Star spots are more common among red giant stars than previously thought. In Astronomy & Astrophysics, researchers led by the Max Planck Institute for Solar System Research (MPS) in Germany report that approximately 8% of red giants have such spots. This is an 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 fast rotation of the star. Although red giants are usually seen as slowly rotating stars, those that have starspots are apparently the exception.
Among the most striking features of the Sun are its sunspots, relatively dark areas, some of which are visible from the Earth even without magnification. Many other stars that, like the Sun, are in their prime, are also stained. On the other hand, in red giants, which 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 conductive plasma currents and rotation generates a magnetic field of the star, which is then washed out onto its surface. In some places, especially strong magnetic fields do not allow hot plasma to flow upward. These areas appear dark and makeup starspots.
Until now, researchers have suggested that almost all red giants rotate quite slowly around their axis. In the end, stars expand dramatically when they turn into red giants by 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 the measurement data of about 4,500 red giants recorded by the NASA space telescope from 2009 to 2013, to identify spots.
In a second step, scientists investigated why spotted giants rotate so fast. How do they collect the necessary energy?
To answer this question, scientists had to determine as many properties of the stars as possible, and then make up the big 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 some stars change over time. This allows us to draw conclusions about their exact movement. The team also examined fast brightness fluctuations that overlap with slower ones caused by starspots. Faster fluctuations are an expression of pressure waves propagating through the interior of a star to its surface.
Analysis showed that approximately 15% of the giants discovered belong to close binary stellar 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, the slower red giant is gaining momentum and spinning faster than without a companion star.
Other red giants with starspots, about 85%, are without a star pair, and yet they spin quickly. Those whose mass is approximately equal to the mass of the Sun probably merged with another star or planet during their evolution and thus gained speed.
Several heavier red giants, whose mass is two to three times the mass of the Sun, had a different development. During the heyday of their life, their internal structure prevented the creation of a global magnetic field, which gradually carries away particles from the star. Their rotation probably never slowed down significantly. Even as red giants, they still spin almost as fast as in their youth.
In total, three groups of rapidly rotating stars were found, each of which has a completely different explanation of the spots.
Studies such as this shed light on the evolution of the rotation and magnetic activity of stars. As well as their complex interactions, including the impact on the habitability of their possible planetary systems. This is one of the main goals of the mission of the European Space Agency in PLATO (PLAnetary Transits and Oscillations of stars), the launch of which is expected by the end of 2026.