Researchers at the University of Victoria's Astronomy Research Centre and the University of Minnesota report that stellar rotation provides the missing link, by dramatically enhancing the mixing of material across a barrier layer that separates the nuclear-burning interior from the outer convective envelope.

As Sun-like stars exhaust hydrogen in their cores, they expand into red giants that can grow to about 100 times their original size. Since the 1970s, astronomers have observed changes in the surface composition of these stars during this expansion phase, including a drop in the ratio of carbon 12 to carbon 13 that cannot be explained without efficient transport of material from deep inside the star up to the surface.

Standard models include a stable layer between the core and the convective envelope that acts as a barrier to mixing, leaving open the question of how newly produced elements cross this region. Scientists knew that internal waves generated by the churning motions in the convective envelope could pass through the barrier, but earlier simulations indicated that these waves carried very little material and could not account for the observed chemical signatures.

Using new high resolution three dimensional hydrodynamical simulations, lead researcher and University of Victoria postdoctoral fellow Simon Blouin and his colleagues quantified the impact of stellar rotation on these internal waves. They found that rotation amplifies the ability of the waves to mix material across the otherwise stable layer to a degree that matches the compositional changes measured at the surfaces of typical red giant stars.

According to the team, the mixing rates in the rotating red giant they studied can exceed those in comparable non rotating stars by more than a factor of 100, and the efficiency of this process increases for faster rotating stars. Because red giants represent a future evolutionary phase of stars like the Sun, the work also offers a window on how the chemical structure of our own star will change as it ages.

To obtain these results, the researchers carried out large scale hydrodynamical calculations that follow how stellar material flows and interacts in three dimensions. The size and detail of the simulations required some of the most advanced supercomputers currently available to the academic community.

The team used computing resources at the Texas Advanced Computing Centre at the University of Texas at Austin, along with the new Trillium supercomputing cluster at SciNet at the University of Toronto. Trillium, which launched in August 2025 as part of the Digital Research Alliance of Canada, is one of the most powerful Canadian systems for large parallel simulations, and its expanded computing capacity was essential for completing the work.

Project lead Falk Herwig, a professor of physics and astronomy and director of the Astronomy Research Centre, notes that the simulations make it possible to isolate subtle physical effects inside stars and connect them directly to astronomical observations. He describes the calculations as the most computationally intensive simulations of stellar convection and internal gravity waves performed to date.

The numerical methods developed for this project have applications beyond stellar interiors. Similar approaches can help researchers study complex flows in other natural systems, including ocean circulation, atmospheric dynamics and even blood flow, enabling cross disciplinary advances in how scientists model and understand turbulence and wave driven transport.

Blouin plans to extend the investigation of rotation driven mixing to a broader range of stars. Future work will examine how different internal rotation profiles modify the efficiency of mixing and whether rotationally enhanced wave transport also operates in other types of stars and at other evolutionary stages.

Research Report:Wave-driven mixing enhanced by rotation in red giant branch stars

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University of Victoria
Stellar Chemistry, The Universe And All Within It