Beyond the Nyquist frequency: Asteroseismic catalog of undersampled Kepler late subgiants and early red giants

TL;DR

This paper introduces a new method to analyze undersampled Kepler data, enabling seismic characterization of late subgiants and early red giants near the Nyquist frequency, and derives stellar ages with implications for stellar evolution models.

Contribution

The authors developed a novel PyA2Z-based technique to identify super-Nyquist oscillators and estimate their seismic parameters, expanding the seismic study to previously inaccessible stars.

Findings

Seismic characterization of 453 stars including 285 super-Nyquist and 168 near-Nyquist.

Good agreement between predicted and observed stellar positions on the HR diagram.

Discrepancies in dredge-up timing and magnitude suggest model or calibration uncertainties.

Abstract

Subgiants and early red giants are crucial for studying the first dredge-up, a key evolutionary phase where the convective envelope deepens, mixing previously interior-processed material and bringing it to the surface. Yet, very few have been seismically characterized with Kepler because their oscillation frequencies are close to the 30 minute sampling frequency of the mission. We developed a new method as part of the new PyA2Z code to identify super-Nyquist oscillators and infer their global seismic parameters, $\nu_\mathrm{max}$ and large separation, $\Delta\nu$. Applying PyA2Z to 2 065 Kepler targets, we seismically characterize 285 super-Nyquist and 168 close-to-Nyquist stars with masses from 0.8 to 1.6 M$_\odot$. In combination with APOGEE spectroscopy, Gaia spectro-photometry, and stellar models, we derive stellar ages for the sample. There is good agreement between the predicted and actual positions of stars on the HR diagram (luminosity vs. effective temperature) as a function of mass and composition. While the timing of dredge-up is consistent with predictions, the magnitude and mass dependence show discrepancies with models, possibly due to uncertainties in model physics or calibration issues in observed abundance scales.