Eight percent of red giants exhibit starspots

Gigantic, red and full of spots.

Starspots are more common among red giant stars than recently suspected. They are the expression of strong magnetic fields at the stellar surface. These magnetic fields are made somewhere inside the star in a procedure that requires convection and fast rotation of the star. Albeit red giants are generally viewed as gradually pivoting stars, those with starspots are an exemption.

A new study by the Max Planck Institute for Solar System Research (MPS) in Germany reports that sunspot-like, dark areas cover about eight percent of red giants. This new study offers an exhaustive analysis of the explanations behind their short revolution periods running from forced synchronization with another, intently neighboring star, to the swallowing of a star or planet, to a fast beginning turn speed in an early phase of development.

Dr. Federico Spada of MPS, co-author of the new study, said, “Rotation and convection are both crucial ingredients for the formation of surface magnetic fields and starspots. Stars with outer convective layers have the potential to generate surface magnetic fields via dynamo action, but only when the star rotates fast enough, the magnetic activity becomes detectable.”

Until now, scientists suspected that almost all red giants rotate rather slowly around their axis. After all, stars expand dramatically when they develop into red giants towards the end of their lives. As a result, their Rotation slows down, like a figure skater doing a pirouette with his arms stretched out.

This new study now suggests that about eight percent of the observed red giants rotate quickly enough for starspots to form.

Scientists scoured the estimation information of around 4500 red giants recorded by NASA’s Kepler space telescope from 2009 to 2013 for evidence of spots. Such spots decrease the measure of light that a star emits into space. Since they typically change somewhat more than a while, they step by step pivot out of the telescope’s field of view – and afterward return after some time. This produces typical, consistently repeating brightness fluctuations.

Scientists, later on, explored why spotted giants rotate so quickly and how they muster the required energy. To find out the answer, scientists determine as many of the stars’ properties as possible and then put together an overall picture.

They studied how the wavelengths of starlight from some of the stars change over time. This allows conclusions about their exact movement. 

Scientists also observed rapid fluctuations in brightness, which are superimposed on the slower ones caused by starspots. The faster fluctuations are the expression of pressure waves propagating through a star’s interior to its surface. They contain information on many internal properties, such as the star’s mass and age.

The study uncovered that almost 15 percent of the spotted giants belong to close binary star systems, usually constituted of a red giant with a small and less massive companion.

Dr. Patrick Gaulme, the lead author of the publication, said, “In such systems, the rotational speeds of both stars synchronize over time until they rotate in unison like a pair of figure skaters. The slower red giant thus gains momentum and spins faster than it would have without a companion star.”

The other red giants with starspots, about 85 percent, are on their own? And yet they rotate quickly. 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. The somewhat heavier ones, whose masses are two to three times that of the Sun, look back on different development. In the heyday of their lives, before they became red giants, their internal structure prevented the creation of a global magnetic field that gradually carries particles away from the star. Unlike their magnetic counterparts, which therefore rotate slower and slower over time, their Rotation has probably never slowed down significantly. Even as red giants, they still rotate almost as quickly as they did in their youth.

Gaulme said“In total, behind the common observational feature that some red giants have spots, we find three groups of rapidly rotating stars, each of which has a very different explanation. So it’s no wonder that the phenomenon is more widespread than we previously thought.”

Journal Reference:
  1. Patrick Gaulme, Jason Jackiewicz, Federico Spada, et al. Active red giants: close binaries versus single rapid rotators, Astronomy & Astrophysics, Volume 639, A63 (July 2020). arXiv:2004.13792

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