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 16
70 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.