Kyoto, Japan -- From birth to death, stars generally slow by 100 to 1000 times their initial rotation rates; in other words, they spin down. The Sun's total angular momentum has declined as material is gradually blown off at the surface as solar wind. By observing this, astronomers have theorized the interaction between magnetic fields and plasma flow to be the most efficient way to spin down stars.
Why and how this happens has long interested astronomers, and recently an observational technique called astroseismology, which measures a star's natural oscillation frequencies, has made it possible to measure the internal rotation rates and magnetic fields of other stars in our galaxy. From this huge population, a picture of how stellar rotation decreases with stellar age has emerged, one that suggests that current theory is insufficient to explain the dramatic decrease in rotation.
Fascinated by astroseismology and by other researchers' 3D simulations of the solar convective zone, a team of researchers at Kyoto University was inspired to investigate how magnetic fields affect rotation inside massive stars..
"Our coauthors in Australia and the UK have already performed 3D magnetohydrodynamic simulations for massive stars before core-collapse. We suspected that the flow inside the massive star's convective zone may evolve analogously with the solar convective zone," says team leader Ryota Shimada.
With a 3D simulation of a massive star, the researchers were able to directly investigate the complex interplay between violent convection, rotation, and magnetic fields. They confirmed that the internal rotation and magnetic field coevolve akin to the solar dynamo: the energy process that sustains our Sun's magnetic field. With these equations in hand, the team was able to mathematically predict the evolution of the star's internal rotation in time.
Their simulation reveals that the speed and direction of convective motions were influenced by rotation and magnetic fields over short timescales, which in turn changes the rotation, causing it to spin down or -- in some cases -- up. The team was able to formulate the interaction between convection, rotation, and magnetic fields as a model for radial transport of angular momentum outwards and inwards, showing that this transport in later burning phases is directly related to the geometry of the magnetic field.
"We were surprised to discover that some configurations of the magnetic fields actually spin the core up, suggesting that the final spin rate will be unique to the star's properties," says co-author Lucy McNeill. "Slow rotation might even be forbidden in some classes of massive stars."
Their discovery of magnetic angular momentum transport during advanced burning phases suggests that the theory developed to describe rotation in solar-type stars may be universal. Next, the team plans to create stellar evolution simulations depicting the whole lifetimes of various low to high mass stars to predict their rotation rates during various evolutionary stages.
