Astron. Astrophys. 355, 922-928 (2000)

Acknowledgements

We wish to thank Dan Maoz for valuable comments that helped improve our manuscript. The anonymous referee is thanked for his/her critical comments and kind help to improve our English presentation. We are very grateful to Wei Zheng for his careful reading of the manuscript. This work is supported by grants from the NSF of P.R. China, and grants from the Ascent Project of the State Scientific Commission of P.R. China.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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