Abstract. We present the complete photometric database and the color-magnitude diagrams for 74 Galactic globular clusters observed with the HST/WFPC2 camera in the F439W and F555W bands. A detailed discussion of the various reduction steps is also presented, and of the procedures to transform instrumental magnitudes into both the HST F439W and F555W flight system and the standard Johnson B and V systems. We also describe the artificial star experiments which have been performed to derive the star count completeness in all the relevant branches of the color magnitude diagram. The entire photometric database and the completeness function will be made available on the Web immediately after the publication of the present paper.

We have used WFPC2 to construct B, V color-magnitude diagrams of four metal-rich globular clusters, NGC 104 (47 Tuc), NGC 5927, NGC 6388, and NGC 6441. All four clusters have well populated red horizontal branches (RHB), as expected for their metallicity. However, NGC 6388 and 6441 also exhibit a prominent blue horizontal-branch (BHB) extension, including stars reaching as faint in V as the turnoff luminosity. This discovery demonstrates directly for the first time that a major population of hot horizontal-branch (HB) stars can exist in old, metal-rich systems. This may have important implications for the interpretation of the integrated spectra of elliptical galaxies.The cause of the phenomenon remains uncertain. We examine the possibility that NGC 6388 and 6441 are older than the other clusters, but a simple difference in age may not be sufficient to produce the observed distributions along the HB. The high central densities in NGC 6388 and 6441 suggest that the existence of the BHB tails might be caused by stellar interactions in the dense cores of these clusters, which we calculate to have two of the highest collision rates among globular clusters in the Galaxy. Tidal collisions might act in various ways to enhance loss of envelope mass and therefore populate the blue side of the HB. However, the relative frequency of tidal collisions does not seem large enough (compared to that of the clusters with pure RHBs) to account for such a drastic difference in HB morphology. While a combination of an age difference and dynamical interactions may help, prima facie the lack of a radial gradient in the BHB͞RHB star ratio seems to argue against dynamical effects playing a role.

We present distributions of colors of stars along the horizontal branch (HB) of the globular cluster NGC 2808, from Hubble Space Telescope Wide Field Planetary Camera 2 imaging in B, V, and an ultraviolet filter (F218W). This cluster's HB is already known to be strongly bimodal, with approximately equal-sized HB populations widely separated in the color-magnitude diagram. Our images reveal a long blue tail with two gaps, for a total of four nearly distinct HB groups. These gaps are very narrow, corresponding to envelope-mass differences of only 10.01 M J. This remarkable multimodality may be a signature of mass-loss processes, subtle composition variations, or dynamical effects; we briefly summarize the possibilities. The existence of narrow gaps between distinct clumps on the HB presents a challenge for models that attempt to explain HB bimodality or other peculiar HB structures. Subject headings: globular clusters: individual (NGC 2808)-stars: horizontal-branch 1 Based on observations with the NASA͞ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.

We present observations of the center of the Galactic globular cluster NGC 6273, obtained with the Hubble Space T elescope Wide Field Planetary Camera 2 as part of the snapshot program GO-7470. A BV color-magnitude diagram (CMD) for D28,000 stars is presented and discussed. The most prominent feature of the CMD, identiÐed for the Ðrst time in this paper, is the extended horizontal-branch blue tail (EBT) with a clear double-peaked distribution and a signiÐcant gap. The EBT of NGC 6273 is compared with the EBTs of seven other globular clusters for which we have a CMD in the same photometric system. From this comparison, we conclude that all the globular clusters in our sample with an EBT show at least one gap along the horizontal branch, which could have similar origins. A comparison with theoretical models suggests that at least some of these gaps may be occurring at a particular value of the stellar mass, common to a number of di †erent clusters. From the CMD of NGC 6273 we obtain a distance modulusWe also estimate an average reddening \ 16.27^0.20. though the CMD is strongly a †ected by di †erential reddening, with the relative reddening spanning a *E(B[V ) D 0.2 mag in the WFPC2 Ðeld. A luminosity function for the evolved stars in NGC 6273 is also presented and compared with the most recent evolutionary models.

Using the Faint Object Camera on the repaired Hubble Space Telescope, we have observed two fields in the globular cluster M15: the central density cusp, and a field at r = 20 ′′ . These are the highest-resolution images ever taken of this cluster's dense core, and are the first to probe the distribution of stars well below the main-sequence turnoff. After correction for incompleteness, we measure a logarithmic cusp slope (d log σ/d log r) of −0.70 ± 0.05 (1-sigma) for turnoff (∼ 0.8M ⊙ ) stars over the radial range from 0. ′′ 3 to 10 ′′ ; this slope is consistent with previous measurements. We also set an approximate upper limit of ∼ 1. ′′ 5 (90% confidence limit) on the size of any possible constant-surface-density core, but discuss uncertainties in this limit that arise from crowding corrections. We find that fainter stars in the cusp also have power-law density profiles: a mass group near 0.7M ⊙ has a logarithmic slope of −0.56 ± 0.05 (1-sigma) over the radial range from 2 ′′ to 10 ′′ . Taken together, the two slopes are not well matched by the simplest core-collapse or black-hole models. We also measure a mass function at r = 20 ′′ , outside of the central cusp. Both of the FOC fields show substantial mass segregation, when compared with a mass function measured with the WFPC2 at r = 5 ′ . In comparing the overall mass functions of the two FOC fields and the r = 5 ′ field, we find that the radial variation of the mass function is somewhat less than that predicted by a King-Michie model of the cluster, but greater than that predicted by a Fokker-Planck model taken from the literature.