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The Revolution Revolution!

posted Oct 11, 2019, 11:10 AM by Jeremy Drake   [ updated Oct 11, 2019, 11:13 AM ]
The magnetic field generating capability of stars like the Sun is driven by rotation acting in concert with convection. Convective motions in a rotating fluid sphere - which is essentially what a star is - lead to a pattern of differential rotation, where different regions of the stellar convective envelope are rotating at different rates. It is the shear between adjacent layers that stretches and wraps the magnetic field, amplifying it in the process. Magnetic field in a stellar interior is buoyant and rises to the surface, producing an array of interesting physical phenomena that have been reported in several previous posts. Examples include stellar winds, coronal mass ejections, and X-ray emission

Stellar winds take away angular momentum from the star, slowing its spin. The Sun, whose equatorial rotation period is about 25 days, for example, would have been spinning with a rotation period of only a couple of days or so when it was born. We can see stellar spin-down at work by observing the rotation periods of stars in open clusters in which all cluster members are essentially the same age.  These rotation periods are distributed in a puzzling bimodal way, with some stars remaining as rapid rotators for many millions of years longer than others. Angular momentum evolution theory has struggled for decades to explain this. 

Smithsonian Astrophysical Observatory and Harvard Institue for Applied Computational Science scientist Cecilia Garraffo has lead a study published in the 2018 July 20 Astrophysical Journal that can now finally explain this rotation behaviour.  The answer appears to lie in the distribution of the magnetic field over the surface of the star. If the field is disorganized and split into many components, or ``multipolar", the wind is closed down and angular momentum loss greatly reduced compared with the case of magnetic field organized into a single dipole.  Observations indicate that magnetic field complexity of Sun-like stars is originally quite high when they are born but decreases rapidly as they spin down. This growth of dipolar field applies a strong magnetic brake that rapidly slows the star's spin, leading to two populations: the remaining fast rotators and a population of slower rotators as observed.  A revolution in the understanding of stellar revolution.