We review the results on distances and absolute ages of Galactic globular clusters (GCs) obtained after the release of the Hipparcos catalog. Several methods aimed at the deÐnition of the Population II local distance scale are discussed, and their results compared, exploiting new results for RR Lyraes in the Large Magellanic Cloud (LMC). We Ðnd that the so-called short distance and long distance scales may be reconciled whether or not a consistent reddening scale is adopted for Cepheids and RR Lyrae variables in the LMC. Emphasis is given in the paper to the discussion of distances and ages of GCs derived using Hipparcos parallaxes of local subdwarfs. We Ðnd that the selection criteria adopted to choose the local subdwarfs, as well as the size of the corrections applied to existing systematic biases, are the main culprit for the di †erences found among the various independent studies that Ðrst used Hipparcos parallaxes and the subdwarf Ðtting technique. We also caution that the absolute age of M92 (usually considered one of the oldest clusters) still remains uncertain due to the lack of subdwarfs of comparable metallicity with accurate parallaxes. Distances and ages for the nine clusters discussed in a previous paper by Gratton et al. are rederived using an enlarged sample of local subdwarfs, which includes about 90% of the metal-poor dwarfs with accurate parallaxes (*n/n ¹ 0.12) in the whole Hipparcos catalog. On average, our revised distance moduli are decreased by 0.04 mag with respect to Gratton et al. The corresponding age of the GCs is t \ 11.5^2.6 Gyr, where the error bars refer to the 95% conÐdence range. The relation between the zero-age horizontal branch (ZAHB) absolute magnitude and metallicity for the nine program clusters turns out to be Thanks.12) to Hipparcos the major contribution to the total error budget associated with the subdwarf Ðtting technique has been moved from parallaxes to photometric calibrations, reddening, and metallicity scale. This total uncertainty still amounts to about^0.12 mag.We then compare the corresponding (true) LMC distance modulus mag with k LMC \ 18.64^0.12 other existing determinations. We conclude that at present the best estimate for the distance of the LMC is suggesting that distances from the subdwarf Ðtting method are D1 p too k LMC \ 18.54^0.03^0.06, long. Consequently, our best estimate for the age of the GCs is revised to Age \ 12.9^2.9 Gyr (95% conÐdence range). The best relation between ZAHB absolute magnitude and metallicity is M V (ZAHB) \ (0.18^0.09)([Fe/H] ] 1.5) ] (0.63^0.07). Finally, we compare the ages of the GCs with the cosmic star formation rate recently determined by studies of the Hubble Deep Field (HDF), exploiting the determinations of and provided by Type Ia supernovae surveys. We Ðnd that the epoch ) M \ 0.3 ) " \ 0.7 of formation of the GCs (at z D 3) matches well the maximum of the star formation rate for elliptical galaxies in the HDF as determined by Franceschini et al.

A catalog including a set of the most recent Color Magnitude Diagrams (CMDs) is presented for a sample of 61 Galactic Globular Clusters (GGCs). We used this data-base to perform an homogeneous systematic analysis of the evolved sequences (namely, Red Giant Branch (RGB), Horizontal Branch (HB) and Asymptotic Giant Branch (AGB)). Based on this analysis, we present: (1) a new procedure to measure the level of the ZAHB (V ZAHB ) and an homogeneous set of distance moduli obtained adopting the HB as standard candle; (2) an independent estimate for RGB metallicity indicators and new calibrations of these parameters in terms of both spectroscopic ([Fe/H] CG97 ) and global metallicity ([M/H], including also the α−elements enhancement). The set of equations presented can be used to simultaneously derive a photometric estimate of the metal abundance and the reddening from the morphology and the location of the RGB in the (V, B − V )-CMD. (3) the location of the RGB-Bump (in 47 GGCs) and the AGB-Bump (in 9 GGCs). The dependence of these features on the metallicity is discussed. We find that by using the latest theoretical models and the new metallicity scales the earlier discrepancy between theory and observations (∼ 0.4 mag) completely disappears.

