M. B. Nielsen, L. Gizon, H. Schunker, C. Karoff
Aims: We aim to measure starspot rotation periods of stars in the Kepler field as a function of spectral type and extend reliable rotation measurements from F-, G-, and K- to M-type stars. Methods: Using the Lomb-Scargle periodogram we search for periodic starspot modulations in the light curves of more than 150,000 stars. We measure rotation periods between 1 and 30 days in eight Kepler quarters (three-month data sets). To exclude false positives, we select stable rotation periods, i.e. periods that do not vary from the median by more than one day in at least six of the eight quarters. Results: We report on the stable starspot rotation periods of 12,151 Kepler stars. We then average the periods for each stellar spectral class according to B-V color and compare the results to archival vsini data, using stellar radii estimates. We find a good agreement between starspot velocities and v sin i data for all F-, G- and early K-type stars. However, starspot velocities are an order of magnitude smaller for late K- and M-type stars compared to the v sin i estimates. Conclusions: Since starspot rotation measurements are much less subject to observational bias toward fast rotators than vsini measurements, they are better suited to determine the rotation periods of late-type stars. The rotation periods of K5, M0, and M5 stars in our sample are 14.1^{+3.2}_{-2.1} days, 14.7^{+1.8}_{-1.5} days, and 16.6^{+1.4}_{-4.4} days respectively.
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http://arxiv.org/abs/1305.5721
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