under the same conditions, such as initial mass and metallicity, and it also lowers the model limit for WNL star formation. This is attributed to the k−ω model's ability to expand the mixing zone of materials within the star. Furthermore, the researchers considered the factor of rotation and found that rotation may play an important role in the formation of lower-mass and metal-poor WNL stars; this effect is more pronounced in the previous model.

This comparison has provided new insights into the formation of observed lower-mass WNL stars. Additionally, the study revealed that the new convective overshooting model can effectively explain the distribution of WNL star samples observed in the Galaxy on the Hertzsprung-Russell diagram.

By utilizing a new model for convective overshooting, the research provides new perspectives and more accurate results for revealing the evolutionary patterns of special stellar phases like the WNL stages. Moreover, the differences in results between the two convective overshooting models offer researchers alternative model choices for inferring the initial evolutionary environments and distributions of the observed samples.

Contact:

SI Jijuan

Yunnan Observatories, CAS

E-mail: sijijuan@ynao.ac.cn