Christoffer edited untitled.tex  over 8 years ago

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at the Mount Wilson Observatory were terminated in 2003, but complementary observations of the  97 stars began in 1996 at the Lowell Observatory and are still ongoing \cite{Hall_2007, Hall_2009}. Unfortunately, these two sets of observations have never been combined. In fact non of the Mount Wilson observations are public and \cite{Baliunas_1995} only include observations up to 1992, so in somewhere a decade of unpublished observations is laying around just waiting to me published.  The results from \cite{Baliunas_1995} can be summerise as follows: out of 112 stars with spectral class between F2 and M2 including the Sun 52 showed cycles with periods between 2.5 to 25 years, 29 showed variability, but no cycles and 31 showed no variability or only a linear trend. For stars with spectra class between G0 to K5 V a pattern of changes in the rotation and chromospheric activity on an evolutionary timescale was indetified. This pattern suggested that these stars could be separated into three distinct groups: 1) stars younger than 1 Gyr that were characterized by fast rotation and high average activity levels. These stars often show large variability, but rarly cycles; 2) stars of intermediate age that were characterized by moderate rotation rates and activity levels. These stars often had shooth cycles and 3) stars as old as the Sun or older that were characterized by slow rotation and low activity levels. Some of these stars showed smooth cycles and some showed flat activity levels.  \section{Rotation}  The study of stellar rotation was pioneered Robert P. Kraft in a series of papers \cite{Kraft_1965a, Kraft_1965b, Anderson_1966, Kraft_1967a, Kraft_1967b}. These observations, together with the early observation from Odie Wilson led to the famous Skumanich spin-down law \cite{Skumanich_1972}.