Seismic constraints on the radial dependence of the internal rotation profiles of six Kepler subgiants and young red giants^⋆

¹ Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse, France
e-mail: sebastien.deheuvels@irap.omp.eu
² CNRS, IRAP, 14 avenue Edouard Belin, 31400 Toulouse, France
³ High Altitude Observatory, National Center for Atmospheric Research, PO Box 3000, Boulder CO 80307, USA
⁴ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁵ LESIA, UMR8109, Observatoire de Paris, Université Pierre et Marie Curie, Université Denis Diderot, CNRS, 5 place Jules Janssen, 92195 Meudon Cedex, France
⁶ Institut d’Astrophysique Spatiale, UMR8617, Université Paris XI, Bâtiment 121, 91405 Orsay Cedex, France
⁷ Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, NSW 2006 Sydney, Australia
⁸ Aarhus Katedralskole, Skolegyde 1, 8000 Aarhus C, Denmark
⁹ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
¹⁰ Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, The Australian National University, ACT 2611 Mont Stromlo, Australia
¹¹ Laboratoire AIM Paris-Saclay, CEA/DSM-CNRS-Université Paris Diderot, IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette Cedex, France
¹² Royal Observatory of Belgium, Ringlaan 3, 1180 Ukkel, Belgium
¹³ Department of Astronomy, Beijing Normal University, 100875 Beijing, PR China
¹⁴ Institut d’Astrophysique et de Géophysique de l’Université de Liège, allée du 6 Août 17, 4000 Liège, Belgium
¹⁵ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
¹⁶ Universidad de La Laguna, Dpto de Astrofísica, 38206 La Laguna, Tenerife, Spain
¹⁷ Institut für Astrophysik, Georg-August-Universität Göttingen, 37077 Göttingen, Germany
¹⁸ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
¹⁹ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Suisse
²⁰ Max-Planck-Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany
²¹ Instytut Astronomiczny Uniwersytetu Wrocławskiego, ul. Kopernika 11, 51-622 Wrocław, Poland
²² Department of Astronomy, the Ohio State University, Columbus OH 43210, USA

Received: 2 October 2013
Accepted: 24 January 2014

Abstract

Context. We still do not understand which physical mechanisms are responsible for the transport of angular momentum inside stars. The recent detection of mixed modes that contain the clear signature of rotation in the spectra of Kepler subgiants and red giants gives us the opportunity to make progress on this question.

Aims. Our aim is to probe the radial dependence of the rotation profiles for a sample of Kepler targets. For this purpose, subgiants and early red giants are particularly interesting targets because their rotational splittings are more sensitive to the rotation outside the deeper core than is the case for their more evolved counterparts.

Methods. We first extracted the rotational splittings and frequencies of the modes for six young Kepler red giants. We then performed a seismic modeling of these stars using the evolutionary codes Cesam2k and astec. By using the observed splittings and the rotational kernels of the optimal models, we inverted the internal rotation profiles of the six stars.

Results. We obtain estimates of the core rotation rates for these stars, and upper limits to the rotation in their convective envelope. We show that the rotation contrast between the core and the envelope increases during the subgiant branch. Our results also suggest that the core of subgiants spins up with time, while their envelope spins down. For two of the stars, we show that a discontinuous rotation profile with a deep discontinuity reproduces the observed splittings significantly better than a smooth rotation profile. Interestingly, the depths that are found to be most probable for the discontinuities roughly coincide with the location of the H-burning shell, which separates the layers that contract from those that expand.

Conclusions. We characterized the differential rotation pattern of six young giants with a range of metallicities, and with both radiative and convective cores on the main sequence. This will bring observational constraints to the scenarios of angular momentum transport in stars. Moreover, if the existence of sharp gradients in the rotation profiles of young red giants is confirmed, it is expected to help in distinguishing between the physical processes that could transport angular momentum in the subgiant and red giant branches.