MEMBERSHIP DETERMINATION OF STARS USING PROPER MOTIONS IN THE REGION OF THE OPEN CLUSTER M11

Relative proper motions of 872 stars in the open cluster M 11 region a re reduced using 10 plate pairs taken over time baselines of 16 simila r to 70 years with the double astrograph telescope of Shanghai Observa tory. The scale is 30 ''/mm. The plates were measured with the PDS mac hines in the Purple Mountain Observatory in Nanjing and the Institute of Technology and Communication in Luoyang, China. The average proper motion accuracy is about 1.1 mas/yr with 85% of the data better than 1 mas/yr. Membership probabilities of 785 stars within 25' centred on M 11 are determined based on their proper motions. The method used is s uggested by Su ct al. (1995) with some improvements of Zhao & He (1990 ), in which the space distribution and magnitude dependencies for clus ter stars are taken into account. The results are significantly good. The total integrated membership probabilities for all these stars is 5 47 and the number of stars with probabilities higher than 0.7 is 541. It can be found after the membership determination that there exists m ass segregation in M 11. Some comparisons and discussion are also give n.