Spin down of the core rotation in red giants^⋆

¹ LESIA, CNRS, Université Pierre et Marie Curie, Université Denis Diderot, Observatoire de Paris, 92195 Meudon Cedex, France
e-mail: benoit.mosser@obspm.fr
² Georg-August-Universität Göttingen, Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
³ Instituut voor Sterrenkunde, K. U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁴ Department of Astronomy, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA
⁵ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
⁶ Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁷ Institut d’Astrophysique et de Géophysique de l’Université de Liège, Allée du 6 Août 17, 4000 Liège, Belgium
⁸ Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA
⁹ Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
¹⁰ Laboratoire AIM, CEA/DSM CNRS – Université Denis Diderot IRFU/SAp, 91191 Gif-sur-Yvette Cedex, France
¹¹ Orbital Sciences Corporation/NASA Ames Research Center, Moffett Field, CA 94035, USA
¹² SETI Institute/NASA Ames Research Center, Moffett Field, CA 94035, USA
¹³ High Altitude Observatory, NCAR, PO Box 3000, Boulder, CO 80307, USA

Received: 26 July 2012
Accepted: 13 September 2012

Abstract

Context. The space mission Kepler provides us with long and uninterrupted photometric time series of red giants. We are now able to probe the rotational behaviour in their deep interiors using the observations of mixed modes.

Aims. We aim to measure the rotational splittings in red giants and to derive scaling relations for rotation related to seismic and fundamental stellar parameters.

Methods. We have developed a dedicated method for automated measurements of the rotational splittings in a large number of red giants. Ensemble asteroseismology, namely the examination of a large number of red giants at different stages of their evolution, allows us to derive global information on stellar evolution.

Results. We have measured rotational splittings in a sample of about 300 red giants. We have also shown that these splittings are dominated by the core rotation. Under the assumption that a linear analysis can provide the rotational splitting, we observe a small increase of the core rotation of stars ascending the red giant branch. Alternatively, an important slow down is observed for red-clump stars compared to the red giant branch. We also show that, at fixed stellar radius, the specific angular momentum increases with increasing stellar mass.

Conclusions. Ensemble asteroseismology indicates what has been indirectly suspected for a while: our interpretation of the observed rotational splittings leads to the conclusion that the mean core rotation significantly slows down during the red giant phase. The slow-down occurs in the last stages of the red giant branch. This spinning down explains, for instance, the long rotation periods measured in white dwarfs.