1. Department of Earth and Space Science, Graduate School of Science, Osaka University, Osaka 560-0043, Japan
2. Department of Physics & Astronomy, University of Utah, Salt Lake City, UT 84112-0830, USA
3. Argelander Institut fuer Astronomie, Auf dem Haegel 71, D-53121 Bonn, Germany
4. Department of Physics & Astronomy, University of Kentucky, Lexington, KY 40506-0055, USA