Abstract.We have identified in the 2MASS database 693 known and candidate globular clusters in M 31. The 2MASS J, H, K magnitudes of these objects have been transformed to the same homogeneous photometric system of existing near infrared photometry of M 31 globulars, finally yielding J, H, K integrated photometry for 279 confirmed M 31 clusters, 406 unconfirmed candidates and 8 objects with controversial classification. Of these objects 529 lacked any previous estimate of their near infrared magnitudes. The newly assembled near infrared dataset has been implemented into a revised version of the Bologna Catalogue of M 31 globulars, with updated optical (UBVRI) photometry taken, when possible, from the most recent sources of CCD photometry available in the literature and transformed to a common photometric system. The final Revised Bologna Catalogue (available in electronic form) is the most comprehensive list presently available of confirmed and candidate M 31 globular clusters, with a total of 1164 entries. In particular, it includes 337 confirmed GCs, 688 GC candidates, 10 objects with controversial classification, 70 confirmed galaxies, 55 confirmed stars, and 4 H  regions lying within ∼3• from the center of the M 31 galaxy. Using the newly assembled database we show that the V − K color provides a powerful tool to discriminate between M 31 clusters and background galaxies, and we identify a sample of 83 globular cluster candidates, which is not likely to be contaminated by misclassified galaxies.

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We have made a detailed study of the color distribution of the main sequence of the globular cluster (GC) NGC 2808, based on new deep HST WFPC2 photometry of a field in the uncrowded outskirts of the cluster. The observed color distribution of main-sequence stars is not Gaussian and is wider than expected for a single stellar population, given our (carefully determined) measurement errors. About 20% of the sample stars are much bluer than expected and are most plausibly explained as a population having a much larger helium abundance than the bulk of the main sequence. Using synthetic color-magnitude diagrams based on new stellar models, we estimate that the helium mass fraction of these stars is Y $ 0:4. The newly found anomaly on the main sequence gives credence to the idea that GCs like NGC 2808 have undergone self-enrichment and that different stellar populations were born from the ejecta of the intermediate-mass asymptotic giant branch (AGB) stars of the first generation. The enhancement and spread of helium among the stars in NGC 2808 have recently been suggested as a simple way to explain the very peculiar morphology of its horizontal branch. We find that if in addition to the Y ¼ 0:40 stars, roughly 30% of the stars have Y distributed between 0.26-0.29, while 50% have primordial Y, this leads to a horizontal-branch morphology similar to that observed. In this framework, three main stages of star formation are identified, the first with primordial helium content Y ' 0:24, the second born from the winds of the most massive AGBs of the first stellar generation ($6-7 M ), with Y $ 0:4, and a third born from the matter ejected from less massive AGBs ($3.5-4.5 M ), with Y $ 0:26-0.29. There could have been a long hiatus (several times 10 7 yr), between the second and third generation in which no star formed in the protocluster. We suggest that during this period, star formation has been inhibited by the explosion of late Type II supernovae deriving from binary evolution.

Stars in globular clusters are generally believed to have all formed at the same time, early in the Galaxy's history. 'Blue stragglers' are stars massive enough that they should have evolved into white dwarfs long ago. Two possible mechanisms have been proposed for their formation: mass transfer between binary companions and stellar mergers resulting from direct collisions between two stars. Recently the binary explanation was claimed to be dominant. Here we report that there are two distinct parallel sequences of blue stragglers in M 30. This globular cluster is thought to have undergone 'core collapse', during which both the collision rate and the mass transfer activity in binary systems would have been enhanced. We suggest that the two observed sequences are a consequence of cluster core collapse, with the bluer population arising from direct stellar collisions and the redder one arising from the evolution of close binaries that are probably still experiencing an active phase of mass transfer.

We present high quality near infrared Color Magnitude Diagrams of 10 Galactic Globular Clusters (GCs) spanning a wide metallicity range (−2.15 < [F e/H] < −0.2). This homogeneous data-base has been used to perform a detailed analysis of the Red Giant Branch (RGB), adopting a variety of observables to describe its physical and chemical properties.First, a set of metallicity indicators have been measured, namely: (i) the RGB (J-K) and (V-K) colors at different magnitude levels; (ii) the RGB K magnitude at different colors; (iii) the RGB slope. For these parameters we present new calibrations in terms of both spectroscopic iron abundance and global metallicity, including the α-element enhancement. These relations can be used to derive a photometric estimate of the GC metal content from the RGB morphology and location.Second, the location in luminosity of the main RGB features (namely, the Bump and the Tip) and their dependence on metallicity have been studied, yielding quantitative observational relationships.Finally, adopting new transformations between the observational and theoretical quantities, the mean ridge lines for the clusters of our sample have been reported in

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