Study of KIC 8561221 observed by Kepler: an early red giant showing depressed dipolar modes^⋆

¹ Laboratoire AIM, CEA/DSM – CNRS – Univ. Paris Diderot – IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette Cedex, France
e-mail: rgarcia@cea.fr
² Kavli Institute for Theoretical Physics, University of California , Santa Barbara CA 93106-4030, USA
³ Instituto de Astrofísica de Canarias, 38205, La Laguna, Tenerife, Spain
⁴ Universidad de La Laguna, Dpto de Astrofísica, 38206 Tenerife, Spain
⁵ Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW, 2006 Sydney, Australia
⁶ Stellar Astrophysics Centre, Dpt. of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus, Denmark
⁷ CNRS, Institut de Recherche en Astrophysique et Planétologie, 14 avenue Edouard Belin, 31400 Toulouse, France
⁸ Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
⁹ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
¹⁰ High Altitude Observatory, National Center for Atmospheric Research, PO Box 3000, Boulder CO 80307, USA
¹¹ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
¹² Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan
¹³ LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
¹⁴ Astronomy Department, Ohio State University, Columbus, Ohio 43210, USA
¹⁵ Institut d’Astrophysique et Géophysique de l’Université de Liège, Allée du 6 Août 17, 4000 Liège, Belgium
¹⁶ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 9 October 2013
Accepted: 27 November 2013

Abstract

Context. The continuous high-precision photometric observations provided by the CoRoT and Kepler space missions have allowed us to understand the structure and dynamics of red giants better using asteroseismic techniques. A small fraction of these stars show dipole modes with unexpectedly low amplitudes. The reduction in amplitude is more pronounced for stars with a higher frequency of maximum power, ν_max.

Aims. In this work we want to characterise KIC 8561221 in order to confirm that it is currently the least evolved star among this peculiar subset and to discuss several hypotheses that could help explain the reduction of the dipole mode amplitudes.

Methods. We used Kepler short- and long-cadence data combined with spectroscopic observations to infer the stellar structure and dynamics of KIC 8561221. We then discussed different scenarios that could contribute to reducing the dipole amplitudes, such as a fast-rotating interior or the effect of a magnetic field on the properties of the modes. We also performed a detailed study of the inertia and damping of the modes.

Results. We have been able to characterise 36 oscillations modes, in particular, a few dipole modes above ν_max that exhibit nearly normal amplitudes. The frequencies of all the measured modes were used to determine the overall properties and the internal structure of the star. We have inferred a surface rotation period of ~91 days and uncovered a variation in the surface magnetic activity during the last 4 years. The analysis of the convective background did not reveal any difference compared to “normal” red giants. As expected, the internal regions of the star probed by the ℓ = 2 and 3 modes spin 4 to 8 times faster than the surface.

Conclusions. With our grid of standard models we are able to properly fit the observed frequencies. Our model calculation of mode inertia and damping give no explanation for the depressed dipole modes. A fast-rotating core is also ruled out as a possible explanation. Finally, we do not have any observational evidence of a strong deep magnetic field inside the star.