The mode selection problem in delta Scuti stars

Inspection of the periodograms of 9597 TESS delta Scuti stars indicates that there is little, if any, correlation between the frequencies and location of the stars in the H-R diagram. A quantitative measure of similarity is described which can be used to compare two frequency spectra. When applied to non-adiabatic pulsation models, the expected strong correlation is found between the predicted pulsation frequencies for models with similar temperatures and luminosities. The correlation is maintained when rotational splitting is included. However, no correlation is found between the frequencies of TESS delta Scuti stars with similar effective temperatures and luminosities. As suspected, the observed frequencies do not depend on the stellar parameters, contradicting the models. This result suggests that an unknown mode selection process is active, presenting a fundamental problem in our current understanding of pulsational driving or the structure of the outer layers of stars with radiative atmospheres.