SIROCO = SeIsmology, ROtation and COnvection with the COROT satellite
In December 2006, the European space mission COROT was successfully launched. The first calibrations of the instrument show levels of precision even better than expected. This mission has two goals: detecting planets orbiting around stars other than the Sun, and probing the internal physics of stars by a method called asteroseismology. Our project concerns the seismic part of the COROT mission. This satellite will observe the micro-variations of the light emitted by specific stars without interruptions and with very high precision. More precisely, the SISMO field of view will observe 10 well-chosen stars during each run. The first 2-month initial run of observations began in February 2007. Then alternating long runs of 150 days of continuous observations and short runs of 30 days will be performed during at least 2.5 years. With these observations, we will be able to detect numerous oscillation modes of stars with very high precision and use them to study stellar interiors with unprecedented precision, a method called "asteroseismology".
Asteroseismology is indeed a new and very promising branch in astrophysics, which proceeds as follows. The oscillation modes of a star carry with them precise information about the physics of stellar interiors. More precisely, the spectrum of normal mode frequencies is directly related to the values of important quantities such as the sound speed (directly related to the temperature), the density and the rotation rate at each specific layer of a star. Hence, the determination and interpretation of the oscillation frequencies of stars allow us to probe the physics of stars in each layer from their very centre to their surface, giving us a unique opportunity to constrain the macrophysics of stellar interiors - mixing processes (diffusion, ...), transport of momentum, detailed description of the turbulent convection - as well as their microphysics (equation of state, ...). In the direct method, the model best reproducing the observed frequency spectrum is obtained in an iterative way. In inverse methods, the rotation, sound speed and density profiles are deduced from the frequencies by inverting integral equations. This last method has been applied with much success to the Sun.
A particularly important and difficult aspect to modelling stars is the influence of rotation. First, it breaks the spherical symmetry by the action of the centrifugal force. Second, differential rotation induces transport of chemicals and angular momentum in stellar interiors, via large scale meridional circulations and small scale turbulent motions. Third, rotation directly modifies the dynamic of stellar oscillations, e.g. by the action of the Coriolis acceleration. As a consequence, the frequency pattern of oscillation modes is strongly affected and becomes much more difficult to interpret. This difficulty has so far prevented the use of asteroseismology's full potential in large fractions of the upper main sequence. For example, Be stars (the study of this class of stars is a speciality of our GEPI team) represent 10% of all stars observed by COROT in seismology and the interpretation of their COROT data will be particularly challenging. Asteroseismology first relies on the ability to compute accurate theoretical frequencies for a given stellar model. While various codes could perform this calculation when the rotation rate is small enough for perturbative methods, an oscillation code able to fully take into account the effects of rotation on the oscillations was much needed. The Toulouse team started to develop such a code in 2000 and succeeded in computing, for the first time, acoustic modes in polytropic stellar models with an accuracy larger by various orders of magnitude than the best accuracy attainable with COROT. This unique tool already provided important new results namely: the first determination of the domain of validity of perturbative methods, mode visibilities in stark contrast to the non-rotating case and the discovery of new regular patterns in the frequency spectrum. Much work must still needs to be done however to start confronting the computed frequency with the observed ones. Not only do we have to use the oscillation code with more realistic stellar models, but we also need to derive efficient diagnostics allowing us to use them for the interpretation of observed frequency spectra.
Besides the frequencies, quantities such as the amplitudes, the phases, or simply the range in frequencies of the observed modes allow us to strongly constrain energetic aspects of the oscillations. First, the analysis of the driving mechanism of stellar oscillations can be done (a speciality of our LESIA team). The point here is to understand the motor mechanisms leading to the pulsation of different type of stars. These mechanisms are typically related to the opacity of stellar matter in specific regions (the "kappa-mechanism"). This opacity-mechanism is enhanced by the presence of some elements (Fe, He, ...) in their partial ionization zones. Transport processes such as microscopic diffusion (segregation effect of gravitation), meridional circulation and turbulence associated with differential rotation can lead to such accumulations. Comparisons of the predicted and observed ranges of oscillation frequencies allow us to strongly constrain these processes. In massive stars (a speciality of our GEPI team) the "kappa--mechanism" produces not only p-modes (e.g. in beta Cep and early-Be stars) but also g-modes (e.g. in SPB and mid- and late Be stars). Up to now, only p-modes have been introduced in models such as the ones developed by our team in Toulouse. However, introducing g-modes in these models is essential to study the driving mechanisms and transport processes of massive stars. Finally, in the case of Be stars, angular momentum is lost through the ejection of material from the star. These ejections, most probably due to the combined effect of rapid rotation and beating of oscillation modes, influence the chemical composition of the stellar surface (and interior?).
Other driving mechanisms are related to the interaction between convection and oscillations. Constraints on the delicate description of time-dependent convection can be obtained in this case. The energetic aspects of oscillations are of a completely different nature in solar-like oscillators. In this case, the excitation of the acoustic modes, called "stochastic excitation", comes from the very vigorous convective movements near the stellar surface. A difficult problem in this context is, typically, the determination of correlation products associated with turbulence. New closure models have been proposed for this purpose by our LESIA team. 3D hydrodynamic simulations can also help in this context. Comparisons with the observed mode amplitudes and line-widths allow us to strongly constrain the description of turbulence near the top of the convective envelope and its coherent and incoherent interaction with oscillations.
Only one other team in the COROT community has begun to develop energetic pulsation models in collaboration with us (Garrido & Moya, Spain). However, we remain the only team in the world to include time-dependent convection in our models of non-radial oscillations. The effort we propose to make will benefit the international community at large since (1) our models and tools will allow the interpretation all COROT data, (2) they can also be used by anyone for all types of seismic data (not only COROT data), (3) our models will provide information on the interiors of stars that will likely revolutionize the current status of stellar physics and thus have consequences on a large domain of astronomy.
Although some results obtained from the ground and with the Canadian microsatellite MOST have already shown the efficiency of asteroseismology as a probe of stellar internal structure, the revolution is coming with COROT. With long continuous observation runs, periodic signals with amplitudes down to ~ 1 ppm will be detectable, which is completely impossible from the ground or with MOST. Thanks to these levels of accuracy, many more modes will be detected with a very high precision on the frequencies. With the techniques mentioned above, we will be able to strongly constrain the physical description of stellar interiors. We emphasize that the extreme physical conditions inside stars cannot be reproduced in laboratories. With the technique of asteroseismology, we transform the stars themselves into laboratories, allowing us to improve our knowledge of fundamental physics in extreme conditions (interaction between matter and radiation, thermonuclear reactions, neutrino physics, turbulence,...).
Our project enters directly in this framework. One of our laboratories, the LESIA, has taken an important part in the design of the instrument. The 3 partners of this project took a central part in the scientific preparation of the COROT mission for more than 10 years. It is now fundamental and urgent to finish the development of tools to obtain the highest benefits from the COROT observations. We decided to join our efforts to obtain the best scientific results from the very precise data expected with this instrument in the forthcoming months. Our 3 groups are experts in different aspects of asteroseismology and have already developed analysis and modelling tools, which will be very useful in this context. But this is not yet sufficient to be able to face the very challenging data to come with COROT. That is why our 3 groups decided to work together to largely improve our current tools and deliver a detailed picture of the interiors and transport processes of stars. This will represent a major step forward in our understanding of stars since such results currently exist only for our Sun. The outcome of our work combined with the COROT data will thus open a new area in stellar physics.