Differential Rotation of the Active G5 V Star κ1 Ceti: Photometry from the MOST Satellite

About 30.5 days of nearly uninterrupted broadband photometry of the solar‐type star κ1 Ceti, obtained with the MOST (Microvariability and Oscillations of Stars) satellite, shows evidence for two large starspots with different rotation periods of 8.9 and ∼9.3 days (ΔΩ/Ω≃4%). Ground‐based measurements of Ca ii H and K emission in 2002 and 2003 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 yr. 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 κ1 Ceti photometry, with a noise level around 7–9 μmag at frequencies in the range of 0.5–4 mHz (3 σ detection limit of 21–27 μmag). There were no flares or planetary transits during 30.5 days of MOST monitoring with light amplitudes greater than 2 mmag (durations >200 minutes) and 3 mmag (2–200 minute durations). While this rules out any close‐in planets with Jupiter diameters ≥0.5 and orbital inclinations close to 90°, the scatter in differential radial velocities permit a close giant planet in a more highly inclined orbit.