Astron. Astrophys. 359, 907-931 (2000)

1. Introduction

Recent works have emphasized that globular clusters (GCs) are stellar systems which exhibit strong dynamical evolution in the potential well of their host galaxy (Gnedin & Ostriker 1997, hereafter GO97; Murali & Weinberg 1997). Since their birth, GCs suffer internal evolution on their own: at the very beginning GCs evolve rapidly because of the fast stellar evolution of their massive stars (Vesperini & Heggie 1997; Portegies Zwart et al. 1997). The finite size of the GCs produces two-body relaxation between the member stars; equipartition of energy heats the lighter stars which diffuse outwards into the halo while the heavier stars sink slowly towards the contracting core (Spitzer & Hart 1971). Typically, the relaxation time (hereafter we will refer to the relaxation time at the half mass radius; see, e.g., Binney & Tremaine 1987) in a GC is of the order of yr (GO97), which is significantly less than the age of all galactic globular clusters.

Owing to the negative specific heat in self-gravitating stellar systems (Antonov 1962; Lynden-Bell & Wood 1968), related to the virial equilibrium, the core is contracting monotonically during a process called the "gravothermal catastrophe", leading to core collapse, characterized by very high stellar densities (up to ). It has been eventually recognized that this process is not so catastrophic after all, since the cluster core does not collapse for ever but bounces back towards lower stellar density phases (Hénon 1975). The key role of binaries (either primordial or formed via encounters during the high stellar density phase) has been emphasized in the internal dynamics of the GCs: these binaries act as a heating source and slow down the core collapse (Goodman & Hut 1989) and even reverse it. Many authors have studied numerically the post core-collapse phase, using conducting-gas-sphere, Fokker-Planck, and N-body codes, in simulations computed well into core collapse and beyond, leading to the discovery of possible post-collapse gravothermal oscillations (see Meylan & Heggie 1997 for a review). Globular clusters evolve dynamically, even when considering only relaxation, which causes stars to escape, consequently cluster cores to contract and envelopes to expand.

Any galaxy through its gravitational potential well influences the dynamical evolution of its globular clusters, accelerating their destruction. The stars in the globular cluster halo are stripped by the tidal field of the galaxy: the outward diffusion of the stars towards the sub-thermalised halo is speeded up and the core contracts even more (Spitzer & Chevalier 1973). Moreover the gravitational shocks heat up the outer parts of the globular cluster, increasing its loss of stars (Aguilar et al. 1988; Weinberg 1994; Kundic & Ostriker 1995; Leon et al. 2000). Shocks are caused by the tidal field of the galaxy: interactions with the disk, the bulge and, at a lower level, with the giant molecular clouds (GMCs, see Spitzer 1958), heat up the outer regions of each star clusters. The disk-shocking occurs when the GC crosses the thin disk where it is compressed by the varying z-component of the galactic plane potential; this has been found to dominate the heating of GCs (Chernoff et al. 1986). A GC globally gains energy during the crossing and exhibits peculiar transient deformation (Leon et al. 2000). The shocks with the bulge and the GMCs are similar in their physical processes: the GCs suffer an elongation aligned parallel to the density gradient in the bulge or the GMC. These processes combined with the internal dynamical evolution have probably destroyed an important fraction of the primordial GCs and are still currently at play. GO97 estimate that "half of the present clusters are to be destroyed within the next Hubble time" .

All GCs are expected to have already lost an important fraction of their mass, deposited in the form of individual stars in the halo of the Galaxy. The mass-loss rate is a function of the total mass of the cluster, its structural parameters like the concentration c (where c = log () with and are the tidal and core radii), and its orbital motion around the galactic center. Till recently, the only way to investigate the orbital history of a globular cluster, apart from proper motions, was to derive its tidal radius , which gives an indication of its perigalactic distance. Unfortunately, there are difficulties in defining the tidal radius, both theoretically and observationally; and, as expected, there are discrepancies between the theoretical and observational values of the tidal radius (see, e.g., Odenkirchen et al. 1997; Scholz et al. 1998).

N-body simulations of globular clusters embedded in a realistic galactic potential (Oh & Lin 1992; Johnston et al. 1997; Combes et al. 1999, hereafter CLM99) have been performed in order to study the amount of mass loss for different kinds of orbits and different kinds of clusters, along with the dynamics and the mass function in tidal tails. The 2-D structure of such tidal tails appears to be a good tracer of gravitational shocks and should be a tracer of the potential well. Moreover the detection of unbound stars released by the clusters is the only way to measure directly the mass loss rate of the cluster.

Grillmair et al. (1995) in an analysis of star-count in the outer parts of a few galactic globular clusters found extra-cluster overdensities that they associated partly with stars stripped into the Galaxy field. Similar tidal interaction remnants around globular clusters have also been found in external galaxies: Grillmair et al. (1996) observed three clusters in M31 which exhibit departures from a King profiles. Leon et al. (1999) find tidal extensions in the outskirts of interacting binary clusters and isolated clusters in the Large Magellanic Cloud (LMC). Not surprisingly, galactic tidal forces play also an essential role in the evolution of smaller-scale galactic clusters, the open clusters: they exhibit tidal tails in their neighborhood (Odenkirchen 1998; Bergond et al. 2000).

In this work we study the 2-D structure of the tidal tails associated with 20 galactic globular clusters by using the wavelet transform to detect weak structures at large scale and filter the strong background noise for the low galactic latitude clusters. We analyze with great care the observational bias which can be very important. In Sect. 2 we present the observations, in Sect. 3 we describe the data reduction, in Sect. 4 we present the results with, for each globular cluster, detailed comments about related observational bias. Sect. 5 presents the general discussion of all results.

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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