[1] Natl Radio Astron Observ, Socorro, NM 87801 USA
[2] Nobeyama Radio Observ, Minamisa Ku, Nagano 3841305, Japan
[3] Natl Astron Observ, Tokyo 181, Japan
[4] Kyoto Univ, Dept Astron, Sakyo Ku, Kyoto 6068502, Japan
[5] Max Planck Inst Radioastron, D-53121 Bonn, Germany
[6] Univ Massachusetts, Dept Phys & Astron, Lederle Grad Res Ctr 640, Amherst, MA 01003 USA
[7] New Mexico Inst Min & Technol, Socorro, NM 87801 USA
[8] Univ Tokyo, Inst Astron, Tokyo 113, Japan