1 in 10 Red Giants are Covered in Spots, and They Rotate Surprisingly Quickly

Sunspots are common on our Sun. These darker patches are cooler than their surroundings, and they’re caused by spikes in magnetic flux that inhibit convection. Without convection, those areas cool and darken.

Lots of other stars have sunspots, too. But Red Giants (RGs) don’t. Or so astronomers thought.

A new study shows that some RGs do have spots, and that they rotate faster than thought.

The new study is titled “Active red giants: Close binaries versus single rapid rotators.” Lead author is Dr. Patrick Gaulme from the Max Planck Institute. The paper is published in the journal Astronomy and Astrophysics.

Red Giant stars are at a late stage of stellar evolution. All stars rotate, but as RGs lose mass and expand, that rotation slows down, like a figure skater who stretches out their arms. That slower rotation calms the dynamo process inside the star, and that dynamo process is what fuels the star’s magnetic activity. Less magnetic activity means fewer spots.

Or it should.

But this new study finds that some RGs don’t conform to this understanding. The study shows that about eight percent of RGs rotate rapidly and produce starspots.

This image tracks the life of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant star on the right. On the left the star is seen as a protostar, embedded within a dusty disc of material as it forms. It later becomes a star like our Sun. Eventually, it’ll enter a helium-burning phase, expand, and turn red. Our Sun has no binary companion, so likely will not rotate rapidly and produce sunspots anymore. Image Credit: By ESO/M. Kornmesser – http://www.eso.org/public/images/eso1337a/, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=27981948

“Rotation and convection are both crucial ingredients for the formation of surface magnetic fields and starspots,” explains Dr. Federico Spada of MPS, co-author of the new study, in a press release. “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,” he adds.

But now there are exceptions to that understanding of RG stars.

Between 2009 and 2013, NASA’s Kepler spacecraft gathered data on about 4,500 red giant stars. The team of researchers behind this study examined all of that data. Though Kepler was designed to ferret out dips in star brightness caused by planets transiting in front of the star, it gathered data on all dips in brightness, including those caused by starspots. RGs typically rotate slowly, so these dips in brightness from starspots rotate in and out of view over the course of a few months. And they repeat.

When the team found that about 8% of the stars are rotating more rapidly than expected, they wondered how the stars got the energy to do so. “To answer this question, we had to determine as many of the stars’ properties as possible and then put together an overall picture,” said lead author Dr. Patrick Gaulme.

Artist’s impression of the structure of a solar-like star and a red giant, not to scale. Image Credit: ESO

The researchers examined a number of things. They looked at the wavelengths of light from the stars and how they changed over time. They looked at rapid fluctuations in the stars’ brightness. Those rapid fluctuations are caused by pressure waves coming from the star’s center to the surface, and they’re superimposed on the dips caused by starspots. The pressure wave fluctuations are like a window into the interior of the star, and they hold clues about the star’s mass and age.

But the most interesting finding was that about 15% of the rapidly rotating RGs with starspots are in close binary relationships with another star, usually one that’s smaller and less massive.

“In such systems, the rotational speeds of both stars synchronize over time until they rotate in unison like a pair of figure skaters,” says Gaulme. So the slowly-rotating Red Giant gains rotational energy from its binary partner. It spins faster than a RG without a binary buddy.

But that doesn’t explain the other 85% of RGs with starspots that are also rotating more rapidly than expected. Those stars don’t have binary companions to gain their rotational energy from. What gives?

“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.”

Dr. Patrick Gaulme, Lead Author, Max Planck Institute

To start with, the researchers divided those stars into two groups: stars that have a mass roughly equal to the Sun, and stars that have two to three times as much mass as the Sun.

The team says that the stars roughly equal to our Sun likely merged with another star, or maybe a planet, and gained rotational energy that way. The stars in the more massive group developed differently. They likely had an internal structure that was quite different from the rest. They may never have created the kind of global magnetic field that, over a long period of time, carries mass away from the star as particles. Without that mass loss, their rotation never slowed, and they rotate more quickly to this day.

The study found that there are three different paths Red Giants can follow to produce starspots. Image Credit: MPS / hormesdesign.de

“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,” said Gaulme.

What does this mean for exoplanets orbiting RGs? The study exposes some of the detail in Red Giant stars, and the complexity that governs their impact on the habitability of any planets around them. But any conclusions or rules that can be applied to them or their planets will have to wait for missions like the ESA’s PLATO (PLAnetary Transits and Oscillations of stars) mission. PLATO will examine the properties of terrestrial exoplanets in the habitable zones around their Sun-like stars. It’ll also study the seismic activity in those stars, further exploring the relationships between habitable zone planets and their stars.

“We look forward to having the PLATO mission in space; with its unique long-duration observations we will be able to extend the study to other regions of the Milky Way,” concluded study co-author Spada.

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Evan Gough

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