Low frequencies in Kepler δ Scuti stars
Abstract
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 δ Scuti stars, even those well outside the blue edge of the γ Doradus instability strip. All δ Sct stars are essentially δ Sct/γ 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 γ Dor instability strip. Stars which do not have high frequencies characteristic of δ Sct pulsations also do not have low frequencies. The amplitudes of the low frequencies correlate with the amplitudes of the δ Sct pulsations. The low frequencies have long lifetimes similar to those of the δ 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 δ Sct modes. By comparing the observed distribution of frequencies with those from δ Sct models, we show that low-frequency peaks cannot be explained as a result of rotational perturbation of high-frequency δ 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.