1. Department of Astronomy, China West Normal University, Nanchong, Sichuan 637002, China
2. Instituto de Física e Química, Universidade Federal de Itajubá, 37500-903 Itajubá, MG, Brazil
3. Instituto de Física, Universidade de São Paulo, CP 66318, 05315-970 São Paulo, SP, Brazil
4. Department of Physics, Chongqing University, Chongqing 400030, China