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

We examine the outer structure of 12 Galactic globular clusters usingstar-count analyses. Deep, two-color, photographic photometry is used to selectand count stars with colors and magnitudes consistent with cluster-specific,color-magnitude sequences. The resulting reduction in the number ofcontaminating foreground stars allows us to push the star counts tosignificantly lower surface densities than has previously been possible. Wefind that most of our sample clusters show extra-tidal wings in their surfacedensity profiles. The form of the surface density profiles is consistent withrecent numerical studies of the tidal stripping of globular clusters.Two-dimensional surface density maps for several clusters are consistent withthe expected appearance of tidal tails, with allowance for the effects of orbitshape, orbital phase, and orientation of our line of sight. We identify theextra-tidal material with stars which are still in the process of being removedfrom the clusters. The extra-tidal stars effectively limit the accuracy towhich we can resolve the ``tidal'' radii of globular clusters. However, bytracing the orbital paths of their parent clusters, these stars may alsofacilitate a renewed attack on the problem of determining globular clusterorbits 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)