The prevalence of solar-like differential rotation in slowly rotating solar-type stars
摘要
For more than 45 years, scientists have believed in the existence of two classes of differential rotation (DR): solar-like and anti-solar. The Sun rotates differentially with a fast equator and slow poles, called solar-like DR. Theoretical studies suggest that this DR becomes anti-solar in slowly rotating stars, where the poles rotate faster. The DR topology substantially affects the stellar magnetic activity and long-time stellar evolution. According to recently reported observations, however, there are some indications that challenge this solar-like and anti-solar picture in several aspects. We carry out unprecedentedly high-resolution magnetohydrodynamic simulations for the slowly rotating solar-type stars. The simulated solar-type stars consistently exhibit solar-like DR even at remarkably slow rotation rates in our simulations. While anti-solar DR has been predicted in previous models, we do not observe it under the conditions explored here. The strong magnetic field maintains the solar-like DR in all cases. The rotation has a less substantial influence on the magnetic field strength than that on the turbulence anisotropy. Our results show that the magnetic field monotonically decreases over the stellar lifetime, indicating a weakening of the magnetic braking. This trend is also consistent with recent observational results of the stellar rotation evolution. Taken together, this work revises the conventional picture of DR in slowly rotating stars and highlights magnetism as a key ingredient in stellar rotational and magnetic evolution.