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Astron. Astrophys. 356, 127-133 (2000)

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1. Introduction

The tidal radii of globular clusters (GCs) are important tools for understanding the complex interactions of GCs with the Galaxy. In fact, they have traditionally been used to study the mass distribution of the galactic halo (Innanen et al. 1983), or to deduce GCs orbital parameters (Freeman & Norris 1981; Djorgovski et al. 1996). Tidal radii have usually been estimated (only in few cases directly measured), by fitting King models to cluster density profiles rarely measured from the inner regions out to the tidal radius, because of the nature of the photographic material, that prevented any measure in the cluster center, and the small format of the first digital cameras. Only in the last few years, the advent of deep digitized sky surveys and wide field digital detectors has allowed us to deal with the overwhelming problem of contamination from field stars and to probe the outer region of GCs directly (Grillmair et al. 1995, hereafter G95; Zaggia et al. 1995; Zaggia et al. 1997; Lehman & Scholz 1997). The study of tidal tails in galactic satellites is gaining interest for many applications related to the derivation of the galactic structure and potential, the formation and evolution of the galactic halo, as well as the dynamical evolution of the clusters themselves. Recent determinations of proper motion for some globular clusters with HIPPARCOS have made it possible to estimate the orbital parameters of a good number of them (Dinescu et al. 1999). This helps to clarify the nature and structure of tidal extensions in GCs.

In principle, available tools to enhance cluster star counts against field stars rely on the color-magnitude diagram (CMD), proper motions, radial velocities, or a combination of the three techniques. The application of these techniques to GCs have led to the discovery that tidal or extra-tidal material is a common feature: Grillmair (1998), for instance, reported the discovery of tidal tails in 16 out of 21 globular clusters. Interestingly, signature of the presence of tidal tails in GCs has also been found in four GC's in M31 (Grillmair et al., 1996). For galactic clusters, the discovery was made by using a selection in the CMD of cluster stars on catalogs extracted from digitized photographic datasets. The CMD selection technique is an economical and powerful method to detect GC tails, since it significantly decreases the number of background and/or foreground objects.

In order to test the feasibility of a survey of most GCs present in the Northern hemisphere, we applied the CMD technique to the galactic globular cluster M 92 (NGC 6341), with the aim of measuring the tidal radius and searching for the possible presence of extra-tidal material. We used plates from the Digitized Second Palomar Sky Survey (hereafter DPOSS), in the framework of the CRoNaRio (Caltech-Roma-Napoli-Rio de Janeiro) collaboration (Djorgovski et al. 1997; Andreon et al. 1997; Djorgovski et al. 1999). A previous account on this work was given in Zaggia et al. (1998). This is the first of a series of papers dedicated to the subject -an ideal application for this kind of all-sky surveys.

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© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000