E. Dalessandro

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. I. Overview of the Project and Detection of Multiple Stellar Populations

In this paper we describe a new UV-initiative HST project (GO-13297) that will complement the existing F606W and F814W database of the ACS Globular Cluster (GC) Treasury by imaging most of its clusters through UV/blue WFC3/UVIS filters F275W, F336W and F438W. This magic trio of filters has shown an uncanny ability to disentangle and characterize multiple-population (MP) patterns in GCs in a way that is exquisitely sensitive to C, N, and O abundance variations. Combination of these passbands with those in the optical also gives the best leverage for measuring helium enrichment. The dozen clusters that had previously been observed in these bands exhibit a bewildering variety of MP patterns, and the new survey will map the full variance of the phenomenon. The ubiquity of multiple stellar generations in GCs has made the formation of these cornerstone objects more intriguing than ever; GC formation and the origin of their MPs have now become one and the same problem. In the present paper we will describe the data base and our data reduction strategy, as well as the uses we intend to make of the final photometry, astrometry, and proper motions. We will also present preliminary color-magnitude diagrams from the data so far collected. These diagrams also draw on data from GO-12605 and GO-12311, which served as a pilot project for the present GO-13297.

The Hubble Space TelescopeUV Legacy Survey of Galactic Globular Clusters – V. Constraints on formation scenarios

We build on the evidence provided by our Legacy Survey of Galactic globular clusters (GC) to submit to a crucial test four scenarios currently entertain ed for the formation of multiple stellar generations in GCs. The observational constraints on multiple generations to be fulfilled are manifold, including GC specificity, ubiquity, variety, predominance, discreteness, supernova avoidance, p-capture processing, helium enrichment and mass budget. We argue that scenarios appealing to supermassive stars, fast rotating m assive stars and massive interactive binaries violate in an irreparable fashion two or more among such constraints. Also the scenario appealing to AGB stars as producers of the material for next generation stars encounters severe diffi culties, specifically concerning the mass budget problem an d the detailed chemical composition of second generation stars. We qualitatively explore ways possibly allowing one to save the AGB scenario, specifically appealing to a possible revis ion of the cross section of a critical reaction rate destroying sodium, or alternative ly by a more extensive exploration of the vast parameter space controlling the evolutionary behavior of AGB stellar models. Still, we cannot ensure success for these efforts and totally new scenarios may have to be invented to understand how GCs formed in the early Universe.

CECI N'EST PAS A GLOBULAR CLUSTER: THE METALLICITY DISTRIBUTION OF THE STELLAR SYSTEM TERZAN 5*

We present new determinations of the iron abundance for 220 stars belonging to the stellar system Terzan 5 in the Galactic bulge. The spectra have been acquired with FLAMES at the Very Large Telescope of the European Southern Observatory and DEIMOS at the Keck II Telescope. This is by far the largest spectroscopic sample of stars ever observed in this stellar system. From this dataset, a subsample of targets with spectra unaffected by TiO bands was extracted and statistically decontaminated from field stars. Once combined with 34 additional stars previously published by our group, a total sample of 135 member stars covering the entire radial extent of the system has been used to determine the metallicity distribution function of Terzan 5. The iron distribution clearly shows three peaks: a super-solar component at [Fe/H]

NO EVIDENCE OF CHEMICAL ANOMALIES IN THE BIMODAL TURNOFF CLUSTER NGC 1806 IN THE LARGE MAGELLANIC CLOUD

simeq0.25

FIRST EVIDENCE OF FULLY SPATIALLY MIXED FIRST AND SECOND GENERATIONS IN GLOBULAR CLUSTERS: THE CASE OF NGC 6362*

dex, accounting for 29% of the sample, a dominant sub-solar population at [Fe/H]

BLUE STRAGGLER MASSES FROM PULSATION PROPERTIES. I. THE CASE OF NGC 6541

simeq-0.30

NON-LOCAL THERMODYNAMICAL EQUILIBRIUM EFFECTS ON THE IRON ABUNDANCE OF ASYMPTOTIC GIANT BRANCH STARS IN 47 TUCANAE*

dex, corresponding to 62% of the total, and a minor (6%) metal-poor component at [Fe/H]

THE WFPC2 ULTRAVIOLET SURVEY: THE BLUE STRAGGLER POPULATION IN NGC 5824*

simeq-0.8

Chemical and kinematical properties of galactic bulge stars surrounding the stellar system Terzan 5

dex. Such a broad, multi-modal metallicity distribution demonstrates that Terzan 5 is not a genuine globular cluster but the remnant of a much more complex stellar system.

CONSTRAINING THE TRUE NATURE OF AN EXOTIC BINARY IN THE CORE OF NGC 6624

We have studied the chemical composition of NGC 1806, a massive, intermediate-age globular cluster that shows a double main sequence turnoff. We analyzed a sample of high-resolution spectra (secured with FLAMES at the Very Large Telescope) for eight giant stars, members of the cluster, finding an average iron content of [Fe/H] = –0.60 ± 0.01 dex and no evidence of intrinsic star-to-star variations in the abundances of light elements (Na, O, Mg, Al). Also, the (m {sub F814W}; m {sub F336W} – m {sub F814W}) color-magnitude diagram obtained by combining optical and near-UV Hubble Space Telescope photometry exhibits a narrow red giant branch, thus ruling out intrinsic variations of C and N abundances in the cluster. These findings demonstrate that NGC 1806 does not harbor chemically distinct sub-populations, at variance with what was found in old globular clusters. In turn, this indicates that the double main sequence turnoff phenomenon cannot be explained in the context of the self-enrichment processes usually invoked to explain the chemical anomalies observed in old globulars. Other solutions (i.e., stellar rotation, merging between clusters, or collisions with giant molecular clouds) should be envisaged to explain this class of globulars.