Pulsating red giant stars in eccentric binary systems discovered from Kepler space-based photometry. A sample study and the analysis of KIC 5006817

Beck, P. G. and Hambleton, K. and Vos, J. and Kallinger, T. and Bloemen, S. and Tkachenko, A. and García, R. A. and Østensen, R. H. and Aerts, C. and Kurtz, D. W. and De Ridder, J. and Hekker, S. and Pavlovski, K. and Mathur, S. and De Smedt, K. and Derekas, Aliz and Corsaro, E. and Mosser, B. and Van, Winckel H. and Huber, D. and Degroote, P. and Davies, G. R. and Prsa, A. and Debosscher, J. and Elsworth, Y. and Németh, Péter and Siess, L. and Schmid, V. S. and Pápics, P. I. and de Vries, B. L. (2014) Pulsating red giant stars in eccentric binary systems discovered from Kepler space-based photometry. A sample study and the analysis of KIC 5006817. ASTRONOMY & ASTROPHYSICS, 564. ISSN 0004-6361

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Abstract

Context. The unparalleled photometric data obtained by NASA’s Kepler space telescope led to an 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, exhibiting ellipsoidal modulations, have been detected with Kepler.Aims. We aim to study the properties of eccentric binary systems containing a red giant star and derive the parameters of the primary giant component.Methods. We apply asteroseismic techniques to determine masses and radii of the primary component of each system. For a selected target, light and radial velocity curve modelling techniques are applied to extract the parameters of the system and its primary com- ponent. Stellar evolution and its effects on the evolution of the binary system are 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 program with the HERMES spectrograph reveal an eccentricity range of e = 0.2 to 0.76. As a case study we present a de- tailed analysis of KIC 5006817, whose rich oscillation spectrum allows for a 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, from our binary modelling we find 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 .

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