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Register a new userMetallicity distribution of Omega Cen Red Giants based on the Stroemgren m1 metallicity index
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We adopted uvby Stroemgren photometry to investigate the metallicity distribution of Omega Cen Red Giant (RG) stars. We provided a new empirical calibration of the Stroemgren m1 = (v-b)-(b-y) metallicity index based on cluster stars. The new calibration has been applied to a sample of Omega Cen RGs. The shape of the estimated metallicity distribution is clearly asymmetric, with a sharp cut-off at low metallicities ([Fe/H] < -2.0) and a metal-rich tail up to [Fe/H] ~ 0.0. Two main metallicity peaks have been identified, around [Fe/H] ~ -1.9 and -1.3 dex, and a metal-rich shoulder at ~ 0.2 dex.
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We performed a new calibration of the Stroemgren metallicity index m1 based on the b-y color of cluster red giant stars. The current Metallicity-Index-Color relation is not linear in the color range 0.40 < b-y < 1.0, but provides iron abundances of cluster and field red giants with an accuracy of ~ 0.25 dex.
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).
In this paper, we present low resolution (R=500) near-infrared spectra for selected and serendipitous sources in six inner in-plane Galactic fields, with the aim of analysing the stellar content present. From the equivalent widths of the main features of the K band spectra (the NaI, CaI and CO bandheads) we have derived the metallicities of the sources by means of the empirical scale obtained by Ramirez et al. (2000) and Frogel et al. (2001) for luminous red giants. Our results show how the mean metallicity of the sample varies with Galactic longitude. We find two groups of stars, one whose [Fe/H] is similar to the values obtained for the bulge in other studies (Molla et al. 2000; Schultehis et al 2003), and a second one with a metallicity similar to that of the inner parts of the disc (Rocha-Pinto et al. 2006). The relative density of both groups of stars in our sample varies in a continuous way from the bulge to the disc. This could hint at the existence of a single component apart from the disc and bulge, running from l=7 to l=27 and able to transport disc stars inwards and bulge stars outwards, which could be the Galactic bar that has been detected in previous works as an overdensity of stars located at those galactic coordinates (Hammersley et al. 1994, 2000; Picaud et al. 2003).
We present a new calibration of the Stroemgren metallicity index m1 using red giant (RG) stars in 4 globular clusters (GCs:M92,M13,NGC1851,47Tuc) with metallicity ranging from [Fe/H]=-2.2 to -0.7, marginally affected by reddening (E(B-V)<0.04) and with accurate u,v,b,y photometry.The main difference between the new metallicity-index-color (MIC) relations and similar relations available in the literature is that we adopted the u-y/v-y colors instead of the b-y.These colors present a stronger sensitivity to effective temperature, and the MIC relations show a linear slope. The difference between photometric estimates and spectroscopic measurements for RGs in M71,NGC288,NGC362,NGC6397, and NGC6752 is 0.04+/-0.03dex (sigma=0.11dex). We also apply the MIC relations to 85 field RGs with metallicity raning from [Fe/H]=-2.4 to -0.5 and accurate reddening estimates. We find that the difference between photometric estimates and spectroscopic measurements is-0.14+/-0.01dex (sig=0.17dex). We also provide two sets of MIC relations based on evolutionary models that have been transformed into the observational plane by adopting either semi-empirical or theoretical color-temperature relations. We apply the semi-empirical relations to the 9 GCs and find that the difference between photometric and spectroscopic metallicities is 0.04+/-0.03dex (sig=0.10dex).A similar agreement is found for the sample of field RGs, with a difference of -0.09+/-0.03dex (sig=0.19dex).The difference between metallicity estimates based on theoretical relations and spectroscopic measurements is -0.11+/-0.03dex (sig=0.14dex) for the 9 GGCs and -0.24+/-0.03dex (sig=0.15dex) for the field RGs. Current evidence indicates that new MIC relations provide metallicities with an intrinsic accuracy better than 0.2dex.
We present results from the largest CaII triplet line metallicity study of Small Magellanic Cloud (SMC) field red giant stars to date, involving 3037 objects spread across approximately 37.5 sq. deg., centred on this galaxy. We find a median metallicity of [Fe/H]=-0.99+/-0.01, with clear evidence for an abundance gradient of -0.075+/-0.011 dex / deg. over the inner 5 deg. We interpret the abundance gradient to be the result of an increasing fraction of young stars with decreasing galacto-centric radius, coupled with a uniform global age-metallicity relation. We also demonstrate that the age-metallicity relation for an intermediate age population located 10kpc in front of the NE of the Cloud is indistinguishable from that of the main body of the galaxy, supporting a prior conjecture that this is a stellar analogue of the Magellanic Bridge. The metal poor and metal rich quartiles of our RGB star sample (with complementary optical photometry from the Magellanic Clouds Photometric Survey) are predominantly older and younger than approximately 6Gyr, respectively. Consequently, we draw a link between a kinematical signature, tentatively associated by us with a disk-like structure, and the upsurges in stellar genesis imprinted on the star formation history of the central regions of the SMC. We conclude that the increase in the star formation rate around 5-6Gyr ago was most likely triggered by an interaction between the SMC and LMC.
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