Gravitational tidal effects on galactic open clusters

We have investigated the 2-D stellar distribution in the outer parts of thr ee nearby open clusters: NGC 2287 (=M 41), NGC 2516, and NGC 2548 (=M 48). Wide-field star counts have been performed in two colours on pairs of digit ized ESO and SRC Schmidt plates, allowing us to select likely cluster membe rs in the colour-magnitude diagrams. Cluster tidal extensions were emphasiz ed using a wavelet transform. Taking into account observational biases, nam ely the galaxy clustering and differential extinction in the Galaxy, we hav e associated these stellar overdensities with real open cluster structures stretched by the galactic gravitational field. As predicted by theory and s imulations, and despite observational limitations, we detected a general el ongated (prolate) shape in a direction parallel to the galactic Plane, comb ined with tidal tails extended perpendicularly to it. This geometry is due both to the static galactic tidal field and the heating up of the stellar s ystem when crossing the Disk. The time varying tidal field will deeply affe ct the cluster dynamical evolution, and we emphasize the importance of adia batic heating during the Disk-shocking. In the case of NGC 2548, our dating of the last shocking with the Plane (based on a tidal clump) is consistent with its velocity. During the 10-20 Z-oscillations experienced by a cluste r before its dissolution in the Galaxy, crossings through the galactic Disk contribute to at least 15% of the total mass loss. Using recent age estima tions published for open clusters, we find a destruction time-scale of abou t 600 Myr for clusters in the solar neighbourhood.