We report on HST/WFPC2 U,V and far-ultraviolet observations of two Galactic
Globular Clusters (GGCs), NGC 6205 = M13 and NGC 6093 = M80. Both of these
clusters have horizontal-branch (HB) tails that extend to the helium-burning
main sequence, with the hottest stars reaching theoretical effective
temperatures above 35,000 K. In both clusters, groups of stars are found to be
separated by narrow gaps along the blue HB sequence. These gaps appear at
similar locations in the color-magnitude diagrams of the two clusters. While
stochastic effects may give rise to variations in the color distribution along
the HB, the coincidence of gaps in different clusters effectively rules this
out as the primary cause. The comparison among the clusters strongly suggests
that there are separate physical processes operating during the earlier
red-giant phase of evolution to produce mass loss.Comment: 13pp LaTeX, uses AAS Latex version 4. Includes 7 postscript figures.
Postscript file available for download at http://www.astro.virginia.edu/~bd4
Using HST-WFPC2 observations in two ultraviolet (UV) filters (F225W and
F336W) of the central region of the high density Galactic Globular cluster
(GGC) M80 we have identified 305 Blue Straggler Stars (BSS) which represents
the largest and most concentrated population of BSS ever observed in a GGC. We
also identify the largest, clean sample of evolved BSS yet found. The high
stellar density alone cannot explain the BSS, and we suggest that in M80 we are
witnessing a transient dynamical state, during which stellar interactions are
delaying the core-collapse process leading to an exceptionally large population
of collisional-BSS.Comment: 15 pages, 5 figures, Astrophysical Journal Letters, in pres
The central regions of six Galactic Globular Clusters (GGCs) (M3, M80, M10, M13, M92 and NGC 288) have been imaged using HST-WFPC2 and the ultraviolet (UV) filters (F255W, F336W). The selected sample covers a large range in both central density (log ρ 0 ) and metallicity ([Fe/H]). In this paper, we present a direct cluster-to-cluster comparison of the Blue Stragglers Stars (BSS) population as selected from (m 255 , m 255 − m 336 ) Color Magnitude Diagrams (CMDs). We have found: (a) BSS in three of the clusters (M3, M80, M92) are much more concentrated toward the center of the cluster than the red giants; because of the smaller BSS samples for the other clusters we can only note that the BSS radial distributions are consistent with central concentration; (b) the specific frequency of BSS varies greatly from cluster to cluster. The most interesting result is that the two clusters with largest BSS specific frequency are at the central density extremes of our sample: NGC 288 (lowest central density) and M80 (highest). This evidence together with the comparison with theoretical collisional models suggests that both stellar interactions in high density cluster cores and at least one other alternate channel operating low density GGCs play an important role in the production of BSS. We also note a possible connection between HB morphology and blue straggler luminosity functions in these six clusters.
We used a set of archived HST/WFPC2 images to probe the stellar population in the core of the nearby Galactic Globular Cluster (GGC) 47 Tuc. From the ultraviolet (UV) Color Magnitude Diagrams (CMDs) obtained for ∼ 4 000 stars detected within the Planetary Camera (PC) field of view we have pinpointed a number of interesting objects: (i) 43 blue stragglers stars (BSSs) including 20
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