KIC 7955301: A hierarchical triple system with eclipse timing variations and an oscillating red giant

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: gaulme@mps.mpg.de
² Department of Astronomy, New Mexico State University, PO Box 30001, MSC 4500, Las Cruces, NM 88003-8001, USA
³ Baja Astronomical Observatory of University of Szeged, Szegedi út Kt. 766, 6500 Baja, Hungary
⁴ ELKH-SZTE Stellar Astrophysics Research Group, Szegedi út Kt. 766, 6500 Baja, Hungary
⁵ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
⁶ ELTE Gothard Astrophysical Observatory, Szent Imre h. u. 112, 9700 Szombathely, Hungary
⁷ MTA-ELTE Exoplanet Research Group, Szent Imre h. u. 112, 9700 Szombathely, Hungary
⁸ Université Paris-Saclay, Institut d’Astrophysique Spatiale, UMR 8617, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
⁹ Department of Physics, Faculty of Science, University of Zagreb, Bijenička cesta 32, 10000 Zagreb, Croatia
¹⁰ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹¹ Department of Astronomy, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA
¹² Dipartimento di Fisica e Astronomia Augusto Righi, Universitá degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
¹³ INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
¹⁴ School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK
¹⁵ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
¹⁶ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 92195 Meudon, France
¹⁷ Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
¹⁸ Laboratoire AIM, DRF/IRFU/SAp, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
¹⁹ Department of Physics, Montana State University, PO Box 173840, Bozeman, MT 59717-3840, USA
²⁰ Monterey Institute for Research in Astronomy, 200 8th Street, Marina, CA 93933, USA

Received: 28 June 2022
Accepted: 6 October 2022

Abstract

KIC 7955301 is a hierarchical triple system with clear eclipse timing and depth variations that was discovered by the Kepler satellite during its original mission. It is composed of a non-eclipsing primary star at the bottom of the red giant branch (RGB) on a 209-day orbit with a K/G-type main-sequence (MS) inner eclipsing binary (EB), orbiting in 15.3 days. This system was noted for the large amplitude of its eclipse timing variations (ETVs, over 4 h), and the detection of clear solar-like oscillations of the red-giant (RG) component, including p-modes of degree up to l = 3 and mixed l = 1 modes. The system is a single-lined spectroscopic triple, meaning that only spectral lines from the RG are detected. We performed a dynamical model by combining the 4-year-long Kepler photometric data, ETVs, and radial-velocity data obtained with the high-resolution spectrometers ARCES, of the 3.5 m ARC telescope at Apache Point observatory, and SOPHIE, of the 1.93 m telescope at Haute-Provence Observatory. The “dynamical” mass of the RG component was determined with a 2% precision at 1.30_−0.02^+0.03 M_⊙. We performed asteroseismic modeling based on the global seismic parameters and on the individual frequencies. Both methods provide an estimate of the mass of the RG that matches the dynamical mass within the uncertainties. Asteroseismology also revealed the rotation rate of the core (≈15 days), the envelope (∼150 days), and the inclination (∼75°) of the RG. Three different approaches led to an estimation of the age between 3.3 and 5.8 Gyr, which highlights the difficulty of determining stellar ages despite the exceptional wealth of information available for this system. On short timescales, the inner binary exhibits eclipses with varying depths during a 7.3-year long interval, and no eclipses during the consecutive 11.9 years. This is why Kepler could detect its eclipses but TESS cannot, and the future ESA PLATO mission should detect these. In the long term, the system appears to be stable and owes its evolution to the evolution of its individual components. This triple system could end its current smooth evolution by merging by the end of the RGB of the primary star because the periastron distance is ≈142 R_⊙, which is close to the expected radius of the RG at the tip of the RGB.