1 Introduction

The open clusters constitute a system of young objects of great value for the study of the dynamics of the Galaxy, because they span a relatively wide range of ages, and their age can be determined with higher precision than any other spiral arm tracer, with the help of the HR diagram. They are key objects to understand the motion of spiral arms and of the moving groups of stars, to derive the rotation curve and to distinguish between star formation processes. To investigate the orbits of this system, it is essential to have at our disposal accurate proper motion and radial velocities.

Many works containing observations of proper motion of open clusters can be found in the literature. However, most measurements performed in the past were relative proper motions. Only recently, after the Hipparcos mission (ESA 1997; Kovalevsky 1998), has a reference frame for absolute proper motion become available, and according to the IAU General Assembly, the Hipparcos Catalogue is considered as the realization of the ICRS at optical wavelengths. It is not clear whether the previous measurements can be corrected in a straightforward manner. The recent works of Platais et al. (1998), De Zeeuw et al. (1999), Robichon et al. (1999) and of Baumgart et al. (2000, hereafter BDW) are examples of measurements of proper motions of large samples of open clusters in the Hipparcos system. All these works are based on data directly taken from the Hipparcos Catalogue. Among these, Platais et al. focus on nearby and extended clusters, while BDW exclude clusters closer than 200 pc. The cluster proper motions derived by BDW are based on only a few stars in most cases, due to the restriction in magnitude of the Hipparcos data.

On the other hand, positions and proper motions of large sets of faint stars have been determined in the Hipparcos system, using ground-based observations, e.g., the Tycho-2 (Hog et al. 2000) and UCAC1 (Zacharias et al. 2000) Catalogues. These Catalogues have allowed various small field investigations to be performed in the new system (e.g., Dias et al. 2000; Teixeira et al. 2000).

In the present work, we determine proper motions of open clusters based on the Tycho2 Catalogue. We explore clusters up to a distance of 1 kpc, using the statistical method of Sanders (1971) to derive probable membership, in order to obtain the mean proper motion based on relatively large samples of stars for each cluster. We are aware, however, that it is possible to find open clusters at larger distances ($d \geq 1$ kpc) with detectable proper motion. Examples are Westerlund 02, Rupretch 49 and Pismis 11, among others. The clusters more distant than 1 kpc will be addressed in a forthcoming paper (Dias et al. 2001).