An international group of astronomers led by Dr. Rachael Roettenbacher from the University of Michigan has used cutting-edge techniques to create direct images of surface structures on a primary member of the nearby binary star system Zeta Andromedae.
Surface images of Zeta Andromedae from September 2013 with fourteen nights of data. Image credit: R.M. Roettenbacher et al.
Zeta Andromedae, also known as HD 4502 and 34 Zeta Andromedae, is a tidally locked close binary system consisting of an orange K-type giant star and an unseen lower-mass companion star.
It is located in the northern constellation Andromeda, approximately 181 light-years away.
The visible member of this binary is about 16 times the size of the Sun in diameter.
In order to image its surface during one of its 18-day rotations, Dr. Roettenbacher and co-authors used a method called interferometry, where the light of physically separate telescopes is combined in order to create the resolving power of a 330-m telescope.
The astronomers discovered the star showed signs of ‘starspots’ – the equivalent of sunspots found within our own Solar System.
The pattern of these spots differs significantly from those found on the Sun.
“These results challenge current understandings of how magnetic fields of stars influence their evolution,” Dr. Roettenbacher and co-authors said.
Furthermore, the team believes that the findings offer a rare glimpse of how the Sun behaved in its infancy, while the Solar System was first forming.
Zeta Andromedae. Image credit: Centre de Données astronomiques de Strasbourg / SIMBAD.
“Most stars behave like giant rotating magnets and starspots are the visible manifestation of this magnetic activity,” said co-author Prof. Stefan Kraus, from the University of Exeter, UK.
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. However, while on the Sun spots only form in bands just above and below its equator, the scientists found something quite different on Zeta Andromedae.
As well as one starspot in the star’s northern polar region, there were also several additional spots that spread across lower latitudes.
“It is a very different star than the Sun. It is a binary star, that is, two stars orbiting each other. This causes the stars to rotate more quickly,” said co-author Dr. Heidi Korhonen, from the Niels Bohr Institute at the University of Copenhagen, Denmark.
“The Zeta Andromeda star, which is the larger of the two stars, rotates at 40 km per second. The Sun rotates at 2 km per second.”
“It is the rapid rotation that creates a different and very strong magnetic field. The strong magnetic field gives a more complicated dynamo effect that resembles what you see at the stage where a new star is being created. Here we are seeing the same effect in an old active star that is in its final stage.”
The findings are published in the journal Nature on May 4, 2016.
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R.M. Roettenbacher et al. No Sun-like dynamo on the active star ζ Andromedae from starspot asymmetry. Nature, published online May 4, 2016; doi: 10.1038/nature17444
