6 Acknowledgments

This work has been supported by grant ATM-0447159 to Lowell Observatory from the National Science Foundation. Writing this review was greatly facilitated by the resources available from NASA’s Astrophysics Data System and the SIMBAD Astronomical Database, operated at CDS, Strasbourg, France. The author thanks Drs. Mark Giampapa and Tom Ayres for their time and thoughtfulness in reading a lengthy manuscript and returning excellent reviews with a number of useful suggestions that improved the final paper.

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