Sandra Laly

In early October, the CNES Board of Administration gave the go-ahead for the COROT (COnvection, ROtation and planetary Transits) science mission. COROT is the third mission of CNES’s low-cost science mission program after Jason and Picasso-Cena. Annie Baglin at the Meudon Observatory and Claude Catala (now at the Midi-PyrÈnÈes Observatory) first proposed the mission in 1993. Many European partners are providing technical and scientific cooperation for the project. The first studies for COROT got under way in January 1994. The mission has two science goals, viz.,
– To study the internal structure and dynamics of stars by observing their natural oscillation modes
– To search for extrasolar planets

COROT is the third satellite built around PROTEUS (reconfigurable platform for observation, telecommunications, and science use) – a new platform design for satellites belonging to the weight class of 500 kilograms. COROT is the main tool for a very-high-precision stellar photometry mission. The instrument is a white light photometer with a reflector telescope, a dioptric imaging objective, and large defocused CCD sensors. Pointing accuracy will be to the order of one hundredth of a degree – a genuine step forward in the field.

An Asteroseismology Program Broken Down into Two Parts

The mission has two complementary goals. Asteroseismology is the goal in the very promising field of space astronomy. The mission will attempt to detect and measure the properties of star oscillation modes. The oscillations are acoustic or gravity waves propagating through each star. However, the lack of spatial resolution on the stellar disk limits the amount of observable modes, meaning that only the lowest degree modes can be detected.

During recent experiments (IPHIR aboard the PHOBOS satellite, VIRGO aboard SOHO), up to 30 modes were detected. COROT is expected to obtain comparable results for stars with high variations. On the other hand, there will be fewer observed modes for solar-type stars that have weaker amplitudes. However, a 0.1 micro-hertz resolution may be reached by continuous and stable observation over a period of 150 days. This would be impossible to do with Earth observation facilities unless there were a network of 8 telescopes with an eight-meter diameter staggered at regular intervals across the planet -and assuming that data linkage was solved.

Mission findings should contribute to the design of stellar hydrodynamic models. To do so, the asteroseismology program will be broken down into two parts. One will address an exploratory program to uncover the seismological properties of different types of stars. Part two is a central program tackling the detailed study of the selected objects. The first program will explore different types of stars, ranging from the more massive (spectral type O and B) to the less massive (spectral type G and K). To date, there is no projection that the inner modes of the stars are different from the Sun. Accordingly, observations lasting 10 to 20 days leading to frequency resolutions of 0.6 micro-hertz should make it possible to assess the size of the convective core of the stars massing more than the Sun.

The mission of the central program will be to observe a small number of already well known stars, to take very fine measurements of their oscillation mode frequencies and of all their typical magnitudes. Information will be collected on the main dynamic processes (convective core structure, size and composition of the outer convective zone, rotation profile, excitation and damping processes).

The Search for Exoplanets – The Most Fascinating Part of the Mission

Today, projections say that at least 5 percent of the stars have orbiting planets. Most of the planets that have been discovered mass much more than Earth and travel an orbit that is very close to their own stars, explaining why they are called ‘hot Jupiters’. The second COROT goal and undoubtedly the most fascinating will be to discover Earth-like planets that might support the development of Life. To do so, the occulations of the star caused by the transit of any planets will have to be observed. Projections for the number of expected transits during program life (i.e., two years and a half) range from 20 to 50. At least 30,000 stars of the main sequence will be observed. At the same time, relative flux variations ranging from 7.10-4 to 5.10-3 for magnitudes between 15.5 and 11 will be measured.

COROT is the first space mission of stellar seismology and telluric planet search. The mission should uncover several dozens of planets, including Earth-like objects or at least such is the hope of the many research scientists involved in the mission, slated for launching in 2004.