ANN ARBOR—Astronomers at the University of Michigan have taken close-up pictures of a nearby star that show starspots—sunspots outside our solar system.
[Link to paper in its entirety.]( http://dx.doi.org/10.1038/nature17444 ) The researchers have used a technique called interferometry to build essentially the first time lapse of zeta Andromedae across one of its 18-day rotations. Zeta Andromedae is about 181 light-years away in the northern constellation Andromeda. A paper on the findings is published in the current issue of Nature.
"While imaging sunspots was one of the first things that Galileo did when he started using the newly-invented telescope, it has taken more than 400 years for us to make a powerful-enough telescope that can image spots on stars beyond the sun," said John Monnier, professor of astronomy in the U-M College of Literature, Science, and the Arts. The pattern of spots the astronomers saw on the star is very different from how they're typically arranged on our sun. The researchers say the findings challenge current theories of how stars' magnetic fields influence their evolution. At the same time, they give scientists a glimpse of how the sun likely behaved in its infancy, when the solar system was forming billions of years ago. "It's important to understand the sun's history because that dictates the Earth's history—its formation and the development of life," said Rachael Roettenbacher, a postdoctoral researcher in astronomy who conducted this research as part of her doctoral thesis at U-M. "The better we can constrain the conditions of the solar environment when life formed, the better we can understand the requirements necessary for the formation of life. These are the highest quality images of a star we have other than the sun." Sunspots and starspots are cooler, darker areas of a star's outer shell that form when stronger regions of the magnetic field block the flow of heat and energy in patches. On the sun, spots only form in bands just above and below its equator. Not so on zeta Andromedae. The new images showed one starspot in the star's northern polar region and several additional spots that spread across lower latitudes. Both findings are important, the researchers say. While previous studies that used indirect approaches to find spots suggested that stars with strong magnetic fields could, in fact, harbor them near their poles, that couldn't be verified. Now, it has been. "Our images unambiguously show polar starspots on zeta Andromedae for the first time," Roettenbacher said. "Now we can see that the spots aren't restricted to forming only in symmetric bands around the equator as sunspots are. We see the starspots in both hemispheres and at all different latitudes. This can't be explained by extrapolating theories about the sun's magnetic field." And the additional, lower latitude spots are spread over such an extended cool region that the scientists say they've found evidence that magnetic fields can suppress heat flow across a large part of the star's surface, rather than just in spots. Astronomers use star temperatures to estimate their ages, so they need to know if anything, such as these extended cool regions, is throwing off those temperature measurements.
Others have speculated that a similar mechanism generating the magnetic field of zeta Andromedae might be at work in young stars, which spin much faster than the sun. The new study connects the two. Zeta Andromedae is a binary system—a bright giant star locked in orbit with a smaller unseen companion. The bright giant has a radius 15 times larger than the sun. Interactions with its companion have sped its spinning so much that it finishes a full rotation in less than three weeks (Earth time) despite its additional girth, when compared to the sun. It has unusually strong magnetic activity. New, growing stars also spin fast as they take in material from disks of dust and gas around them and contract in the process. Nowadays, the sun rotates about once every 24 days, and the number of sunspots ebbs and flows along with the sun's 11-year magnetic activity cycle. More sunspots signal more magnetic activity and, on Earth, greater potential for geomagnetic storms that could damage satellites and the electrical grid. But astronomers believe the sun spun a lot faster when it was younger. "Observing spots on solar-like stars, young and old, helps us understand the fundamental physics behind magnetic field generation and how that changes over time," said Alicia Aarnio, U-M assistant research scientist. "Solar magnetic activity can greatly impact our lives today, so this work is important for developing our picture of the early, current and future behavior of the sun's magnetic field. Not only does this have implications for the beginnings of life, but its continuation as we know it." Interferometry allows the researchers to transform six telescopes into one super zoom lens. The CHARA Array, located on Mt. Wilson, Calif., is the most powerful optical interferometer in the world and is operated by Georgia State University. In order to take full advantage of CHARA, U-M's Monnier developed the Michigan InfraRed Combiner to give dimension to astronomical objects like stars that had long been mere points of light, even to the most powerful telescopes. "Zeta Andromedae represents a critical first step in studying the magnetic storms on sun-like stars," said Fabien Baron, assistant professor of astronomy at Georgia State University. "The CHARA Array will soon create images of many other stars to learn about the diversity of their surfaces." The paper is titled "No Sun-like dynamo on the active star ζ Andromedae from starspot asymmetry." Also contributing were researchers from the Finnish Centre for Astronomy with ESO, the University of Copenhagen, Ohio Wesleyan University, the Hungarian Academy of Sciences, the University of Exeter, Georgia State University, the Harvard-Smithsonian Center for Astrophysics and Pennsylvania State University. The research was funded by the National Science Foundation.