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 Physics , 1998, Abstract: The globular cluster Omega Centauri (NGC 5139) is the most massive and brightest cluster in our Galaxy. It has also a moderately high mass to light ratio (3.6) and an anomalous flattening (0.83) for a globular cluster. This cluster is also very interesting because it is one of a few examples of globular clusters with a measurable spread in the metal abundance (see Da Costa & Willumsen 1981, Norris et al. 1996, and Suntzeff and Kraft 1996 and references therein) and then it offers a unique, big sample of nearby stars having all the same distance and reddening but showing different metallicity (and age ?) effects. A recent paper by Norris et al. (1997) shows also an interesting correlation between kinematics and metal abundance.
 Physics , 2001, Abstract: We explore the metallicity distribution function (MDF) of red giant stars in Omega Centauri from a catalogue of Washington M, T_2 and DDO51 photometry covering over 1.1 deg^2 outside the cluster core. Using updated calibrations of giant branch isometallicity loci in this filter system, photometric metallicities, guided by previously published spectroscopic abundances, are derived. Several methods are employed to correct the MDF for contamination by Galactic stars, including: (1) use of the surface gravity sensitivity of the (M-DDO51) color index to eliminate foreground dwarf stars, (2) radial velocities, and (3) membership probabilities from proper motions. The contamination-corrected MDF for Omega Cen shows a range of enrichment levels spanning nearly 2 dex in [Fe/H], and with peaks at [Fe/H]=-1.6, -1.2, and -0.9.
 Physics , 1999, Abstract: We have observed (with vby filters) a field north of the core of the most massive globular cluster in our galaxy, Omega Centauri. We have found a correlation of age and metallicity in a region which avoids the dense core and the inhomogeneous foreground dust emission shown by the IRAS satellite. Our observations show that the comparatively metal-rich stars (as defined by the (b-y) and m_1 colors) are younger than the metal-poor stars by at least 3 Gyr. This correlation of metallicity with age suggests that Omega Cen has enriched itself over a timescale of about 3 Gyr, and possibly longer. It is remarkable that ejecta from stellar winds combined with supernovae of type II failed to disperse the cluster's interstellar matter at an earlier epoch, but were captured by the cluster instead. Star formation would have ceased as type Ia supernovae dispersed the remaining interstellar matter. This work and other recent evidence suggests that Omega Cen could have been part of a small satellite galaxy in which all the activity occurred before it was captured by the Milky Way.
 Physics , 1996, DOI: 10.1086/117930 Abstract: We present spectroscopic abundances and radial velocities for giant stars in the Galactic globular cluster omega Centauri based on the CaII infrared triplet. Two samples of stars were observed: 234 stars at M_V = 1.25 on the lower giant branch at radial distances between 8 and 23arcmin, and 145 stars at M_V = -1.3 at radial distances between 3 and 22arcmin. Previous metallicity studies found a non-gaussian metallicity distribution containing a tail of metal-rich stars. We confirm these results except our unbiased cluster metallicity distributions are significantly narrower. They contain the following key features: (1) No very metal-poor stars, (2) a sudden rise in the metal-poor distribution to a modal [Fe/H] value of --1.70 consistent with an homogeneous metallicity unresolved at the 0.07 dex level, (3) a tail to higher metallicities with more stars than predicted by simple chemical evolution models, and (4) a weak correlation between metallicity and radius such that the most metal-rich stars are concentrated to the cluster core. The unresolved metal-weak tail implies that the gas out of which omega Cen formed was well-mixed up to the modal metallicity of the cluster. Therefore, omega Cen like other Galactic globular clusters, seems to have formed in a pre-enriched and homogenized (up to the modal metallicity) environment. The existence of a weak metallicity gradient supports the idea that omega Cen self-enriched, with the enriched gas sinking to the cluster center due to gas dissipation processes. We also note, however, that the metal-rich stars are more massive than the bulk of the stars in the cluster, and may have sunk to the center by dynamical mass segregation over the lifetime of the cluster.
 Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15839.x Abstract: To study the possible origin of the huge helium enrichment attributed to the stars on the blue main sequence of Omega Centauri, we make use of a chemical evolution model that has proven able to reproduce other major observed properties of the cluster, namely, its stellar metallicity distribution function, age-metallicity relation and trends of several abundance ratios with metallicity. In this framework, the key condition to satisfy all the available observational constraints is that a galactic-scale outflow develops in a much more massive parent system, as a consequence of multiple supernova explosions in a shallow potential well. This galactic wind must carry out preferentially the metals produced by explosive nucleosynthesis in supernovae, whereas elements restored to the interstellar medium through low-energy stellar winds by both asymptotic giant branch (AGB) stars and massive stars must be mostly retained. Assuming that helium is ejected through slow winds by both AGB stars and fast rotating massive stars (FRMSs), the interstellar medium of Omega Centauri's parent galaxy gets naturally enriched in helium in the course of its evolution.
