Rotational light variations in Kepler observations of A-type stars

Abstract

We analyse the light curves of over 9000 A–F stars in the first public release of Kepler data and examine the noise properties in constant or nearly constant stars. For the A stars, we find a correlation between the excess power in certain frequency regions and the effective temperature which may be due to granulation. The majority of A–F stars vary with low frequencies (<5 d−1) and low amplitudes (40–150 ppm). The low-frequency variation extends to the hottest A-type stars where the typical amplitude is about 40 ppm. We find that about 8 per cent of A8–A0 stars have light curves resembling those usually attributed to starspots in cool stars, including a few exhibiting travelling waves usually interpreted as differentially rotating starspots. A further 20 per cent of A stars have dominant low frequencies which are visible in the periodogram. If we assume that the dominant low frequency in A–F stars is the rotation frequency, we can calculate the distribution of equatorial rotational velocities given the stellar radii. The resulting distribution matches the distribution of equatorial rotational velocities in field stars of the same spectral type and luminosity class. However, the A8–A0 stars have an excess of slow rotators which can be explained as contamination from horizontal-branch stars. We conclude that the light variations in A-type stars may possibly be due to starspots or other corotating structures and that A-star atmospheres may not be quiescent as previously supposed. We also analyse low frequencies in Kepler A-type δ Sct stars which are too hot to be due to γ Dor pulsations. These do not appear to be due to simple combinations of high-frequency δ Sct modes. Unlike normal A-type stars, the dominant low frequency is close to twice the rotational frequency. In a significant proportion of δ Sct stars there is, in fact, a frequency of smaller amplitude at exactly half the dominant low frequency. There is clearly a quadrupole surface brightness distribution in a significant fraction of these stars, but the amplitudes seem to be too low to be explained as a proximity effect in a binary.