| 基金项目： | Acknowledgements This work has been partly supported by the National Basic Research Program of China （973 Program, 2009CB24900）, the Guangxi Natural Science Foundation （0991018Z）, and the Science and Technology Commission of Shanghai Municipality （06DZ22101）. Tao An has been supported by a grant from the Chinese Academy of Sciences and a visitor grant from the The Netherlands Science Foundation. The University of Michigan Radio Astronomy Observatory is sup- ported by funds from the NSF, NASA, and the University of Michigan. |
