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The red-giant oscillation universal pattern

1 November 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: Typical red-giant oscillation spectrum. Degrees 0 to 3 are indicated. Mixed modes surrounding l=1 modes are also identified. The synthetic template used for the mode identification is shown in red.

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: CoRoT red giant power spectra stacked into an image after sorting on the large separation. One line corresponds to one star. The first sorting (A) is based on the large separation before any correction. In B, the blurred aspect disappears once the correction has been performed and reveals a clear comb-structure common to all red giants. The vertical lines at 23.2 microHz are the signature of the low-Earth orbit Click on the image to enlarge it

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)

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