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

No abstract

We present a high-precision, large-sample luminosity function (LF) for the Galactic globular cluster M3. With a combination of ground-based and Hubble Space T elescope data we cover the entire radial extent of the cluster. The observed LF is well Ðtted by canonical standard stellar models from the red giant branch (RGB) tip to below the main-sequence turno † point. SpeciÐcally, neither the RGB LF bump nor subgiant branch LF indicate any breakdown in the standard models. On the main sequence we Ðnd evidence for a Ñat initial mass function and for mass segregation due to the dynamical evolution of the cluster.

We present extensive UBV R photometry of the Galactic globular cluster (GGC) NGC 6712 obtained with the ESO Very Large Telescope (VLT) which reaches down to 2 mag below the main-sequence turno † and allows us for the Ðrst time to determine the age of this cluster. By using the apparent luminosity of the zero age horizontal branch (ZAHB), and the stellar main-sequence (MS) V ZAHB \ 16.32^0.05 turn-o † (TO) magnitude we obtain (a value fully compatible with V TO \ 19.82^0.10, *V TO HB \ 3.5^0.1 that derived for other clusters), which suggests that, at an age of D12 Gyr, NGC 6712 is coeval with other GGC of similar metallicity.We derive interstellar reddening by comparing the position and morphology of the red giant branch (RGB) with a wide variety of reference clusters and Ðnd E(B[V ) \ 0.33^0.05, a value signiÐcantly lower than had been determined previously. Assuming this value for the reddening, we determine a true distance modulus of corresponding to a distance of D8 kpc. (m[M) 0 \ 14.55, We Ðnd a population of 108 candidate blue straggler stars (BSSs), surprisingly large when compared with the typical BSS content of other low-concentration clusters. Moreover, we detect a very bright blue star in the core of NGC 6712 that might be a post-AGB star. These results, combined with those already presented in two companion papers, strongly support the hypothesis that NGC 6712 was, at some early epoch of its history, much more massive and concentrated. The continued interaction with the bulge and the disk of the Galaxy has driven it toward dissolution, and what we now observe is nothing but the remnant core of a cluster that once was probably one of the most massive in the Galaxy.

In this paper, we present a B−V CCD colour–magnitude diagram (CMD) of the globular cluster M13 (NGC 6205), reaching down about 2 mag below the main sequence turn‐off, based on a new, independent photometric calibration. The most notable features of the CMD, including about 5500 stars, are (i) the long extension of the blue tail of the horizontal branch (HB), which can be traced down to V∼19 and (moreover) shows the presence of two gaps; and (ii) the so‐called bump on the red giant branch which is well‐detected at V=14.75±0.05, using both the differential and integral luminosity functions. No similar features have ever been detected in any previous ground‐based photometry of this cluster. A direct comparison between the CMD of M13 and M3, calibrated during the same run with the same apparatus, sets an upper limit of about 1.5 Gyr on their age difference, too small to explain the striking differences between their HB morphologies.