Stars rotate around their axis, similarly as our planet. This angular motion can be measured from the spectrum of a star, or by timing the movements of active features on star’s surface. Theoretical knowledge suggests that this rotation gradually slows down over sufficiently long periods of time and it happens mostly due to the interaction of the magnetic field of a star with its stellar wind. But there is also another way to “tinker” with the angular momentum of a star.
Luckily, this “method” is more of a theoretical kind, since it involves the use of coronal mass ejections (CMEs). These phenomena could play a significant role not only in the evolution of planetary systems, but also could be capable to affect the evolution of a star itself. Certainly, some specific conditions are necessary for that to happen, as note the authors of a study, which was published on arXiv.org this week.
The team claims that our Sun is relatively quiet compared to most of the younger stars, which may be hundreds or even ten thousand times more energetic, according to the amount of energy that is thrown out into open space with each CME eruption. It is relatively easy to comprehend such power by comparing the magnitude of the distance that post-flare loops and prominences reach after extending from the surface of the star: CMEs from the Sun launch the solar matter into the height that typically equals one radius of the Sun; in more rampant stars the same structures may soar at the distances of tens of stellar radii from the surface of the star.
Obviously, such amounts of energy can not simply disappear without leaving any noticeable trace. Thus, the scientists decided to check if there is a specific reduction of the star’s angular momentum caused by CME, significant enough to slow the stellar rotation.
The scientists claim that a large part of the total energy released by a stellar flare is related to the total magnetic energy in a flare loop produced by CME. On the basis of this assumption, they applied the existing models that are used to assess the angular momentum loss due to the stellar wind-related mass and energy transfer.
The results of the calculations indicated, that a high CME mass loss rate (via strong and frequently occurring CME’s) potentially could be large enough to counteract the star’s spin up from contraction that is also the result of the same CMEs, thus they can leave a permanent effect on the stellar spin.
The team notes that more data on stellar activity parameters is needed to assess the potential CME-related slow-down of less active stars, that are more similar to our Sun. This research may also provide some new insights into formation and evolution of young stellar systems, when the host star often forms simultaneously with surrounding planets.
By Alius Noreika, source: www.technology.org