Activity of KOI 877 and KOI 896 observed by the Kepler Space Telescope

Abstract

Observational data obtained by the Kepler Space Telescope are used to investigate the photometric variability of the stars KOI 877 and KOI 896, both of which are multi-planet systems. Surface-temperature inhomogeneities are studied and their evolution traced. 64 810 individual brightness measurements made over 1459.5 days (almost four years) were selected for this analysis. Maps of the surface-temperature inhomogeneities of these stars have been constructed based on fitting to the stellar light curves in a two-temperature approximation. These maps were used to identify the locations of active regions. KOI 877 displayed three states of activity during the observation period. Two active regions were present almost continuously on the surface of KOI 896, with the distance between them in phase being 0.58 at the beginning of the observations and then monotonically decreasing to 0.36. Estimation of the differential-rotation parameters of the stars suggests that KOI 877 is rotating differentially, similar to the Sun, while this parameter is a factor of two higher for KOI 896. Analysis of the variability of the light-curve amplitudes has yielded an independent estimate of the differential-rotation parameters of the two stars. The main properties of KOI 877 and KOI 896 are compared to those of the previously studied star Kepler-32 (KOI 952), whose differential rotation is reanalyzed.