1. Department of Astronomy, University of Washington, Seattle, Washington 98195, United States
2. Department of Physics, Bryn Mawr College, 101N Merion Avenue, Bryn Mawr, Pennsylvania 19010, United States
3. Department of Physics and Astronomy, University of California, Riverside, Riverside, California 92521, United States
4. Department of Physics and Astronomy, Haverford College, 370 Lancaster Avenue, Haverford, Pennsylvania 19041, United States
5. Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, United Kingdom