Open clusters play an important role in understanding stars, stellar systems and our Galaxy. Some cluster regions have become focal points for studies of star formation, for instances, the Trapezium (Prosser et al. 1994), IC 348 (Lada & Lada 1995), NGC 2362 (Wilner & Lada 1991) and Oh (Wilking et al. 1989; Rieke et al. 1989). Secondly, although their HR diagrams are basic for distance determination of celestial bodies, there still are some unsolved questions, such as the mechanisms of blue stragglers, gaps in main sequences, double main sequences, and so on (Milone & Latham 1994; Raboud & Mermilliod 1994). Furthermore, being typical stellar systems, open clusters leave many open questions, although great efforts have been devoted to dynamics, internal motions, etc. (Carraro & Chiosi 1994; Su 1994; Drukier 1995). Finally, open clusters are often considered as special tracers to investigate the structure and evolution of our Galaxy, the relationship between the disk and halo, and whether or not there existed some star formation bursts during the lifetime of the Galactic disk (Janes & Phelps 1994; Phelps & Janes 1994).
As we know, the concentration to the Galactic plane for open clusters is very strong. Their scale heights perpendicular to the Galactic plane are 55 pc and 375 pc for ``young" and ``old" open clusters, respectively (Janes & Phelps 1994). That is to say, there are many field stars mixed in the observational data for clusters. If membership is not reasonably determined for an open cluster, great uncertainties will be brought out in features, such as age, distance, structure, dynamics, stellar mass function, etc. Thus, it is the firststep that membership determination must be done before doing deeper researches.
M 11(NGC 6705, , ) is a famous middle-age open cluster with distance of 1.6 kpc, mass of and age of 2.0 108 yrs (Mathieu 1984; Su 1994; Su et al. 1997). Its Trumple classification is II 2r and/or 2b-a. Because of its large number of members and very good central concentration, M 11 is one of the best-choice candidates to study open clusters.
McNamara et al. (1977) used 15 plate pairs to derive proper motions of 1890 stars in the central region within 0.25 square degree of M 11. Most of these stars have V magnitudes. The completeness of their sample is down to . Using these proper motion data, they also determined M 11 membership. They took 874 stars with membership probabilities higher than 0.5 as M 11 members. This is the best sample of M 11 at that time. McNamara & Sanders (1977) also used this sample to analyze the internal motions of M 11. They found that there exists obvious space-mass segregation, but the velocity- mass segregation is unclear.
There are some other researches for M 11, including photometric (Johnson et al. 1957; Patenaude 1978; Mathieu 1984), analyses of internal motion (Su 1994; Su et al. 1997; Zhao & He 1987), as well as structure and dynamics (Su 1994; Mathieu 1984).
In the present paper, we use 10 plate pairs taken with the double astrograph at the Zo-Se station of Shanghai Observatory to derive proper motion data of 872 stars. The time baselines of the plate pairs are 1670 years. According to the method suggested by Stetson (1979), we can reasonably estimate B magnitudes for individual stars; V magnitude values of stars can be obtained from McNamara et al. (1977) through the cross-identification for common stars. The completeness magnitude for the data is about in the B band. Stars within 25 arcmin centred on the open cluster M 11 region are used to do membership determination, from which some useful information can be found. Because all of our plates were taken at one telescope and the time baselines are also rather long, the accuracy of proper motions will be expected to be good and it is an independent and useful sample of M 11 for further researches.
The descriptions of the plates and the reduction of the proper motion data are in Sect. 2. Membership determination is in Sect. 3. Some comparisons and discussion are given in Sect. 4. The last section is the summary.