Asteroseismology of 35 Kepler and TESS

Aims. The aim of this work is to determine the maximum ages that can be unambiguously established for δ Sct stars using seismic observables and by extension the oldest open clusters that can be dated using this type of star. Methods. I estimated the large frequency separation using various techniques applied to two samples of δ Sct located near the red edge of the instability strip. One sample consists of 18 targets observed by the Kepler mission, and the other comprises 17 targets observed by TESS. I employed a grid of stellar models representative of typical δ Sct parameters, incorporating mass, metallicity, and rotation as independent variables, and computed the first eight radial modes for each model. Using the observed spectroscopic temperature and the estimated large separation, I estimated the age of each star by fitting a weighted probability density function to the age distribution of the models that best match the seismic constraints. Results. To evaluate the performance of the fitting method, it was applied to a synthetic population of 20 δ Sct stars with varying metallicities and ages, generated by randomly selecting models. The analysis indicates that δ Sct stars older than 1 Gyr but that have not yet reached the terminal-age main sequence can in principle be reliably age dated. Nevertheless, when the method is applied to the observational sample, only three out of the 35 stars considered marginally exceed an estimated age of 1 Gyr. Conclusions. From these results, I can say that open clusters older than approximately 1 Gyr cannot be reliably dated using the asteroseismology of δ Sct stars with 1D models, at least not without a more complete treatment of convection and a non-linear treatment of rotation.