The Sun rotates slowly, about once every 24 days at its equator although the hot gas at every latitude rotates at a slightly different rate. Rotation helps to drive the mechanisms that power stellar magnetic fields, and in slowly rotating solar-type stars also helps to explain the solar activity cycle. In the case of solar-type stars that rotate much faster than does the modern-day Sun, the dynamo appears to be generated by fundamentally different mechanisms that, along with many details of solar magnetic field generation, are not well understood. Astronomers trying to understand dynamos across a range of solar-type stars (and how they evolve) have been observing a variety of active stars, both slow and fast rotators, to probe how various physical parameters of stars enhance or inhibit dynamo processes.
Most techniques used to observe stellar magnetism rely on indirect
proxies of the field, for example on characteristics of the radiation
emitted by atoms. Surveys using these proxies have found clear
dependencies between rotation and the stellar dynamo and the star’s
magnetic cycles, among other things. Recent advances in instrumentation
that can sense the polarization of the light extend these methods and
have made it possible to directly measure solar-strength magnetic fields
on other stars.
CfA astronomer Jose-Dias do Nascimento is a member of a team of
astronomers that has just completed the most extensive polarization
survey of stars to date. They detected magnetic fields on sixty-seven
stars, twenty-one of them classified as solar-type, about four times as
many solar-type stars as had been previously classified. The scientists
found that the average field increases with the stellar rotation rate
and decreases with stellar age, and that its strength correlates with
emission from the stars’ hot outer layers, their chromospheres.
Not only
does this paper represent the most extensive survey to date of its
kind, it demonstrates the power of the polarization technique. It
signals that it is possible to greatly expand the study of magnetic
fields in solar-type stars, which efforts will continue to improve our
understanding of the surface fields in the Sun.
Reference(s):
"A
BCool Magnetic Snapshot Survey of Solar-Type Stars," S. C. Marsden, P.
Petit, S. V. Jeffers, J. Morin, R. Fares, A. Reiners, J.-D. do
Nascimento Jr., M. Auriere, J. Bouvier, B. D. Carter, C. Catala, B.
Dintrans, J.-F. Donati, T. Gastine, M. Jardine, R. Konstantinova-Antova,
J. Lanoux, F. Lignieres, A. Morgenthaler, J.C. Ramırez-Velez, S.
Theado, V. Van Grootel and the BCool Collaboration, MNRAS 444, 3517, 2014.
