1. Department of Physics and Astronomy, Texas Tech University , Lubbock, TX 79409, USA
2. Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma , 440 W. Brooks St., Norman, OK 73019, USA
3. Smithsonian Astrophysical Observatory , 60 Garden St, Cambridge, MA 02138, USA
4. Département de Physique, Université de Montréal , C.P. 6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7, Canada