The rate at which stars rotate dictates the amount of energy they put into generating magnetic fields - see the posting "The rotation of stars and their complex magnetism" for a more complete discussion. When they emerge at the stellar surface, magnetic fields interact and dissipate their energy, heating the very outer atmosphere - the "corona" - of the star up to millions of degrees C in the process. The resulting hot plasma radiates mostly in extreme ultraviolet and X-ray wavelengths.  This energetic radiation is important during initial stages of planet formation in the "protoplanetary disk", and also factors later in the evolution of planetary atmospheres by ionizing the outer layers and driving atmospheric evaporation.  The key to understanding the severity of these processes through the lifetime of a planet lies in understanding the rotation speed of the star and how it changes through time.

When they are young - a few tens of million years old - stars are rotating at their fastest, perhaps completing a revolution in less than a day - see the posting on the ultra-fast rotator HD199143 for an example.  As they age, their stellar winds carry away the momentum of this spin and stars slow down.  From a billion years of age onwards, the process is reasonably predictable and rotation rate can even be used to estimate the age of a star.  However, at ages from tens to hundreds of millions of years - crucial times for the evolution of planets - rotation is more difficult to understand and predict.  Some stars keep spinning fast, while others slow down quite quickly. SAO scientists Cecilia Garraffo, Ofer Cohen and I have been interested in seeing how the shape of a star's magnetic field might influence this process. 

The Sun's magnetic field is mostly like that of a simple bar magnet, or "dipole".  Younger stars appear to have more complex fields.  To investigate the spin-down effect of these more complex fields, we synthesized some idealized magnetic field maps (looking like 1960s rug designs...) with varying degrees of complexity and used these to drive a detailed magnetohydrodynamic model routinely used to predict the solar wind conditions from magnetic maps of the Sun.  A dipole field traps the wind in magnetic loops at the equator, creating a wind "dead zone". We found that more complex magnetic fields create many more dead zones over the star, shutting down the wind quite effectively - by a factor of 100 or so for the most complex fields compared to a dipole. Since it is the wind that creates the drag on the stellar spin, the complex field configurations would spin down much more slowly.  We think that some stars persist in spinning fast at early ages because they have more complex magnetic fields.  Once their fields become more simple, they then spin down quite rapidly and eventually, like the Sun, reach rotation periods of 25 days or so at 4-5 billion years old.  Lead by Cecilia Garraffo, this work was published in the 2015 July 1 edition of the Astrophysical Journal Letters.