The Effects of Magnetic Activity and Tidal Forces on Solar-like Oscillations in Red Giant Binaries

Abstract

Of the 18 known red giants in eclipsing binaries (RG/EBs), four do not show solar-like oscillations. These four also have the most signs of magnetic activity and tidal forces. A handful of the rest show suppressed oscillation modes with moderate levels of magnetism and tides. This paper uses Kepler light curves, high-resolution spectra, and stellar evolution modeling to demonstrate this trend and discuss the physical mechanisms responsible. Comments welcome. [Abstract placeholder]

Introduction

\label{intro}
  • Why red giants in eclipsing binaries are useful

  • Why we care about understanding when solar-like oscillations can happen

  • Why we think tides and stellar activity affect solar-like oscillations and how we can use models constrained by observations to investigate

In the absence of external influences, all evolved giant stars with a convective outer layer should theoretically exhibit solar-like oscillations. However, that is clearly not the case. Approximately one fifth of the known RG/EBs do not show any solar-like oscillation activity at all (Gaulme et al., 2014). The fraction of single evolved stars without confirmed binary companions that lack oscillations is unknown. Gaulme et al. (2014) proposed that stronger tidal interactions from short-period binaries and increased magnetic activity on spotty giants are linked to absent or damped solar-like oscillations. Now that the oscillating and non-oscillating binaries alike have been well-characterized globally (Frandsen et al., 2013; Rawls et al., 2016; Gaulme et al., 2016), we can use the available observations to explore how magnetically active each system is, how stellar evolution likely proceeded, and what role tidal forces have played over time.

In this paper, we perform an in-depth study of 18 red giants in eclipsing binaries (hereafter RG/EBs) which exhibit a range of orbital periods and solar-like oscillation behavior. Section \ref{review} revisits the dynamic eclipsing binary models based on Kepler light curves and radial velocity curves as well as the stellar atmosphere models from high-resolution spectra used to derive physical parameters for these RG/EBs. In Section \ref{magnetic}, we quantify each system’s magnetic activity photometrically and spectroscopically. Section \ref{tides} presents 1D stellar evolution models for each system, which are subsequently used to quantify each system’s level of tidal forces acting over time. We discuss how magnetic activity, tides, and solar-like oscillations are linked in Section \ref{discuss}, and Section \ref{conclusion} summarizes our results.

Physical parameters of the RG/EBs

\label{review}

Binary modeling