1. Departamento de Física, CCNE, Universidade Federal de Santa Maria , 97105-900 Santa Maria, RS, Brazil
2. Departamento de Astronomia, IF, Universidade Federal do Rio Grande do Sul , CP 15051, 91501-970 Porto Alegre, RS, Brazil
3. Department of Physics and Astronomy, University of California , 4129 Frederick Reines Hall , Irvine, CA 92697, USA
4. Department of Physics & Astronomy, Bloomberg Center, Johns Hopkins University , 3400 N. Charles St , Baltimore, MD 21218, USA