Asteroseismic Diagnostics for Evolved Stars with Kepler and TESS: Tracing Helium Ignition and the Transition from Subgiants to Red Giants

Asteroseismology of large samples of red giants has been done very successfully using νmax and Δν. However, other global oscillation properties have been comparatively neglected. In a recent Kepler study of∼ 16, 000 red giants, we used an automated collapsed- échelle analysis to measure Δν, ε, δν01, and δν02. The catalogue reveals red-clump and secondary-clump sequences in unprecedented detail, tracing the mass-dependent seismic imprint of helium ignition. Meanwhile ε and δν01 show systematic offsets from stellar-evolution models, pointing to limitations in near-surface and outer-envelope modelling and possible mode-dependent surface terms. We have extended this framework to a TESS sample of bright subgiants and early red giants, targeting an evolutionary regime that was difficult to sample at scale with Kepler long-cadence observations: these stars have νmax above the long-cadence Nyquist frequency and Kepler short-cadence target slots were scarce. For∼ 700 stars, we measure p-mode diagnostics and, where mixed modes are detected, combine them with ΔΠ1, q, and εp,1. These measurements allow us to trace the transition from a few avoided crossings to dense red-giant mixed-mode patterns, refining evolutionary-state and age constraints across the subgiant- to-red-giant boundary. By cross-matching these stars with spectroscopic surveys, we are able to link our seismic measurements to surface-abundance changes associated with first dredge-up and place these bright stars in age-abundance space for local Galactic archaeology.