Hidden oscillations in plain sight: identification of seismically unresolved red-giant asteroseismic binary candidates

TL;DR

This study identifies six potential unresolved red-giant asteroseismic binary candidates in Kepler data, demonstrating how blended oscillations can bias stellar parameter estimates and emphasizing the need for careful analysis of such systems.

Contribution

It introduces a method to detect seismically unresolved binaries by comparing power density spectra across different light curve apertures, revealing biases in stellar parameters.

Findings

Six unresolved AB candidates identified with changing PDS morphologies.

Oscillations from two stars cause biases in seismic and stellar parameters.

Biases lead to up to 3x mass and 2x radius discrepancies.

Abstract

Light curves of oscillating stars provide valuable insights into the stellar interiors. When oscillations from a pair of stars are captured within a single photometric aperture, they can be considered as potential asteroseismic binaries (ABs). If the two stars oscillate at similar frequency ranges, the superpositioned oscillation patterns appear as if from a single star, leading to inaccurate asteroseismic parameters. We investigate seismically unresolved AB candidates consisting of two red-giant stars observed by Kepler. We directly compare the power density spectra (PDSs) of blended and separated oscillations from both stars, and examine the impact of oscillations from two stars on asteroseismic and stellar parameters. We selected APOKASC3 stars with at least one neighboring source within 20 arcsec and show oscillations in similar frequency ranges. We focus on the systems where the light curves from each star in AB candidates are available or can be extracted with a custom mask. We identified 6 seismically unresolved AB candidates whose PDS morphologies change noticeably across light curves extracted with different apertures. Oscillations from two stars in a PDS cause inaccurate mode identification and bias the seismic parameters. These biases propagate into stellar properties: masses and radii for the 6 AB candidates differ by up to about 3 and 2 times relative to the individual stars, respectively. For the AB candidate with the most complex PDS, core properties become unreliable, with the coupling factor often being overestimated. We checked that all 6 AB candidates are chance alignments. Our results indicate that the inconsistencies in asteroseismic and stellar parameters across different studies can be explained by potential seismically unresolved ABs. We highlight the importance of identifying and accurately accounting for such systems in asteroseismic analysis.