Analysis of Kepler B stars: rotational modulation and Maia variables

Abstract

We examine 4-yr almost continuous Kepler photometry of 115 B stars. We find that the light curves of 39 percent of these stars are simply described by a low-frequency sinusoid and its harmonic, usually with variable amplitudes, which we interpret as rotational modulation. A large fraction (28 percent) of B stars might be classified as ellipsoidal variables, but a statistical argument suggests that these are probably rotational variables as well. About 8 percent of the rotational variables have a peculiar periodogram feature which is common among A stars. The physical cause of this is very likely related to rotation. The presence of so many rotating variables indicates the presence of star spots. This suggests that magnetic fields are indeed generated in radiative stellar envelopes. We find five beta Cep variables, all of which have low frequencies with relatively large amplitudes. The presence of these frequencies is a puzzle. About half the stars with high frequencies are cooler than the red edge of the beta Cep instability strip. These stars do not fit into the general definition of beta Cep or SPB variables. We have therefore assumed they are further examples of the anomalous pulsating stars which in the past have been called "Maia" variables. We also examined 300 B stars observed in the K2 Campaign 0 field. We find 11 beta Cep/Maia candidates and many SPB variables. For the stars where the effective temperature can be measured, we find at least two further examples of Maia variables.