Kepler observations of δ Scuti stars

We analyse observations of 1568 delta Scuti stars in the public archive of the Kepler satellite. We compare the location of these stars in the Hertzsprung-Russell (HR) diagram with that calculated by linear, non-adiabatic pulsation models. There is generally a good agreement and it appears that many of the hotter delta Scuti stars are pulsating in overtones as high as radial order n = 8. Models predict unstable modes of intermediate and high spherical harmonic, l, which are trapped in the envelope. The instability strip for these modes extends well beyond the blue edge for l <= 4 and should be visible in the Kepler data. However, stars with these predicted properties are not seen. Moreover, we find that the observed frequency range does not agree with the models. Another challenge is to understand why more than half of the stars in the delta Sct instability strip are not pulsating. The distribution of amplitudes argues strongly against the notion that the pulsation amplitudes are below the Kepler detection limit. The mode density of delta Scuti stars is surprisingly low and suggests that modes of a high degree are not common. We do not find any stars with mode densities as high as that found in the CoRoT observations of HD 50844. The periodograms of stars in the same part of the HR diagram vary widely. However, we have identified a group of delta Scuti stars characterized by a single dominant frequency in which a period-luminosity law is present. In many cases the dominant frequency is accompanied by a close frequency of lower amplitude. The problem of low frequencies in hot delta Scuti stars remains unresolved. We confirm that these frequencies have long lifetimes and are not likely to be of stochastic origin. By carefully examining the periodograms of delta Sct stars in the low-frequency regime, we were able to identify a group of delta Sct stars with low-frequency peaks closely resembling those attributed to pure gamma Dor stars. These stars lie in the region of the HR diagram where ground-based gamma Dor stars are located. None is found hotter than the blue edge of gamma Dor pulsation. We suspect that these are genuine gamma Dor/delta Sct hybrids.