The purpose of stellar seismology is to analyse the vibration modes of the stars which, submitted to forces of gravity, pressure and Coriolis, behave as oscillators with many specific modes. The eigen frequency (between 1 minute and 3 hours), the amplitude (a few ppm in Fourier space) and the lifetime (a few days) of these modes make it possible to determine some important parameters of stellar physics, such as the size and the composition of the core, the limits between radiative and convective zones, or the internal profile of rotation. 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 150 stars will be observed, with magnitude between 6 and 9.
The search for exoplanets (telluric)
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 around. This photometric method, complementary to the radial velocities, has the interest to unveil both orbit period and size (radius) of the planets detected. According to existence hypotheses, the Corot mission should discover, in addition to a large number of giants (hot Jupiters), a few tens of rocky planets (exoEarths). Potential rings or satellites could also be detected around giant extrasolar planets. The chromatric analysis of the CoRoT light curves, thanks to a dispersion device (prism) mounted in front of the exoplanet channel CCDs, allows to help identifying the different families of detected events (transits, stellar activity, eclipsing binaries...).
Till the end of the mission, almost 200 000 stars will be surveyed.
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
It was soon realised that these data could also be used to address several areas of stellar physics in addition to stellar seismology: 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 enabling the discovery and analysis of unexpected stellar behaviours.
It has therefore been decided to group together all seismic studies, and to group together all other other stellar programmes under the heading "Stellar physics (except seismology)", replacing the previous designation of "Additional programmes".
Corot is placed by a Soyuz launcher in a polar inertial circular orbit (90-degree inclination) at an altitude of 896 km. Not to be perturbed by the Earth straylight (scattered by the limb), the viewing zone is in the equatorial direction.
Twice a year, when the Sun gets closer to the orbit plane and is about to blind the telescope, the spacecraft performs a reversal attitude maneuver, dividing the year into two 6-month periods of observation (by convention, summer and winter).
During the observing runs (alternately 20 and 150 days), the spacecraft is 3-axis stabilized with asterocentric pointing. The jitter of stars on the detector is 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 target line of sight and the programming of the payload are elaborated by the Mission Center, whereas the Control Center performs the pointing maneuver and sets the ACS to fine pointing mode.
The right ascension of the orbit plane (12.5°) has been chosen after a ground preparatory observation campaign: Corot will look in the sky at 6:50 in winter and 18:50 in summer. Thanks to the baffle efficiency, it is possible to get closer to the Earth limb direction and thus orientate the satellite inside a cone with a 10-degree radius. When projected onto the sky, this cone draws the 2 eyes of CoRoT, where will be selected the stellar fields to be observed.
A slight orbit drift (by a housekeeping maneuver of inclination change) will make it possible to broaden the viewing zone and to put the stellar fields observed closer to the eye center, where the level of straylight is the lowest. The instrument performance is 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 are rotated every 14 days.
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