Astronomers have long been aware that stars progressively slow their spin rate throughout their evolution, shedding anywhere from one hundred to a thousand times their initial angular velocity. This deceleration is clearly visible when observing the Sun, which sheds angular momentum via the solar wind carrying matter away from its surface. It is generally accepted that the interplay between magnetic fields and plasma streams plays a crucial part in this mechanism. Clarity on this matter began to emerge with the advancement of asteroseismology—a technique allowing for the measurement of a star’s internal oscillations, thereby enabling the determination of its internal structure, rotational speed, and magnetic field configuration. Observations of numerous stars have demonstrated that current theoretical frameworks fail to fully account for the pronounced reduction in rotation speed correlated with stellar age. A research consortium from Kyoto University embarked on investigating this discrepancy, focusing specifically on the influence of magnetic fields deep within massive stars. The scientists drew inspiration from both asteroseismic data and prior three-dimensional simulations of the Sun’s convective zone, hypothesizing that analogous processes might be occurring in more massive luminaries during the later phases of their lives. Utilizing three-dimensional modeling, the researchers were able to directly examine, for the first time, the intricate coupling between turbulent convection, rotation, and magnetic fields inside a massive star. The findings indicated that the internal rotation and the magnetic field evolve in a coupled fashion, reminiscent of the solar dynamo—the engine that sustains the Sun’s magnetic field. This breakthrough allowed researchers to mathematically delineate how a star’s rotation profile changes over time. It transpired that the speed and orientation of convective flows can undergo rapid shifts, influenced by rotation and magnetic fields, which in turn impacts the rotation itself. Consequently, a star might either decelerate or, under certain circumstances, accelerate its spin. Thus, the process governing rotational evolution proved to be substantially more intricate and dynamic than previously supposed. The team also formulated an angular momentum transport model illustrating how this momentum can flow both outward and inward through the star. It was determined that during the later stages of thermonuclear burning, this transport becomes directly dependent on the geometry of the magnetic field. A surprising outcome was the discovery that particular magnetic field alignments possess the capacity to speed up the rotation of the stellar core, rendering the final spin rate unique to each celestial object. Furthermore, for certain categories of massive stars, achieving a slow rotation might prove entirely unattainable. The gathered evidence suggests that the angular momentum transport mechanisms previously established for solar-type stars may be universally applicable across a broad spectrum of stellar masses. This discovery opens new avenues for comprehending stellar evolution and their ultimate fates, including core collapse and the formation of compact remnants. Moving forward, the team intends to construct more comprehensive stellar evolution models, encompassing the entirety of a star’s lifecycle—from its genesis to its demise—to achieve more precise predictions regarding changes in rotational velocity at various developmental junctures.
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