Low frequencies in Kepler δ Scuti stars

We use Kepler photometry to examine the properties of low-frequency oscillations in A stars. We find that multiple peaks in the range 0-5 d(-1) are found in nearly all delta Scuti stars, even those well outside the blue edge of the gamma Doradus instability strip. All delta Sct stars are essentially delta Sct/gamma Dor hybrids. The reason why this fact was not recognized from ground-based observations is due to the fact that the low frequencies reach maximum amplitude close to the blue edge of the gamma Dor instability strip. Stars which do not have high frequencies characteristic of delta Sct pulsations also do not have low frequencies. The amplitudes of the low frequencies correlate with the amplitudes of the delta Sct pulsations. The low frequencies have long lifetimes similar to those of the delta Sct modes. They are therefore not excited by a stochastic mechanism. We show that the low frequencies cannot be explained as non-linear combinations of delta Sct modes. By comparing the observed distribution of frequencies with those from delta Sct models, we show that low-frequency peaks cannot be explained as a result of rotational perturbation of high-frequency delta Sct modes. We find that the number of low-frequency peaks increases with rotation, but Rossby and Kelvin modes can be ruled out as possible candidates. The presence of low frequencies in A stars would be explained if the convective blocking mechanism continues to be active even in the hottest A stars.