CoRoT focuses on stars' oscillations. Just like the musical instruments, each star has its own oscillation patterns depending on its spherical structure1. One of the mission's objectives is to detect those oscillations, and in particular the sunlike oscillations. By measuring their frequencies, amplitudes and life times, scientists obtain information on the star's internal physical state. This technique, called "stellar seismology", offers a benefit examining remote stars at much bigger distances than the usual observation techniques.
The article's authors could thus analyse over time the variations of those oscillations in the star HD49933, located at 100 light years in the Unicorn constellation, West of Orion. The infinitesimal variations of the frequencies and amplitudes have a periodicity of around 120 days. The scientists think those variations are due to the magnetic activity which causes structural modifications to stars and in particular to the starspots distribution at the surface, similar to sunspots. Even if such phenomena had already been observed on some stars, it was the first time stellar seismology made those observations possible.
"We discovered a magnetic activity cycle on this star, similar to the Sun's one. This will allow us to potentially observe hundreds of stars", explained Jérôme Ballot, co-author of the article.
1All bodies, and in particular stars, can be host of periodic vibrations. If those vibrations have "audible" frequencies, i.e. frequencies the human can hear, we call them sounds. The musical instruments are the most enjoyable example. The vibrations' frequencies inform us about the body's structure. We do know that a cracked-plate "sounds" quite different than a plate in good condition. We also know that by changing the tension of a violin string, we change the sound it produces when played with the bow.
A promising technique
This technique paves the way for the observation of the magnetic activity of several hundreds other stars. It will help to better understand how the magnetic activity cycles change from one star to another and to perceive the underlying mechanisms. The objective is also to shine a light on the processes behind the Sun's magnetic activity and to improve the forecasts of the solar cycle and geomagnetic storms, which are responsible of important disruptions in the electricity grid and communications. Scientists also want a better view on the effects on Earth and eventually on climate.
"For the first time, seismology is showing its capacity to provide tools to reach the long-term and big scale indicators of the magnetic activity. This pioneer result will be completed by current and future missions", explained Annie Baglin, co-author of the article and Principal Investigator for the CoRoT mission in LESIA (Laboratory of Space Studies and Instrumentation in Astrophysics, Observatoire de Paris).
The team also hopes to understand how the stars could host planets, in particular those which could be able to host life. "The understanding of the activity of stars hosting planets will be necessary because the magnetic conditions at the star's surface could influence the area where life could develop", explained Rafael Garcia, principal author of the study and investigator in the CEA.
The magnetic cycle of HD49933 IS SHORTER THAN OU SUN'S
The scientists examined 187 days of data gathered by CoRoT during two periods performed one year apart.
CoRoT had already shown that the star HD49933 was bigger and hotter than the Sun. It shows now than its magnetic cycle is much shorter. It is important because of the possibility to observe an entire cycle more quickly than the Sun's which lasts eleven years and thus gather information more quickly on magnetic cycles.
The scientists expect to extend the observations to other stars with CoRoT, but also with the NASA's Kepler mission launched in March 2009. The latter will provide continuous data during 3 to 5 years on thousands stars.
"If the short magnetic cycle turns out to be common to all stars, it will be possible to fully observe a large number of stars during the CoRoT and Kepler missions", said the authors of the document..
CoRoT reveals a magnetic activity cycle in a Sun-like star, Science review, Brevia, August 27th, 2010.