Power density spectra morphologies of seismically unresolved red-giant asteroseismic binaries

¹ Heidelberg Institute for Theoretical Studies (HITS), Schloss-Wolfsbrunnenweg 35, D-69118 Heidelberg, Germany
² Heidelberg University, Centre for Astronomy, Landessternwarte, Königstuhl 12, D-69117 Heidelberg, Germany

^⋆ Corresponding author: jeongyun.choi@h-its.org

Received: 23 April 2025
Accepted: 31 May 2025

Abstract

Context. Asteroseismic binaries are two oscillating stars detected in a single light curve. These systems provide robust constraints on stellar models from the combination of dynamical and asteroseismical stellar parameters. Predictions suggested that approximately 200 asteroseismic binaries may exist among the long-cadence Kepler data, and the majority of them consist of two red-clump (core helium burning) stars. However, detecting these systems is challenging when the binary components exhibit oscillations at similar frequencies that are indistinguishable (i.e., unresolved asteroseismic binaries).

Aims. In this study, we predict the morphologies of power density spectra (PDSs) of seismically unresolved red-giant asteroseismic binaries to provide examples that can be used to identify the systems among observed stars.

Methods. We created 5000 artificial asteroseismic binary (AAB) systems by combining the KASOC light curves of red giants with oscillations at similar frequency ranges. To quantify the complexity of the oscillation patterns, we used the maximum signal-to-noise ratio of the background-normalized PDS and Shannon entropy. Additionally, we identified the radial and quadrupole mode pairs for the individual binary component and determined their impact on the PDS morphologies of AABs.

Results. Our results reveal that the majority of AABs (∼47%) consist of the two red-clump stars. The PDSs of AABs generally exhibit increased Shannon entropy and decreased oscillation power compared to individual components. We focused on the ∼8% of AABs whose stellar components have a similar brightness and classified them into four distinct morphologies: (i) single star-like PDSs, whereby oscillations from one component dominate, (ii) aligned, whereby the dominant oscillations in the stars that form the AAB appear at similar frequencies, (iii) partially aligned, whereby some oscillation modes of component stars are aligned while others are not, and (iv) PDSs containing complex structures with unclear mode patterns caused by the misalignment of the mode frequencies of both components.

Conclusions. We found that most AABs with detectable oscillations from both components show complex oscillation patterns. Therefore, unresolved asteroseismic binaries with a low oscillation power and complex oscillation patterns as characterized by high Shannon entropy offer a potential explanation to understand the observed stars with complex PDSs.