1. School of Astronomy and Space Science, Nanjing University , Nanjing 210093 , P. R. China
2. Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education , Nanjing 210093 , P. R. China
3. School of Physical Science and Technology, Guangxi University , Nanning 530004 , P. R. China
4. Institut de Radioastronomie Millimetrique , 300 rue de la Piscine, Domaine Universitaire de Grenoble, 38406, Saint-Martin d’Hères , France