2006ApJ...648..607C

The Microvariability and Oscillations of STars (MOST) photometric satellite observed three rotations of ε Eri continuously in late 2005. We detected two spots (Δm∼0.01) at different latitudes (20°.0, 31°.5) revolving with different periods (11.35 days, 11.55 days), from which we derive a differential rotation coefficient, k=0.11^+.03_–.02, in agreement with the prediction by Brown and coworkers for a young Sun-like star having roughly twice the solar angular velocity. The light curve was analyzed with the program StarSpotz, a modification of SPOTMODEL by Ribárik and coworkers. The best-fitting value for the inclination angle i=30°±3° is compatible with inclinations already estimated for the disk (∼25°) and planetary orbit (26°.2). The inclination also leads to an equatorial rotation speed of 3.42 km/s and the photometric value of vsini=1.7 km/s. When compared with spectroscopically determined values, the photometric vsini allows, in principle, an independent estimate of the macroturbulent velocity. Both spots would have distorted the radial velocity curve ~±10 m/s by the Rossiter-McLaughlin effect, which is similar to the stellar radial velocity ``noise'' detected by others. Details of the StarSpotz model and of the uniqueness tests that we applied in order to arrive at a best solution and realistic estimates of errors in the derived parameters are given.