The Differential Rotation of κ¹ Ceti as Observed by MOST^*

We first reported evidence for differential rotation of κ¹ Ceti in Paper I. In this paper we demonstrate that the differential rotation pattern closely matches that for the Sun. This result is based on additional MOST observations in 2004 and 2005, as well as those from 2003. Using StarSpotz, a program developed specifically to analyze MOST photometry, we have solved for k, the differential rotation coefficient, and P_eq, the equatorial rotation period using the light curves from all three years. The absolute range in spot latitudes is 10°-75° and k = 0.090, less than the solar value but consistent with the younger age of the star; k is also well constrained by the independent spectroscopic estimate of v sin i. We demonstrate independently that the pattern of differential rotation with latitude is indeed solar. Details are given of the parallel tempering formalism used in finding the most robust solution, which gives P_eq = 8.77 days, smaller than that usually adopted, implying an age <750 My. Our values of P_eq and k can explain the range of rotation periods others have found by spots or activity at a variety of latitudes. Historically, Ca II activity seems to occur consistently between latitudes 50° and 60°, which might indicate a permanent magnetic feature. Knowledge of k and P_eq is key to understanding the dynamo mechanism and rotation structure in the convective zone, as well assessing age for solar-type stars. We recently published values of k and P_eq for Eri based on MOST photometry and expect to analyze MOST light curves for several more spotted, solar-type stars.