2 Determination of the mean absolute proper motions

Following the procedure described in Paper I, we selected in the DAML2002 database all the clusters catalogued with distance greater than 1 kpc.

We searched for the selected clusters in the Tycho2 Catalogue (Hog et al. 2000), using the central coordinates and the radius taken from the database. All the stars situated within the limits of the clusters were investigated. For open clusters with an apparent diameter less than $15^{\prime}$ we searched for stars in an area 4 times the cluster area, and if $D \geq 15^{\prime}$ we used the area covered by the cluster.

In Paper I we argued that the Tycho2 Catalogue is the main source of data, at least in large scale, to be used in this kind of investigation (see Dias et al. 2001; Sanner et al. 2001). However it is important to note that because of its limiting magnitude (about 13 mag), this catalogue is not very dense. This is the reason why it is specially difficult to obtain satisfactory results for open clusters with $D \leq 6^{\prime}$. Actually, we were able to determine the mean proper motion of 94 clusters, which is only about $40\%$ of the total investigated.

When a sufficient number of stars was found in the region of the cluster, we performed the statistical analysis of Sanders (1971) to segregate field and clusters stars.

The method fits a two-dimensional Gaussian distribution to the vector point diagram (VPD) and the best parameters are determined by a maximum likelihood procedure. The details are given by Sanders (1971), and discussions on improvements of the method, can be found in Zhao & He (1990).