Researchers from the Yunnan Observatories of the Chinese Academy of Sciences have advanced the theoretical understanding of massive star evolution. The team performed model simulations of the angular momentum transfer mechanism of a special class of massive stars—Wolf-Rayet (WR) stars. They successfully extended theoretical models traditionally used for low- and intermediate-mass stars to the domain of massive stars, providing new theoretical support for understanding the evolution of internal stellar structures.

Massive stars with initial masses between 8 and 70 times that of the Sun undergo intense mass loss after evolving off the main sequence stage. This leads to the loss of a significant portion of their hydrogen-rich envelopes, exposing their inner helium cores. When these stars enter the red supergiant stage, the surface hydrogen envelope is typically completely lost, revealing internal nucleosynthesis products such as carbon, nitrogen, and oxygen. When the surface temperature exceeds 10000 K, these stars enter the WR stage. Theoretical models predict that such stars usually exhibit slow rotation.

Theoretically, as a star evolves to the end of the main sequence, core contraction leads to increased rotation, while the expansion of the outer envelope slows it down. When the star enters the red supergiant phase and sheds a significant portion of its hydrogen-rich outer envelope, a specific mechanism transfers the angular momentum produced by core contraction to the outer layers; this angular momentum is then carried away along with the ejected material. Ultimately, a slowly rotating helium core remains, thus forming a slowly rotating WR star.

For decades, mechanisms such as hydrodynamic processes, magnetic dynamo theory, and internal gravity waves have been proposed to explain how angular momentum transfer within stars. While these models have been extensively validated for low- and intermediate-mass stars, their systematic application to massive stars has been limited.

The team from Yunnan Observatories applied two modified angular momentum transfer models—the internal gravity wave model optimized based on the k - ω model and the magnetic dynamo model considering magnetic energy dissipation corrections—to the evolutionary simulations of massive WR stars.

Drawing on analogies with low- and intermediate-mass stars where gravity waves excited at the lower boundary of the convective envelope contribute to material mixing, the team extended this framework to the red supergiant stage of massive stars. They systematically studied the respective roles of gravity waves excited at the base of the convective envelope and at the boundary of the convective core in angular momentum redistribution. Their results indicate that internal gravity waves significantly enhance angular momentum transport in massive stars. By adopting 70 km/s as the upper limit for "slow rotation", the researchers constrained the model's free parameter A to values greater than 10.

Additionally, the team applied the modified magnetic dynamo model to massive stars, constraining the reasonable magnitude of the free parameter α to approximately 10^-2. This value is relatively lower compared to findings for low-mass stars. The difference arises because the shear parameter q, which depends on α, is inversely proportional to α. A smaller q more easily excites the Taylor instability, thereby promoting effective angular momentum transfer. Thus, while a smaller α may be insufficient for efficient transport, a larger α would lead to complete rotational braking.

The study confirms that angular momentum transfer mechanisms established for low-mass stars can also be extended to massive stars. The related findings have been published in The Astrophysical Journal.

Contact:
SI Jijuan
Yunnan Observatories, CAS
e-mail:sijijuan@ynao.ac.cn