Scientists Have Discovered One Of The Secrets Of The Red Giants. How Did They Get Spots?
Starspots are more common among red giant stars than previously thought. In the journal Astronomy and Astrophysics, researchers led by the Max Planck Institute for Solar System Research (MPS) in Germany report that about 8% of red giants have such spots; this is the expression of strong magnetic fields on the surface of a star.
They are created deep inside the star in a process that requires, among other things, convection and rapid rotation of the star. Although red giants are usually viewed as slowly rotating stars, those with starspots appear to be an exception.
Among the Sun's most striking features are its sunspots, relatively dark areas, some of which are visible from Earth even without magnification. Many other stars, which, like the Sun, are in the Prime of their years, are also covered with spots. On the other hand, in red giants that are at an advanced stage of stellar evolution, such spots were previously considered rare. The reason for this difference can be found deep inside the stars. The interaction of conducting plasma currents and rotation generates a magnetic field of the star, which is then washed out on its surface. In some places, particularly strong magnetic fields prevent hot plasma from flowing upwards. These areas appear dark and makeup starspots.
Until now, researchers have assumed that almost all red giants rotate quite slowly around their axis. After all, stars expand dramatically when they turn into red giants towards the end of their lives. As a result, their rotation slows down. A new study led by scientists from MPS and the University of New Mexico (USA) is currently changing the way scientists think about this process.
The research team studied measurements of about 4,500 red giants recorded by the NASA space telescope from 2009 to 2013 to identify the spots.
In the second phase, the researchers investigated the question of why the spotted giants spin so fast. How do they collect the necessary energy?
To answer this question, scientists needed to determine as many properties of stars as possible, and then create a General picture, explains the lead author of the publication, Dr. Patrick Golm. For example, at the Apache point Observatory in new Mexico (USA), researchers studied how the wavelengths of starlight from certain stars change over time; this allows us to conclude their exact movement. The team also looked at fast brightness fluctuations that overlap with slower ones caused by starspots. Faster fluctuations are the expression of pressure waves propagating through the inner space of the star to its surface.
The analysis showed that about 15% of the detected giants belong to close binary star systems, usually consisting of a red giant with a small and less massive satellite. In such systems, the rotation speeds of both stars are synchronized in time until they rotate in unison. Thus, a slower red giant gains momentum and rotates faster than without a companion star.
Other red giants with starspots, about 85%, are without a star pair, and yet they rotate rapidly. Those with a mass roughly equal to that of the Sun probably merged with another star or planet in the course of their evolution and thus gained speed.
Several of the heavier red giants, which are two or three times the mass of the Sun, had a different development. During their heyday, their internal structure prevented the creation of a global magnetic field that gradually carries particles away from the star. Their rotation probably never slowed down significantly. Even as red giants, they still spin almost as fast as they did when they were young.
In total, three groups of rapidly rotating stars were found, each with a completely different explanation for the spots.
Studies like this shed light on the evolution of the rotation and magnetic activity of stars. And their complex interactions, including the impact on the habitability of their possible planetary systems; this is one of the main goals of the European Space Agency's PLATO (PLAnetary Transits and Oscillations of stars) mission, which is expected to launch by the end of 2026.