Beyond the Nyquist frequency

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, νmax 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.