From UCSB: “Magnetic Hide and Seek”

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KITP Kavli Institute for Theoretical Physics UCSB

October 22, 2015
Julie Cohen

Researchers at the Kavli Institute for Theoretical Physics develop a new technique to detect magnetic fields inside stars

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This artist’s representation of a red giant star with a strong internal magnetic field shows sound waves propagating in the stellar outer layers, while gravity waves propagate in the inner layers where a magnetic field is present.

Magnetic fields have important consequences in all stages of stellar evolution, from a star’s formation to its demise. Now, for the first time, astrophysicists are able to determine the presence of strong magnetic fields deep inside pulsating giant stars.

A consortium of international researchers, including several from UC Santa Barbara’s Kavli Institute for Theoretical Physics (KITP), used asteroseismology — a discipline similar to seismology — to track waves traveling through stars in order to determine their inner properties. Their findings appear in the journal Science.

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Jim Fuller, Matteo Cantiello and Lars Bildsten Photo Credit: Bill Wolf

“We can now probe regions of the star that were previously hidden,” said co-lead author Matteo Cantiello, a specialist in stellar astrophysics at KITP. “The technique is analogous to a medical ultrasound, which uses sound waves to image otherwise invisible parts of the human body.”

Cantiello’s curiosity and that of his co-authors was sparked when astrophysicist Dennis Stello of the University of Sydney presented puzzling data from the Kepler satellite, a space telescope that measures stellar brightness variations with very high precision.

NASA Kepler Telescope
NASA/Kepler

Cantiello, KITP director Lars Bildsten and Jim Fuller, a postdoctoral fellow at the California Institute of Technology, agreed that this was a mystery worth solving. After much debate, many calculations and the additional involvement of Rafael García, a staff scientist at France’s Commissariat à l’Énergie Atomique, a solution emerged. The data were explained by the presence of strong magnetic fields in the inner regions of these stars.

The puzzling phenomenon was observed in a group of red giants imaged by Kepler. Red giants are stars much older and larger than the sun. Their outer regions are characterized by turbulent motion that excites sound waves, which interact with gravity waves that travel deep into the stellar core. Magnetic fields in the core can hinder the motions produced by the gravity waves.

“Imagine the magnetic field as stiff rubber bands embedded in the stellar gas, which affect the propagation of gravity waves,” Fuller explained. “If the magnetic field is strong enough, the gravity waves become trapped in the star’s core. We call this the magnetic greenhouse effect.”

The trapping occurs because the incoming wave is reflected by the magnetic field into waves with a lower degree of symmetry, which are prevented from escaping the core. As a result, stellar surface oscillations have smaller amplitude compared to a similar star without a strong magnetic field.

“We used these observations to put a limit on — or even measure — the internal magnetic fields for these stars,” Cantiello said. “We found that red giants can possess internal magnetic fields nearly a million times stronger than a typical refrigerator magnet.

“This is exciting as internal magnetic fields play an important role both for the evolution of stars and for the properties of their remnants,” Cantiello added. “For example, some of the most powerful explosions in the universe — long gamma-ray bursts — are associated with the death of some huge stars. These behemoths — 10 or more times more massive than our sun — most likely ended their lives with strong magnetic fields in their cores.”

This work was written collaboratively on the web. A public, open Science version of the published paper can be found on Authorea, including a layman’s summary.

See the full article here .

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The University of California, Santa Barbara (commonly referred to as UC Santa Barbara or UCSB) is a public research university and one of the 10 general campuses of the University of California system. Founded in 1891 as an independent teachers’ college, UCSB joined the University of California system in 1944 and is the third-oldest general-education campus in the system. The university is a comprehensive doctoral university and is organized into five colleges offering 87 undergraduate degrees and 55 graduate degrees. In 2012, UCSB was ranked 41st among “National Universities” and 10th among public universities by U.S. News & World Report. UCSB houses twelve national research centers, including the renowned Kavli Institute for Theoretical Physics.

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