 Physics , 2000, DOI: 10.1086/301304 Abstract: We present new measurements of the metallicity of 131 RR Lyrae stars in the globular cluster Omega Centauri, using the hk index of the Caby photometric system. The hk method has distinct advantages over Delta-S and other techniques in determining the metallicity of RR Lyrae stars, and has allowed us to obtain the most complete and homogeneous metallicity data to date for the RR Lyrae stars in this cluster. For RR Lyrae stars in common with the Delta-S observations of Butler et al. (1978) and Gratton et al. (1986), we have found that our metallicities, [Fe/H](hk), deviate systematically from their Delta-S metallicity, while our [Fe/H](hk) for well observed field RRab stars are consistent with previous spectroscopic measurements. We conclude that this is due to the larger errors associated with the previous Delta-S observations for this cluster. The Mv(RR) - [Fe/H] and period-shift - [Fe/H] relations obtained from our new data are consistent with the evolutionary models predicted by Lee (1991), confirming that the luminosity of RR Lyrae stars depends on evolutionary status as well as metallicity. Using the period - amplitude diagram, we have also identified highly evolved RRab stars in the range of -1.9 < [Fe/H] < -1.5, as predicted from the synthetic horizontal-branch models.
 Physics , 2011, DOI: 10.1111/j.1365-2966.2011.18963.x Abstract: Dust production is explored around 14 metal-poor ([Fe/H] = -1.91 to -0.98) giant stars in the Galactic globular cluster omega Centauri using new Spitzer IRS spectra. This sample includes the cluster's post-AGB and carbon stars and is thus the first representative spectral study of dust production in a metal-poor ([Fe/H] < -1) population. Only the more metal rich stars V6 and V17 ([Fe/H] = -1.08, -1.06) exhibit silicate emission, while the five other stars with mid-infrared excess show only a featureless continuum which we argue is caused by metallic iron dust grains. We examine the metallicity of V42, and find it is likely part of the metal-rich population ([Fe/H] ~ -0.8). Aside from the post-AGB star V1, we find no star from the cluster's bulk, metal-poor ([Fe/H] < -1.5) population - including the carbon stars - to be producing detectable amounts of dust. We compare the dust production to the stars' H-alpha line profiles obtained at the Magellan/Clay telescope at Las Campanas Observatory, finding pulsation shocking in the strongest pulsators (V6, V17 and V42), but evidence of outflow in all other stars. We conclude that the onset of dust production does not signify a fundamental change in the material leaving the star. Our data add to a growing body of evidence that metallic iron dominates dust production in metal-poor, oxygen-rich stars, but that dust is probably not the primary accelerant of winds in this mass-metallicity regime.
 Physics , 1998, DOI: 10.1086/311648 Abstract: The compilation of the metallicity values of $\omega$~Centauri stars led to detection of spatial asymmetry in the metallicity distribution of the bright giants. It was found that the most and least metal-poor objects are separated along galactic latitude. Statistical tests show that the chance occurrence of the phenomenon is very low. The subgiant and the RR~Lyrae samples do not show, however, similar segregation of the different metallicity objects. The physical parameters of the variables calculated by using empirical formulae indicate that the bulk of the RRab stars comprises a very homogeneous group regarding both their metallicity and mass values. These stars are most probably already in the evolved stage of their horizontal branch evolution.
 Physics , 2009, DOI: 10.1088/0004-637X/706/2/1277 Abstract: We present new intermediate-band Stroemgren photometry based on more than 300 u,v,b,y images of the Galactic globular cluster Omega Cen. Optical data were supplemented with new multiband near-infrared (NIR) photometry (350 J,H,K_s images). The final optical-NIR catalog covers a region of more than 20*20 arcmin squared across the cluster center. We use different optical-NIR color-color planes together with proper motion data available in the literature to identify candidate cluster red giant (RG) stars. By adopting different Stroemgren metallicity indices we estimate the photometric metallicity for ~4,000 RGs, the largest sample ever collected. The metallicity distributions show multiple peaks ([Fe/H]_phot=-1.73+/-0.08,-1.29+/-0.03,-1.05+/-0.02,-0.80+/-0.04,-0.42+/-0.12 and -0.07+/-0.08 dex) and a sharp cut-off in the metal-poor tail ([Fe/H]_phot<=-2 dex) that agree quite well with spectroscopic measurements. We identify four distinct sub-populations,namely metal-poor (MP,[Fe/H]<=-1.49), metal-intermediate (MI,-1.49<[Fe/H]<=-0.93), metal-rich (MR,-0.95<[Fe/H]<=-0.15) and solar metallicity (SM,[Fe/H]~0). The last group includes only a small fraction of stars (~8+/-5%) and should be confirmed spectroscopically. Moreover, using the difference in metallicity based on different photometric indices, we find that the 19+/-1% of RGs are candidate CN-strong stars. This fraction agrees quite well with recent spectroscopic estimates and could imply a large fraction of binary stars. The Stroemgren metallicity indices display a robust correlation with alpha-elements ([Ca+Si/H]) when moving from the metal-intermediate to the metal-rich regime ([Fe/H]>-1.7 dex).
 Physics , 1999, DOI: 10.1086/308375 Abstract: The globular cluster omega Centauri contains the largest known population of very hot horizontal branch (HB) stars. We have used the Hubble Space Telescope to obtain a far-UV/optical color-magnitude diagram of three fields in omega Cen. We find that over 30% of the HB objects are extreme'' HB or hot post-HB stars. The hot HB stars are not concentrated toward the cluster center, which argues against a dynamical origin for them. A wide gap in the color distribution of the hot HB stars appears to correspond to gaps found earlier in several other clusters. This suggests a common mechanism, probably related to giant branch mass loss. The diagram contains a significant population of hot sub-HB stars, which we interpret as the blue-hook'' objects predicted by D'Cruz et al. (1996a). These are produced by late He-flashes in stars which have undergone unusually large giant branch mass loss. omega Cen has a well-known spread of metal abundance, and our observations are consistent with a giant branch mass loss efficiency which increases with metallicity.
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