Seismic probing of the first dredge-up event through the eccentric red-giant and red-giant spectroscopic binary KIC 9163796
How different are red-giant stars with a mass ratio of ~1.015?★
Instituto de Astrofísica de Canarias,
La Laguna, Tenerife, Spain
2 Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
3 IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
4 Université Paris Diderot, AIM, Sorbonne Paris Cité, CEA, CNRS, 91191 Gif-sur-Yvette, France
5 Institut für Astronomie der Universität Wien, Türkenschanzstr. 17, 1180 Wien, Austria
6 Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia
7 LUPM, UMR5299, Université de Montpellier, CNRS, 34095 Montpellier cedex 5, France
8 Instituut voor Sterrenkunde, KU Leuven, 3001 Leuven, Belgium
9 LESIA, Obs. de Paris, PSL Research Univ., CNRS, Univ. Pierre et Marie Curie, Univ. Paris Diderot, 92195 Meudon, France
10 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
11 Departamento de Física, Universidade Federal do Rio Grande do Norte, CEP: 59072-970 Natal, RN, Brazil
Accepted: 13 December 2017
Context. Binaries in double-lined spectroscopic systems (SB2) provide a homogeneous set of stars. Differences of parameters, such as age or initial conditions, which otherwise would have strong impact on the stellar evolution, can be neglected. The observed differences are determined by the difference in stellar mass between the two components. The mass ratio can be determined with much higher accuracy than the actual stellar mass.
Aim. In this work, we aim to study the eccentric binary system KIC 9163796, whose two components are very close in mass and both are low-luminosity red-giant stars.
Methods. We analysed four years of Kepler space photometry and we obtained high-resolution spectroscopy with the Hermes instrument. The orbital elements and the spectra of both components were determined using spectral disentangling methods. The effective temperatures, and metallicities were extracted from disentangled spectra of the two stars. Mass and radius of the primary were determined through asteroseismology. The surface rotation period of the primary is determined from the Kepler light curve. From representative theoretical models of the star, we derived the internal rotational gradient, while for a grid of models, the measured lithium abundance is compared with theoretical predictions.
Results. From seismology the primary of KIC 9163796 is a star of 1.39 ± 0.06 M⊙, while the spectroscopic mass ratio between both components can be determined with much higher precision by spectral disentangling to be 1.015 ± 0.005. With such mass and a difference in effective temperature of 600 K from spectroscopy, the secondary and primary are, respectively, in the early and advanced stage of the first dredge-up event on the red-giant branch. The period of the primary’s surface rotation resembles the orbital period within ten days. The radial rotational gradient between the surface and core in KIC 9163796 is found to be 6.9−1.0+2.0. This is a low value but not exceptional if compared to the sample of typical single field stars. The seismic average of the envelope’s rotation agrees with the surface rotation rate. The lithium’abundance is in agreement with quasi rigidly rotating models.
Conclusions. The agreement between the surface rotation with the seismic result indicates that the full convective envelope is rotating quasi-rigidly. The models of the lithium abundance are compatible with a rigid rotation in the radiative zone during the main sequence. Because of the many constraints offered by oscillating stars in binary systems, such objects are important test beds of stellar evolution.
Key words: stars: solar-type / stars: evolution / stars: rotation / stars: oscillations / stars: individual: KIC9163796, KIC4586817 / binaries: spectroscopic
© ESO 2018