Beyond the Nyquist frequency

Asteroseismic catalog of undersampled Kepler late subgiants and early red giants

¹ ENS Paris-Saclay, Université Paris-Saclay, 91190 Gif-sur-Yvette, France
² Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, F-91191 Gif-sur-Yvette, France
³ Instituto de Astrofísica de Canarias (IAC), E-38205 La Laguna, Tenerife, Spain
⁴ Institute of Science and Technology Austria (ISTA), Am Campus 1, 3400 Klosterneuburg, city Austria
⁵ Universidad de La Laguna (ULL), Departamento de Astrofísica, E-38206 La Laguna, Tenerife, Spain
⁶ Department of Astronomy, The Ohio State University, Columbus OH 43210, USA
⁷ Institute of Space Sciences (ICE, CSIC), Carrer de Can Magrans S/N, E-08193 Cerdanyola del Vallès, Spain
⁸ Institut d’Estudis Espacials de Catalunya (IEEC), Carrer Esteve Terradas, 1, Edifici RDIT, Campus PMT-UPC, E-08860 Castelldefels, Spain
⁹ Department of Physics and Astronomy, California State University, Long Beach, Long Beach CA 90840, USA
¹⁰ Department of Astronomy, University of Florida, Gainesville FL 32611, USA
¹¹ Department of Physics, The Ohio State University, Columbus OH 43210, USA

^⋆ Corresponding author.

Received: 15 April 2025
Accepted: 24 July 2025

Abstract

Subgiants and early red giants are crucial for studying the first dredge-up, a key evolutionary phase in which 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 of identifying super-Nyquist oscillators and inferring their global seismic parameters, ν_max and large separation, Δν.

Applying PyA2Z to 2065 Kepler targets, we seismically characterize 285 super-Nyquist and 168 close-to-Nyquist stars with masses from 0.8 to 1.6 M_⊙. In combination with APOGEE spectroscopy, Gaia spectrophotometry, 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.