813 research outputs found
F.R. TOMLINSON: MENS EN WETENSKAPLIKE
No full textThe 1996 F.R. Tomlinson Memorial Lecture was delivered on 23rd May 1996 in Bloemfontein. The author, Prof D.J.G. Smith, chose the subject : F.R. Tomlinson: Man and ScientistTeaching/Communication/Extension/Profession,
F.R. Falconer's House
No full textPhotograph - A man in the backyard of F.R. Falconer's house, Athabasca, Alberta. Across the lane is the nurse's residence, later owned by Mrs. Dan Bilid
Discovery of Another Peculiar Radial Distribution of Blue Stragglers in Globular Clusters: The Case of 47 Tucanae
No full textWe have used the high-resolution Wide Field Planetary Camera (WFPC2) on the Hubble Space Telescope (HST) and wide-field ground-based observations to construct a catalog of blue straggler stars (BSSs) in the globular cluster 47 Tuc spanning the entire radial extent of the cluster. The BSS distribution is highly peaked in the cluster center, rapidly decreases at intermediate radii, and finally rises again at larger radii. The observed distribution closely resembles that discovered in M3 by Ferraro and coworkers. To date, complete BSS surveys covering the full radial extent (from HST for the center and wide-field CCD, ground-based observations for the exterior) have only been performed for these two clusters. Both show a bimodal radial distribution despite their different dynamical properties. BSS surveys covering the full spatial extent of more globular clusters are clearly required to determine how common bimodality is and what its consequences are for theories of BSS formation and cluster dynamic
The Discovery of an Anomalous Subgiant Branch in the Color-Magnitude Diagram of ω Centauri
No full textUsing deep high-resolution multiband images taken with the Very Large Telescope and the Hubble Space Telescope, we discovered a new anomalous sequence in the color-magnitude diagram of omega Cen. This feature appears as a narrow well-defined subgiant branch (SGB-a), which merges into the main sequence of the dominant cluster population at a magnitude significantly fainter than the cluster turnoff (TO). The simplest hypothesis assumes that the new feature is the extension of the anomalous red giant branch (RGB-a) metal-rich population discovered by Lee et al. and Pancino et al. However, under this assumption the interpretation of the SGB-a does not easily fit into the context of a self-enrichment scenario within omega Cen. In fact, its TO magnitude, shape, and extension are not compatible with a young metal-rich population, as required by the self-enrichment process. The TO level of the SGB-a suggests indeed an age as old as the main cluster population, further supporting the extracluster origin of the most metal-rich stars, as suggested by Ferraro, Bellazzini, & Pancino. Only accurate measurements of radial velocities and metal abundances for a representative sample of stars will firmly establish whether or not the SGB-a is actually related to the RGB-a and will finally shed light on the origin of the metal-rich population of omega Cen
The Intermediate-Age Globular Cluster NGC 1783 in the Large Magellanic Cloud
No full textWe present Hubble Space Telescope ACS deep photometry of the intermediate-age globular cluster NGC 1783 in the Large Magellanic Cloud. By using this photometric data set, we have determined the degree of ellipticity of the cluster (epsilon = 0.14 ± 0.03) and the radial density profile. This profile is well reproduced by a standard King model with an extended core (rc = 24.5'') and a low concentration (c = 1.16), indicating that the cluster has not experienced the collapse of the core. We also derived the cluster age using the Pisa Evolutionary Library isochrones, with three different amounts of overshooting (namely, ?os = 0.0, 0.10, and 0.25). From a comparison of the observed color-magnitude diagram and main-sequence luminosity function (LF) with the theoretical isochrones and LFs, we find that only models with the inclusion of some overshooting (?os = 0.10-0.25) are able to reproduce the observables. By using the magnitude difference ?V = 0.90 between the mean level of the He clump and the flat region of the SGB, we derive an age ? = 1.4 ± 0.2 Gyr
Chemical Composition of the Old Globular Clusters NGC 1786, NGC 2210, and NGC 2257 in the Large Magellanic Cloud
No full textThis paper presents the chemical abundance analysis of a sample of 18 giant stars in three old globular clusters in the Large Magellanic Cloud (LMC), NGC 1786, NGC 2210, and NGC 2257. The derived iron content is [Fe/H] = -1.75 ± 0.01 dex (σ = 0.02 dex), -1.65 ± 0.02 dex (σ = 0.04 dex), and -1.95 ± 0.02 dex (σ = 0.04 dex) for NGC 1786, NGC 2210, and NGC 2257, respectively. All the clusters exhibit similar abundance ratios, with enhanced values (~+0.30 dex) of [α/Fe], consistent with the Galactic halo stars, thus indicating that these clusters have formed from a gas enriched by Type II supernovae. We also found evidence that r-process is the main channel of production of the measured neutron capture elements (Y, Ba, La, Nd, Ce, and Eu). In particular, the quite large enhancement of [Eu/Fe] (~+0.70 dex) found in these old clusters clearly indicates a relevant efficiency of the r-process mechanism in the LMC environment
Red giant branch in near-infrared colour-magnitude diagrams - I. Calibration of photometric indices
No full textWe present new high-quality near-infrared photometry of 10 Galactic globular clusters spanning a wide metallicity range (-2.1
The distance of M 33 and the stellar population in its outskirts
No full textWe present deep V, I photometry of two 9.4 arcmin × 9.4 arcmin field in the outer regions of the M 33 galaxy. We obtain a robust detection of the luminosity of the Red Giant Branch Tip (ITRGB=20.72 ± 0.08) from which we derived a new estimate of the distance modulus of M 33, (m-M)0=24.64 ± 0.15, corresponding to a distance D=847 ± 60 kpc. By comparison of the color and magnitude of the observed Red Giant Branch stars with ridge lines of template globular clusters we obtained the photometric metallicity distribution of the considered fields in three different metallicity scales. The derived metallicity distributions are very similar over a range of distances from the galactic center 10 arcmin≤R≤33 arcmin, and are characterized by a well defined peak at [M/H]≃-0.7 ([Fe/H]≃ -1.0, in the Zinn & West scale) and a weak metal-poor tail reaching [M/H]⪅ -2.0. Our observations demonstrate that Red Giant Branch and Asymptotic Giant Branch stars have a radial distribution that is much more extended than the young MS stars associated with the star-forming disc
The “dynamical clock”: dating the internal dynamical evolution of star clusters with Blue Straggler Stars
No full textWe discuss the observational properties of a special class of objects (the so-called “Blue Straggler Stars”, BSSs) in the framework of using this stellar population as probe of the dynamical processes occurring in high-density stellar systems. Indeed, the shape of the BSS radial distribution and their level of central concentration are powerful tracers of the stage of dynamical evolution reached by the host cluster since formation. Hence, they can be used as empirical chronometers able to measure the dynamical age of stellar systems. In addition, the presence of a double BSS sequence in the color–magnitude diagram is likely the signature of the most extreme dynamical process occurring in globular cluster life: the core collapse event. Such a feature can, therefore, be used to reveal the occurrence of this process and, for the first time, even date it
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