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Non-radial oscillation modes with long lifetimes in giant stars

Abstract

Towards the end of their lives, stars like the Sun greatly expand to become red giant stars. Such evolved stars could provide stringent tests of stellar theory, as many uncertainties of the internal stellar structure accumulate with age. Important examples are convective overshooting and rotational mixing during the central hydrogen-burning phase, which determine the mass of the helium core, but which are not well understood1. In principle, analysis of radial and non-radial stellar oscillations can be used to constrain the mass of the helium core. Although all giants are expected to oscillate2, it has hitherto been unclear whether non-radial modes are observable at all in red giants, or whether the oscillation modes have a short or a long mode lifetime3,4,5,6,7, which determines the observational precision of the frequencies. Here we report the presence of radial and non-radial oscillations in more than 300 giant stars. For at least some of the giants, the mode lifetimes are of the order of a month. We observe giant stars with equally spaced frequency peaks in the Fourier spectrum of the time series, as well as giants for which the spectrum seems to be more complex. No satisfactory theoretical explanation currently exists for our observations.

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Figure 1: A stack of power spectra of nine red giant pulsators.
Figure 2: Power density spectrum of the red giant candidate CoRoT-101034881 showing a frequency pattern with a regular spacing.
Figure 3: Echelle diagram of the modes shown in Fig. 2, showing ‘ridges’ related to radial and non-radial modes.
Figure 4: Power density spectrum of the red giant CoRoT-101600807 showing broad profiles.

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Acknowledgements

J.D.R., F.C. and S.H. acknowledge support from the Fund for Scientific Research, Flanders, from the research council of K. U. Leuven, and from the Belgian Federal Science Policy. A.P.H. acknowledges the support of the Deutsches Zentrum für Luft- und Raumfahrt. T.K. and W.W.W. acknowledge support by the Austrian Research Promotion Agency (FFG-ARL). J.D.R. thanks A. Miglio, M.-A. Dupret and C. Aerts for discussions.

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Correspondence to Joris De Ridder.

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This file contains Supplementary Figures 1-4 with Legends, Supplementary Methods, Supplementary Table 1,a Supplementary Discussion, Supplementary Notes and Supplementary References. (PDF 569 kb)

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De Ridder, J., Barban, C., Baudin, F. et al. Non-radial oscillation modes with long lifetimes in giant stars. Nature 459, 398–400 (2009). https://doi.org/10.1038/nature08022

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