Probing the core structure and evolution of red giants using gravity-dominated mixed modes observed with Kepler^⋆

¹ LESIA – Observatoire de Paris, CNRS, Université Pierre et Marie Curie, Université Denis Diderot, 92195 Meudon Cedex, France
e-mail: benoit.mosser@obspm.fr
² Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
³ Georg-August-Universität, Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
⁵ Instituut voor Sterrenkunde, K. U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁶ Laboratoire AIM, CEA/DSM CNRS – Université Paris Diderot IRFU/SAp, 91191 Gif-sur-Yvette Cedex, France
⁷ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁸ Institute for Astronomy (IfA), University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁹ SETI Institute/NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁰ Bay Area Environmental Research Inst./NASA Ames Research Center, Moffett Field, CA 94035, USA

Received: 24 November 2011
Accepted: 5 February 2012

Abstract

Context. There are now more than 22 months of long-cadence data available for thousands of red giants observed with the Kepler space mission. Consequently, we are able to clearly resolve fine details in their oscillation spectra and see many components of the mixed modes that probe the stellar core.

Aims. We report for the first time a parametric fit to the pattern of the ℓ = 1 mixed modes in red giants, which is a powerful tool to identify gravity-dominated mixed modes. With these modes, which share the characteristics of pressure and gravity modes, we are able to probe directly the helium core and the surrounding shell where hydrogen is burning.

Methods. We propose two ways for describing the so-called mode bumping that affects the frequencies of the mixed modes. Firstly, a phenomenological approach is used to describe the main features of the mode bumping. Alternatively, a quasi-asymptotic mixed-mode relation provides a powerful link between seismic observations and the stellar interior structure. We used period échelle diagrams to emphasize the detection of the gravity-dominated mixed modes.

Results. The asymptotic relation for mixed modes is confirmed. It allows us to measure the gravity-mode period spacings in more than two hundred red giant stars. The identification of the gravity-dominated mixed modes allows us to complete the identification of all major peaks in a red giant oscillation spectrum, with significant consequences for the true identification of ℓ = 3 modes, of ℓ = 2 mixed modes, for the mode widths and amplitudes, and for the ℓ = 1 rotational splittings.

Conclusions. The accurate measurement of the gravity-mode period spacing provides an effective probe of the inner, g-mode cavity. The derived value of the coupling coefficient between the cavities is different for red giant branch and clump stars. This provides a probe of the hydrogen-shell burning region that surrounds the helium core. Core contraction as red giants ascend the red giant branch can be explored using the variation of the gravity-mode spacing as a function of the mean large separation.