2011A&A...532A..81F

We present a spectroscopic and photometric analysis of the rapid rotator KIC 8429280, discovered by ourselves as a very young star and observed by the NASA Kepler mission, designed to determine its activity level, spot distribution, and differential rotation. We use ground-based data, such as high-resolution spectroscopy and multicolor broad-band photometry, to derive stellar parameters (vsin i, spectral type, T_eff, logg, and [Fe/H]), and we adopt a spectral subtraction technique to highlight the strong chromospheric emission in the cores of hydrogen Hα and CaII H&K and infrared triplet (IRT) lines. We then fit a robust spot model to the high-precision Kepler photometry spanning 138 days. Model selection and parameter estimation is performed in a Bayesian manner using a Markov chain Monte Carlo method. We find that KIC 8429280 is a cool (K2V) star with an age of about 50Myr, based on its lithium content, that has passed its T Tau phase and is spinning up approaching the ZAMS on its radiative track. Its high level of chromospheric activity is clearly indicated by the strong radiative losses in CaII H&K and IRT, Hα, and Hβ lines. Furthermore, its Balmer decrement and the flux ratio of CaII IRT lines imply that these lines are mainly formed in optically-thick regions similar to solar plages. The analysis of the Kepler data uncovers evidence of at least seven enduring spots. Since the star's inclination is rather high - nearly 70° - the assignment of the spots to either the northern or southern hemisphere is not unambiguous. We find at least three solutions with nearly the same level of residuals. Even in the case of seven spots, the fit is far from being perfect. Owing to the exceptional precision of the Kepler photometry, it is not possible to reach the noise floor without strongly enhancing the degrees of freedom and, consequently, the non-uniqueness of the solution. The distribution of the active regions is such that the spots are located around three latitude belts, i.e. around the star's equator and around ±(50°-60°), with the high-latitude spots rotating slower than the low-latitude ones. The equator-to-pole differential rotation dΩ≃0.27rad/d is at variance with some recent mean-field models of differential rotation in rapidly rotating main-sequence stars, which predict a much smaller latitudinal shear. Our results are consistent with the scenario of a higher differential rotation, which changes along the magnetic cycle, as proposed by other models.