SCYON Abstract

Received on July 16 2001

Gravitational tidal effects on galactic open clusters

Authors	G. Bergond(¹), S. Leon(²), & J. Guibert(¹)
Affiliation	(¹) CAI, Observatoire de Paris, 77 Av. Denfert-Rochereau, 75014 Paris, France (²) I. Physikalisches Institut, Universität zu Köln, Zulpicher Strasse 77, 50937 Koln, Germany
Accepted by	Astronomy & Astrophysics
Contact	gbergond@eso.org
URL	http://dsmama.obspm.fr/
Links	NGC 2287 / NGC 2516 / NGC 2548

Abstract

We have investigated the 2-D stellar distribution in the outer parts of three nearby open clusters: NGC 2287 (= M41), NGC 2516, and NGC 2548 (= M48). Wide-field star counts have been performed in two colours on pairs of digitized ESO and SRC Schmidt plates, allowing to select likely cluster members in the colour-magnitude diagrams. Cluster tidal extensions were emphasized using a wavelet transform. Taking into account observational biases, namely the galaxy clustering and differential extinction in the Galaxy, we have associated these stellar overdensities to real open cluster structures stretched by the galactic gravitational field. As predicted by theory and simulations, and despite observational limitations, we detected a general elongated (prolate) shape in a direction parallel to the galactic Plane, combined 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 system when crossing the Disk. The time varying tidal field will affect deeply the cluster dynamical evolution, and we emphasize the importance of the adiabatic 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 cluster before its dissolution in the Galaxy, crossings through the galactic Disk contribute at least to 15% of the total mass loss. Using recent age estimations published for open clusters, we find a destruction time-scale of about 600 Myr for clusters in the solar neighbourhood.