Red giants are stars in a late phase of stellar evolution, representing the future evolutionary status of our Sun. After having burnt their core hydrogen, stars with intermediate mass (roughly 0.5 to 10 solar masses) greatly expand so that their outer layers cool down and then redden. At the end of their life, red giants eventually expell their outer layers and then participate to the chemical enrichment of our Galaxy.
A red giant star exhibits oscillation modes that are stochastically excited by turbulent motions in the convective outer layers of the star. Some of these oscillations are radial : the star expands and contracts radially and spherical symmetry is preserved during the oscillation cycle. If transverse motions occur in addition to radial motions, one uses the term "non-radial oscillations". See Fig.1 for an example of such oscillations.
Figure 1 : Animation showing an example of a non-radial oscillation. Note that the amplitudes have been amplified more than 100000 compared to what it is observed in real stars. Courtesy of David B. Guenther.
Figure 2 : A stack of power spectra of nine red giant pulsators. The power is expressed in parts per million (p.p.m.) squared divided by 1000. The oscillation frequency peaks are clearly visible, around 75 microHz for the bottom panel, down to 10 microHz for the top panel. The nine-digit numbers given are the CoRoT indentifiers of the targets. Click on the image to enlarge it
The analysis of the CoRot data obtained during the first scientific campaign allows to identify more than 300 red giants candidates. These red giants show a large variety of oscillation spectra as seen in Fig. 2. A particularly interesting oscillation spectrum is shown in Fig. 3 where clearly equidistant peaks are seen. This regular pattern is similar to what is observed in the solar case and is associated to the presence of non-radial modes. The narrow peaks are the results of long-lived modes. We notice also that these modes have a lifetime longer than the solar ones.
In conclusion, thanks to the unprecedented high quality photometric data covering a long quasi uninterrupted period, CoRot reveals, unambiguously, the presence of non-radial modes with long lifetimes in red giants. These results show the great interest of these kind of stars for doing asteroseismology. Red giants are thus excellent laboratories to test our models of stellar interior in completely different conditions than for our Sun.
The team is composed of : Joris De Ridder (1), Caroline Barban (2), Frédéric Baudin (3), Fabien Carrier (1), Artie P. Hatzes (4), Saskia Hekker (5,1), Thomas Kallinger (6), Werner W. Weiss (6), Annie Baglin (2), Michel Auvergne (2), Réza Samadi (2), Pierre Barge (7), Magali Deleuil (7) (1) Instituut voor Sterrenkunde, K.U.Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium. (2) LESIA, UMR8109, Université Pierre et Marie Curie, Université Denis Diderot, Observatoire de Paris, 92195 Meudon Cedex, France. (3) Institut d’Astrophysique Spatiale, Campus d’Orsay, F-91405 Orsay, France. (4) Thüringer Landessternwarte, D-07778 Tautenburg, Germany. (5) Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium. (6) Institute for Astronomy, University of Vienna, Türkenschanzstrasse 17, A-1180 Vienna, Austria. (7) Laboratoire d’Astrophysique de Marseille, OAMP, Université Aix-Marseille & CNRS, 38 rue Frédéric Joliot Curie, 13388 Marseille cedex 13, France.