CoRoT detects non radial modes with long lifetimes in giant stars |
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CoRoT detects non radial modes with long lifetimes in giant stars
In about 5 billion years, our Sun will expand to become a red giant star. This evolutionary status is common to stars like the Sun after they have burned their combustible, hydrogen, in their core. Thus, these stars are particularly interesting objects to test our theories about late stages of stellar evolution. The analysis of the high-performance photometric data obtained by the CoRot space mission made by an international team, including researchers of Paris Observatory, reveals the richness of their oscillations spectra opening the prospect of applying asteroseismic techniques to probe the interior of red giants.
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.
The only way to probe directly the stellar interiors is to measure frequencies of these oscillations; this technique is called: asteroseismology. But the quality of the data obtained before CoRoT (from the ground and from space) was not good enough to fully use this technique to probe the interior of these kind of stars.
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.
The CoRot (COnvection, Rotation and planetary Transit) satellite is a space mission that is led by CNES (the french space agency) in association with ESA and several international partners: Belgium, Austria, Brazil, Germany and Spain. It was launched on 27 december 2006 from the Baikonour cosmodrome on a Soyuz Fregat II-1b launcher. CoRoT has two main scientific objectives: to study the interior and dynamics of the stars using seismology and to detect exoplanets. To fulfill these two scientific purposes, CoRot provides high-performance photometric data over long quasi-uninterrupted periods.
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.
Figure 3: Power density spectrum of the red giant candidate CoRoT-101034881 showing a frequency pattern with a regular spacing.
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.
Reference
Non-radial modes with long lifetimes in giant stars
Nature, 21 May 2009Contact
Caroline Barban (Observatoire de Paris, LESIA, et CNRS)
Annie Baglin (Observatoire de Paris, LESIA, et CNRS)
Michel Auvergne (Observatoire de Paris, LESIA, et CNRS)
Réza Samadi (Observatoire de Paris, LESIA, et CNRS)
