02/11/2016

# Meredith L. Rawls,11Department of Astronomy, New Mexico State University, P.O. Box 30001, MSC 4500, Las Cruces, NM 88003, USA; mrawls@nmsu.edu Patrick Gaulme,22Apache Point Observatory, 2001 Apache Point Road, P.O. Box 59, Sunspot, NM 88349, USA$${}^{,}$$11Department of Astronomy, New Mexico State University, P.O. Box 30001, MSC 4500, Las Cruces, NM 88003, USA Jean McKeever,11Department of Astronomy, New Mexico State University, P.O. Box 30001, MSC 4500, Las Cruces, NM 88003, USA Jason Jackiewicz,11Department of Astronomy, New Mexico State University, P.O. Box 30001, MSC 4500, Las Cruces, NM 88003, USA Jerome A. Orosz,33Department of Astronomy, San Diego State University, 5500 Campanile Drive, San Diego, CA 91945, USA Enrico Corsaro,44Laboratoire AIM, CEA/DSM – CNRS – Univ. Paris Diderot – IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette Cedex, France$${}^{,}$$55Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain$${}^{,}$$66Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain Paul G. Beck,44Laboratoire AIM, CEA/DSM – CNRS – Univ. Paris Diderot – IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette Cedex, France Benoît Mosser,77LESIA, Observatoire de Paris, PSL Research University, CNRS, Université Pierre et Marie Curie, Université Paris Diderot, 92195 Meudon, France David W. Latham,88Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA and Christian A. Latham88Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Abstract

We combine Kepler photometry with ground-based spectra to present a comprehensive dynamical model of the double red giant eclipsing binary KIC 9246715. While the two stars are very similar in mass ($$M_{1}=2.171\begin{subarray}{c}+0.006\\ -0.008\end{subarray}\ M_{\odot}$$, $$M_{2}=2.149\begin{subarray}{c}+0.006\\ -0.008\end{subarray}\ M_{\odot}$$) and radius ($$R_{1}=8.37\begin{subarray}{c}+0.03\\ -0.07\end{subarray}\ R_{\odot}$$, $$R_{2}=8.30\begin{subarray}{c}+0.04\\ -0.03\end{subarray}\ R_{\odot}$$), an asteroseismic analysis finds one main set of solar-like oscillations with unusually low-amplitude, wide modes. A second set of oscillations from the other star may exist, but this marginal detection is extremely faint. Because the two stars are nearly twins, KIC 9246715 is a difficult target for a precise test of the asteroseismic scaling relations, which yield $$M=2.17\pm 0.14\ M_{\odot}$$ and $$R=8.26\pm 0.18\ R_{\odot}$$. Both stars are consistent with the inferred asteroseismic properties, but we suspect the main oscillator is Star 2 because it is less active than Star 1. We find evidence for stellar activity and modest tidal forces acting over the 171-day eccentric orbit, which are likely responsible for the essential lack of solar-like oscillations in one star and weak oscillations in the other. Mixed modes indicate the main oscillating star is on the secondary red clump (a core-He-burning star), and stellar evolution modeling supports this with a coeval history for a pair of red clump stars. This system is a useful case study and paves the way for a detailed analysis of more red giants in eclipsing binaries, an important benchmark for asteroseismology.

Accepted for publication in The Astrophysical Journal, December 2015
Published February 2016, Rawls et al. 2016, ApJ, 818, 108
Blog post summary for the public: http://wp.me/p3H1S0-6X
AAS Nova Highlight: http://aasnova.org/2016/02/17/sizing-up-red-giant-twins

\keywords

stars: activity — binaries: eclipsing — stars: evolution — stars: fundamental parameters — stars: individual (KIC 9246715) — stars: oscillations

## Introduction

\label{intro}

Mass and radius are often-elusive stellar properties that are critical to understanding a star’s past, present, and future. Eclipsing binaries are the only astrophysical laboratories that allow for a direct measurement of these and other fundamental physical parameters. Recently, however, observing solar-like oscillations in stars with convective envelopes has opened a window to stellar interiors and provided a new way to measure global stellar properties. A pair of asteroseismic scaling relations use the Sun as a \revisebenchmark between these oscillations and a star’s effective temperature to yield mass and radius (Kjeldsen et al., 1995; Huber et al., 2010; Mosser et al., 2013).

While both the mass and radius scaling relations