Rapidly-oscillating {\it TESS} A--F main sequence stars

From sector 1--40 {\em TESS} observations, 20 new roAp stars, 97 ostensibly non-peculiar stars with roAp-like frequencies (the "roA" variables) and 617 $\delta$~Scuti stars with independent frequencies typical of roAp stars were found. There is no criterion that can distinguish roAp/roA stars from $\delta$~Sct stars. For expediency, an arbitrary low frequency of 60\,d$^{-1}$ was chosen as the boundary between the $\delta$~Sct and roAp/roA classes. Because an unknown mode selection process is clearly present in $\delta$~Sct stars, the "roAp/roA" stars may be considered as $\delta$~Sct stars in which high frequencies are preferentially selected. This interpretation is supported by the fact that the combined proportion of $\delta$~Sct and roAp stars among Ap stars is the same as among non-Ap stars. Contrary to models, observations show that low frequencies in Ap stars are not suppressed. One of the most puzzling aspects of roAp stars is the large fraction which have short mode lifetimes. The failure of current models to explain these results may be due to an incorrect treatment of the outer layers of these stars.