1. Institute of Astrophysics, Central China Normal University, Wuhan 430079, China
2. School of Physics and Mechanical & Electrical Engineering, Hubei University of Education, Wuhan 430205, China
3. Research Center for Astronomy, Hubei University of Education, Wuhan 430205, China
4. Department of Astronomy, School of physics, Huazhong University of Science and Technology, Wuhan 430074, China
5. Department of Physics, San Diego State University, San Diego, CA 92182, USA
6. Center for Astrophysics and Space Sciences, University of California at San Diego, La Jolla, CA 92093, USA