So far, only 5-10% of these stars were suspected to have a magnetic field up to 10 million times that of Earth. This unexpected discovery demonstrates the importance of a "dynamo effect" able of maintaining intense fields in the interior of stars. These results, obtained using the Kepler satellite data, are published in the journal Nature on 21 January 2016.
Magnetic greenhouse effect
Using data from the NASA Kepler satellite, an international team of researchers was able to study about 13,000 red giant stars through the technique of asteroseismology, the measurement of vibrations inside and on the surface of the stars. Red giants are older and more massive stars than our Sun for which it is possible to measure seismic vibrations propagating down to their heart.
Recently published results from a small sample of red giants had already shown that strong magnetic fields can disrupt the propagation of gravity waves, which then remain trapped in the inner layers of the star in a phenomenon called "magnetic greenhouse effect". It is by measuring the amplitude of the oscillation modes, that the scientists found that in a large number of stars, about 65%, these modes were removed, implying the existence of large internal magnetic fields.
A revolution in the evolution of stars
"This result was completely unexpected," says Rafael García of Astrophysics Department-AIM Laboratory "Until now, only a small number of stars with masses less than 2 solar masses had large measured magnetic fields. These large magnetic fields were completely unnoticed by researchers because they were hidden in the deepest heart of the stars. "
The distribution of these fields as a function of the mass of the star shows that they are likely the result of magnetic fields from a dynamo effect developed in the heart of the convective stars beyond 1.2 solar masses.
"It's very exciting," says Rafael García "because magnetic phenomena will change the properties of the interiors of stars and we have neglected them so far in the current models of stellar structure and evolution."
This major discovery, which affects more than half of the stars in this range of masses, has yet unassessed implications. The results will enable scientists to test more directly their theories about how magnetic fields form and evolve within the stars - a process known as the dynamo effect. This could lead to a better general understanding of the magnetic dynamos, including control of the magnetic dynamo responsible for the 22 years cycle of the Sun, which is known to affect the communication systems and cloud cover on Earth.
A prevalence of dynamo-generated magnetic fields in the cores of intermediate-mass stars, Dennis Stello, Matteo Cantiello, Jim Fuller, Daniel Huber, Rafael A. García, Timothy R. Bedding, Lars Bildsten & Victor Silva Aguirre, Nature, 21 January 2016.
- Scientific contact: rafael.garcia at cea.fr
- Astronomy and astrophysics program scientist, CNES: Olivier.Lamarle at cnes.fr