Differential rotation of the active G5V star Kappa1 Ceti: Photometry from the MOST satellite

Abstract

About 30.5 days of nearly uninterrupted broadband photometry of the solar-type star Kappa1 Ceti, obtained with the MOST (Microvariability & Oscillations of STars) satellite, shows evidence for two large starspots with different rotation periods of 8.9 and approximately 9.3 days (DeltaOmega/Omega~4%). Ground based measurements in 2002 and 2003 of Ca II H & K emission reveal variations in chromospheric activity with a period of about 9.3 days. The data were obtained during the MOST commissioning phase. When the data are combined with historical observations, they indicate that the 9.3-day spot has been stable in its period for over 30 years. The photometry, with a sampling rate of approximately once per minute, was also used to search for acoustic (p-mode) oscillations in the star. We detect no clear evidence for p-modes in the Kappa1 Ceti photometry, with a noise level around 7-9 mu_mag at frequencies in the range 0.5-4 mHz (3-sigma detection limit of 21 - 27 mu_mag). There were no flares or planetary transits during the 30.5 days of MOST monitoring with light amplitudes greater than 2 mmag (durations greater than 200 minutes) and 3 mmag (2-200 min durations). While this rules out any close-in planets of >=0.5 Jupiter diameters with an orbital inclination close to 90 deg, the scatter in differential radial velocities permit a close giant planet in a more highly inclined orbit.