Peakbagging the K2 KEYSTONE sample with PBjam: characterising the individual mode frequencies in solar-like oscillators

TL;DR

This work leverages the KEYSTONE solar-like oscillator sample to perform automated, Bayesian peakbagging of individual mode frequencies using PBjam on K2 data. Through a two-stage process—mode identification and final peakbagging—the authors extract over 6000 mode frequencies, widths, and heights for 168 stars spanning the main sequence, sub-giants, and red-giant branch, yielding improved global asteroseismic parameters. They demonstrate significant gains in radius and mass precision (roughly 49% and 48% reductions in median uncertainties) and provide detailed mode catalogs that enable precise stellar modelling and cross-validation with Gaia and scaling relations. The results validate PBjam’s applicability to current datasets and highlight its potential for upcoming PLATO observations, while also identifying areas to refine prior sampling for future analyses.

Abstract

The pattern of individual mode frequencies in solar-like oscillators provides valuable insight into their properties and interior structures. The identification and characterisation of these modes requires high signal-to-noise and frequency resolution. The KEYSTONE project unlocks the asteroseismic potential of the K2 mission by providing individually reduced, high-quality time series data, global asteroseismic parameters, and spectroscopic analysis for 173 solar-like oscillators. In this work, we build on the KEYSTONE project and present the first analysis of the pattern of individual modes in the oscillation spectra for the K2 KEYSTONE stars. We perform a robust identification and characterisation of the modes through peakbagging methods in the open-source analysis tool PBjam. We present over 6000 mode frequencies, widths, and heights for 168 stars in the sample, covering the HR diagram from FGK dwarfs to sub-giants and the lower red giant branch, providing a significant increase in the number of individual mode frequency detections for main sequence and sub-giant oscillators. This study also presents sample-wide trends of oscillation patterns as a function of the fundamental stellar properties, and improves the precision of the global asteroseismic parameters. These measurements are part of the legacy of the K2 mission, and can be used to perform detailed modelling to improve the precision of fundamental properties of these stars. The results of this analysis provides evidence for the validity of using PBjam to identify and characterise the modes resulting from the observations of the future PLATO mission.

Figures (13)

• Figure 1: HR diagram of the 173 stars in the KEYSTONE sample, which are analyzed in this work. The evolutionary tracks (blue) for masses 1.0-1.5 M$_\odot$ are shown for [Fe/H]=0. The position of the Sun is shown for reference.
• Figure 2: Power density spectrum for EPIC 241011563, showing the increased level of power from the p-mode oscillation envelope. For clarity the spectrum has been smoothed with a Gaussian kernel with widths of $0.1\mu$Hz (light grey) and $0.5\mu$Hz (dark grey).
• Figure 3: Power spectrum of EPIC 212586030, a star with prominent overtones of instrumental variability caused by the roll of the spacecraft. The black arrow shows the location of $\nu_{\rm max}$, and the solid red lines show the location of the instrumental variability overtones, every $\sim47.2\mu$Hz.
• Figure 4: HR diagram of the 173 stars in the KEYSTONE sample. The colours denote the different fitting models used in PBjam, with main sequence (MS) in blue, sub-giant (SG) in orange, and red giant branch (RGB) in magenta. The black points denote the five stars that were removed from the sample (see Sec. \ref{['sec: Global Results']}). The 1.0-1.5 M$_\odot$ evolutionary tracks (grey) are shown for [Fe/H] = 0. The position of the Sun is shown for reference.
• Figure 5: Relation between phase shift, $\varepsilon_{\rm p}$, and effective temperature, $T_{\rm eff}$, for the stars in our sample.