Differential Rotation of ε Eridani Detected byMOST

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Abstract
The Microvariability and Oscillations of STars (MOST) photometric satellite observed three rotations of Eri continuously in late 2005. We detected two spots (Δm ~ 0.01) at different latitudes (200, 315) revolving with different periods (11.35 days, 11.55 days), from which we derive a differential rotation coefficient, k = 0.11, 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 (262). The inclination also leads to an equatorial rotation speed of 3.42 km s-1 and the photometric value of v sin i = 1.7 km s-1. When compared with spectroscopically determined values, the photometric v sin i allows, in principle, an independent estimate of the macroturbulent velocity. Both spots would have distorted the radial velocity curve ~±10 m s-1 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.