Pulsating red giant stars in eccentric binary systems discovered from Kepler space-based photometry

A sample study and the analysis of KIC 5006817

¹ Instituut voor Sterrenkunde, KU Leuven, 3001 Leuven, Belgium
e-mail: paul.beck@ster.kuleuven.be
² Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
³ Institut für Astronomie der Universität Wien, Türkenschanzstr. 17, 1180 Wien, Austria
⁴ Laboratoire AIM, CEA/DSM-CNRS – Université Denis Diderot-IRFU/SAp, 91191 Gif-sur-Yvette Cedex, France
⁵ Department of Astrophysics, IMAPP, University of Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁶ Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁷ Department of Physics, Faculty of Science, University of Zagreb, 10000 Zagreb, Croatia
⁸ Space Science Institute, 4750 Walnut street Suite #205, Boulder CO 80301, USA
⁹ Konkoly Observ., Research Centre f. Astronomy and Earth Sciences, Hungarian Academy of Sciences, 1121 Budapest, Hungary
¹⁰ LESIA, CNRS, Université Pierre et Marie Curie, Université Denis Diderot, Observatoire de Paris, 92195 Meudon Cedex, France
¹¹ NASA Ames Research Center, Moffett Field CA 94035, USA
¹² School of Physics and Astronomy, University of Birmingham, Edgebaston, Birmingham B13 2TT, UK
¹³ Department of Astronomy and Astrophysics, Villanova University, 800 East Lancaster avenue, Villanova PA 19085, USA
¹⁴ Institut d’Astronomie et d’Astrophysique, Univ. Libre de Bruxelles, CP226, boulevard du Triomphe, 1050 Brussels, Belgium
¹⁵ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium

Received: 12 August 2013
Accepted: 16 December 2013

Abstract

Context. The unparalleled photometric data obtained by NASA’s Kepler Space Telescope has led to improved understanding of red giant stars and binary stars. Seismology allows us to constrain the properties of red giants. In addition to eclipsing binaries, eccentric non-eclipsing binaries that exhibit ellipsoidal modulations have been detected with Kepler.

Aims. We aim to study the properties of eccentric binary systems containing a red giant star and to derive the parameters of the primary giant component.

Methods. We applied asteroseismic techniques to determine the masses and radii of the primary component of each system. For a selected target, light and radial velocity curve modelling techniques were applied to extract the parameters of the system and its primary component. Stellar evolution and its effects on the evolution of the binary system were studied from theoretical models.

Results. The paper presents the asteroseismic analysis of 18 pulsating red giants in eccentric binary systems, for which masses and radii were constrained. The orbital periods of these systems range from 20 to 440 days. The results of our ongoing radial velocity monitoring programme with the Hermes spectrograph reveal an eccentricity range of e = 0.2 to 0.76. As a case study we present a detailed analysis of KIC 5006817, whose rich oscillation spectrum allows for detailed seismic analysis. From seismology we constrain the rotational period of the envelope to be at least 165 d, which is roughly twice the orbital period. The stellar core rotates 13 times faster than the surface. From the spectrum and radial velocities we expect that the Doppler beaming signal should have a maximum amplitude of 300 ppm in the light curve. Fixing the mass and radius to the asteroseismically determined values, we find from our binary modelling a value of the gravity darkening exponent that is significantly larger than expected. Through binary modelling, we determine the mass of the secondary component to be 0.29 ± 0.03 M_⊙.

Conclusions. For KIC 5006817 we exclude pseudo-synchronous rotation of the red giant with the orbit. The comparison of the results from seismology and modelling of the light curve shows a possible alignment of the rotational and orbital axis at the 2σ level. Red giant eccentric systems could be progenitors of cataclysmic variables and hot subdwarf B stars.