The purpose of stellar seismology is to analyze the vibration modes of the stars which, submitted to gravity forces, pressure and Coriolis, behave as oscillators with many specific modes. Thanks to the eigen-frequency (period between 1 minute and 3 hours), the amplitude (a few ppm in Fourier space) and the lifetime (a few days) of these modes, some important parameters of stellar physics can be determined, such as the size and the composition of the core, the limits between radiative and convective zones, or the internal rotation profile . These oscillating modes, which generate variations of luminosity at the surface of the star, are the only information, with neutrinos, coming from the depth of the stars. Acquired and collected on stars with different mass, age and chemical composition, the CoRoT light curves bring a significant amount of data of a new kind about stellar evolution.
During the whole mission, thousands of stars, from magnitude 6 to 9, were observed.
The method used is the method of the "planetary transits" which consists in detecting a planet by the small periodic drop of brightness on the disc of the star it orbits. This photometric method, complementary to the radial velocities, has the advantage of unveiling both the orbit period and the size (radius) of the detected planets. The chromatric analysis of the CoRoT light curves, thanks to a dispersion device (prism) mounted in front of the exoplanet channel CCDs, helps identify the different families of detected events (transits, stellar activity, eclipsing binaries, etc.).
By the end of the mission, about 160 000 stars have been scrutinized.
Stellar physics (except seismology)
During initial mission preparation, it was anticipated that up to 200,000 targets would be observed in a magnitude range of 5.5 to 16, with sampling times from 1 second to 15 minutes, and with a relative accuracy of 100ppm per measurement
In addition to stellar seismology, several areas of stellar physics benefit from these data, namely stellar activity and magnetism, intrinsic variability not caused by pulsation, the detection of comets and of small size Kuiper belt objects, the detailed study of binary stars, as well as the seismology of high amplitude pulsators. This has been confirmed; the high quality data lead to the discovery and analysis of unexpected stellar behaviours.
CoRot was placed by a Soyuz launcher on a polar inertial circular orbit (90-degree inclination) at an altitude of 896 km. In order to avoid Earth stray light (scattered by the limb), the observed zone is in the equatorial direction.
Twice a year, when the Sun got closer to the orbit plane and was about to blind the telescope, the spacecraft performed a reversal attitude manoeuvre, dividing the year into two 6-month observation period (by convention, summer and winter).
During the observing runs (alternately 20 and 150 days), the spacecraft was 3-axis stabilized with asterocentric pointing. The jitter of stars on the detector was then less than 0.5 arcsec (0.2 pixel). The seismology channel provides the platform with the angular data feeding the attitude control system. The Mission Centre was responsible for the target line of sight and payload programming, whereas the Control Centre performed the pointing manoeuvre and set the ACS to fine pointing mode.
The right ascension of the orbit plane (12.5°) had been chosen after a ground preparatory observation campaign: CoRot looked in the sky at 6:50 in winter and 18:50 in summer. Thanks to the baffle efficiency, it was possible to get closer to the Earth limb direction and thus to orientate the satellite inside a cone with a 10-degree radius. When projected onto the sky, this cone drew the 2 CoRoT eyes, where the stellar fields to be observed were selected.
A slight orbit drift (by a housekeeping manoeuvre of inclination change) allowed to broaden the viewing zone and to put the stellar fields observed closer to the eye centre, where the level of stray light is the lowest. The instrument performance was thus optimized.
Seen from the satellite, the movement of the Sun is a rotation of 1° every day. To guarantee a correct level of battery charge, the solar wings were rotated every 14 days.
More information about the CoRot mission: