Illustration par défaut

The red-giant oscillation universal pattern

1er novembre 2010

Thanks to CoRoT data (note 2), researchers at the Observatoire de Paris and an international team have demonstrated that red-giants, the fate of our Sun in 5 billions years, present homologous interior structures. They strongly differ from dwarf stars that show much different and varied interiors.

Figure 1 : Spectre d’oscillation typique d’une géante rouge, avec identification des degrés radiaux d’oscillations (de 0 à 3) et des modes mixtes (m) au voisinage des modes de degré 1.

The careful analysis of the large separation has first shown that the red giants showing solar-like oscillations (about a thousand) present in fact very similar solar spectra. The differences are in fact only fortuitous ; they result from the stochastic excitation of the modes. A dedicated tool has been set up for correcting these differences. We then obtain a much less noisy information. With the correction, all red-giant show very similar oscillation patterns (Fig. 2). This shows clearly the structure homology of giants, contrary to dwarf stars which still burn hydrogen in their core.

Figure 2 : Spectres d’oscillation des géantes observées par CoRoT, classés par grande séparation. Le classement A ne tient pas compte de l’identification du motif universel : l’aspect de la figure est brouillé ; en B, après correction et classement selon cette correction, le motif universel apparaît, et c’est cet accord qui valide la méthode et ses conséquences. Cliquer sur l’image pour l’agrandir

This result has important consequences. First, the identification of the universal pattern (really universal, since all red giants showing solar-like oscillation present the same oscillation pattern), the mode identification is obvious. The identification of the mode degrees allows us to benefit from the most precise asteroseismic information. Furthermore, we have then access to the tiny frequency differences that characterize each individual oscillation spectrum, and we can derive from this signature the stellar interior structure. Note 1 : The large separation frequency varies as the square root of the stellar density. Red giants being typically 1000 times less dense than the Sun, their large separation is about 30 times smaller. Values measured with CoRoT are in the range [0.6, 10 microHz], depending on the stellar radius. Giant with a large radius are less dense and present lower oscillation frequency. Note 2 : The CoRoT space mission (for Convection, Rotation and planetary Transits) was launched on 2006 December 27. It was developed and is operated by the (CNES), with participation of the Science Programs of ESA, ESAs RSSD, Austria, Belgium, Brazil, Germany and Spain. CoRoT has a 27-cm telescope, with a 4-CCD detector.

Reference The universal red-giant oscillation pattern ; an automated determination with CoRoT data B. Mosser, K. Belkacem, M.J. Goupil, E. Michel, Y. Elsworth, C. Barban, T. Kallinger, S. Hekker, J. DeRidder, R. Samadi, F. Baudin, F.J.G. Pinheiro, M. Auvergne, A. Baglin, C. Catala, 2010, Accepted in A&A Letters Contact Benoît Mosser (Observatoire de Paris, LESIA et CNRS)

Actualités