KIC 2568888: To be or not to be a binary

TL;DR

This study investigates whether two red giants observed by Kepler form a true binary system using asteroseismology, photometry, and Gaia data, revealing complexities that challenge the binary interpretation.

Contribution

It combines asteroseismic analysis with photometric and Gaia data to assess the binary nature of two red giants, highlighting the rarity and complexity of such systems.

Findings

Detected oscillation modes for two red giants in a single spectrum

Found similar ages and mass ratios close to 1 for the stars

Distances from Gaia and photometry are inconsistent, challenging the binary hypothesis

Abstract

In cases where both components of a binary system show oscillations, asteroseismology has been proposed as a method to identify the system. For KIC 2568888, observed with $Kepler$, we detect oscillation modes for two red giants in a single power density spectrum. Through an asteroseismic study we investigate if the stars have similar properties, which could be an indication that they are physically bound into a binary system. While one star lies on the red giant branch (RGB), the other, more evolved star, is either a RGB or asymptotic-giant-branch star. We found similar ages for the red giants and a mass ratio close to 1. Based on these asteroseismic results we propose KIC 2568888 as a rare candidate binary system ($\sim 0.1\%$ chance). However, when combining the asteroseismic data with ground-based $BVI$ photometry we estimated different distances for the stars, which we cross-checked with $Gaia$ DR2. From $Gaia$ we obtained for one object a distance between and broadly consistent with the distances from $BVI$ photometry. For the other object we have a negative parallax with a not yet reliable $Gaia$ distance solution. The derived distances challenge a binary interpretation and may either point to a triple system, which could explain the visible magnitudes, or, to a rare chance alignment ($\sim 0.05\%$ chance based on stellar magnitudes). This probability would even be smaller, if calculated for close pairs of stars with a mass ratio close to unity in addition to similar magnitudes, which may indeed indicate that a binary scenario is more favourable.