Asteroseismology of 1523 misclassified red giants using $\textit{Kepler}$ data

TL;DR

This study reclassified 1523 Kepler red giants, discovering 626 new oscillators, and improved methods for analyzing oscillations near the Nyquist frequency, enhancing understanding of low-luminosity red giants.

Contribution

It presents a novel classification scheme and analysis of misclassified red giants, increasing the known sample and refining oscillation detection methods near the Nyquist frequency.

Findings

26% increase in known oscillating low-luminosity red giants

Identification of 47 super-Nyquist oscillators

Misclassification mainly due to uncertainties in surface gravity

Abstract

We analysed solar-like oscillations in 1523 $\textit{Kepler}$ red giants which have previously been misclassified as subgiants, with predicted $\nu_{\rm max}$ values (based on the Kepler Input Catalogue) between 280$\mu$Hz to 700$\mu$Hz. We report the discovery of 626 new oscillating red giants in our sample, in addition to 897 oscillators that were previously characterized by Hekker et al. (2011) from one quarter of $\textit{Kepler}$ data. Our sample increases the known number of oscillating low-luminosity red giants by $26\%$ (up to $\sim$ 1900 stars). About three quarters of our sample are classified as ascending red-giant-branch stars, while the remainder are red-clump stars. A novel scheme was applied to determine $\Delta \nu$ for 108 stars with $\nu_{\rm max}$ close to the Nyquist frequency (240$\mu$Hz < $\nu_{\rm max}$ < 320$\mu$Hz). Additionally, we identified 47 stars oscillating in the super-Nyquist frequency regime, up to 387$\mu$Hz, using long-cadence light curves. We show that the misclassifications are most likely due to large uncertainties in KIC surface gravities, and do not result from the absence of broadband colors or from different physical properties such as reddening, spatial distribution, mass or metallicity. The sample will be valuable to study oscillations in low-luminosity red giants and to characterize planet candidates around those stars.