Activity and Rotation of Kepler-17

Magnetic activity on stars manifests itself in the form of dark spots on the stellar surface, which cause modulations of a few percent in the light curve of the star as it rotates. When a planet eclipses its host star, it might cross in front of one of these spots, creating a "bump" in the transit light curve. By modeling these spot signatures, it is possible to determine the physical properties of the spots such as size, temperature, and location. In turn, monitoring of the spots' longitude provides estimates of the stellar rotation and differential rotation. This technique was applied to the star Kepler-17, a solar- type star orbited by a hot Jupiter. The model yields the following spot characteristics: average radius of 49 +/- 10 Mm, temperatures of 5100 +/- 300 K, and surface area coverage of 6 +/- 4%. The rotation period at the transit latitude, - 5., occulted by the planet was found to be 11.92 +/- 0.05 day, slightly smaller than the out-of-transit average period of 12.4 +/- 0.1 day Adopting a solar-like differential rotation, we estimated the differential rotation of Kepler-17 to be Delta Omega = 0.041 +/- 0.005 rd day(-1), which is close to the solar value of 0.050 rd day(-1), and a relative differential rotation of Delta Omega/Omega = 8.0 +/- 0.9%. Because Kepler-17 is much more active than our Sun, it appears that, for this star, larger rotation rate is more effective in the generation of magnetic fields than shear.