Globular Clusters with Tidal Tails: Deep Two-Color Star Counts

Abstract

We examine the outer structure of 12 Galactic globular clusters using star-count analyses. Deep, two-color, photographic photometry is used to select and count stars with colors and magnitudes consistent with cluster-specific, color-magnitude sequences. The resulting reduction in the number of contaminating foreground stars allows us to push the star counts to significantly lower surface densities than has previously been possible. We find that most of our sample clusters show extra-tidal wings in their surface density profiles. The form of the surface density profiles is consistent with recent numerical studies of the tidal stripping of globular clusters. Two-dimensional surface density maps for several clusters are consistent with the expected appearance of tidal tails, with allowance for the effects of orbit shape, orbital phase, and orientation of our line of sight. We identify the extra-tidal material with stars which are still in the process of being removed from the clusters. The extra-tidal stars effectively limit the accuracy to which we can resolve the ``tidal'' radii of globular clusters. However, by tracing the orbital paths of their parent clusters, these stars may also facilitate a renewed attack on the problem of determining globular cluster orbits and the shape of the Galactic potential field.Comment: to appear in AJ, 29 pgs + 17 tables (Latex, AASTEX macros) + 30 pages of uuencoded, gzipped postscript figures. Figures 1 and 13 available on request from Grillmair (carl@sundown.ucsc.edu)

Gravitational tidal effects on galactic open clusters

“…Next, we adopt the method described by Grillmair et al (1995), which consists of selecting regions in the CMD where the "cluster vs. field contrast" is important. For this purpose, we use a so-called colour-magnitude S/N function s(i, j) defined by subdividing the CMD in multiple cells (50×50 bins here, i.e.…”

“…Terlevich (1987) and de la Fuente Marcos (1997 and references therein) performed realistic simulations of open cluster evolution. From the observational point of view, Grillmair et al (1995) and Leon et al (2000) detected important tidal tail extensions around galactic globular clusters using a wide-field star count analysis. Leon et al (1999) also found noticeable tidal tails around several binary star clusters in the Large Magellanic Cloud.…”

Internal dynamics of globular clusters

“…Recent N-body simulations of globular clusters embedded in a realistic galactic potential (Oh & Lin 1992;Johnston et al 1999) were 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 segregation in tidal tails. Grillmair et al (1995) in an observational analysis of star counts 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.…”

“…For each field, a (B vs. B − V ) color-magnitude diagram was constructed, on which a field/cluster star selection was performed, following the method of Grillmair et al (1995), since cluster stars and field stars exhibit different colors. In this way present and past cluster members could be distinguished from the fore-and background field stars by identifying in the CMD the area occupied primarily by cluster stars.…”

“…Finally, these N-body experiments help to understand the recent observations of extended tidal tails around globular clusters (Grillmair et al 1995: the systematic observations of the geometry of these tails should provide much information on the orbit, dynamics, and mass loss history of the clusters, and on the galactic structure as well. The coordinates of G1, viz.…”