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تسجيل مستخدم جديدStromgren photometry of Galactic Globular Clusters. II Metallicity distribution of red giants in Omega Centauri
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A. Calamida
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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).
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In this letter, the results of our low-resolution spectroscopic survey for identifying the hydrogen-deficient (H-deficient) stars in the red giant sample of the globular cluster Omega Cen are reported. Spectral analyses were carried out on the basis
of the strengths of (0,0) MgH band and the Mg b triplet. In our sample, four giants were identified with weak/absent MgH bands in their observed spectra not as expected for their well determined stellar parameters. The Mg abundances for the program stars were determined from subordinate lines of the MgH band to the blue of the Mg b triplet, using the spectral synthesis technique. The derived Mg abundances for the program stars were as expected for the red giants of Omega Cen (Norris & Da Costa 1995), except for the four identified candidates. Determined Mg abundances of these four candidates are much lower than that expected for the red giants of Omega Cen, and are unacceptable based on the strengths of Mg b triplet in their observed spectra. Hence, the plausible reason for the weak/absent MgH bands in the observed spectra of these stars is a relatively lower abundance of hydrogen in their atmospheres. These giants may belong to the group of helium enriched red giants of Omega Cen.
We present abundances of several light, alpha, Fe-peak, and neutron-capture elements for 66 red giant branch (RGB) stars in the Galactic globular cluster Omega Centauri. Our observations lie in the range 12.0<V<13.5 and focus on the intermediate and
metal-rich RGBs. We find that there are at least four peaks in the metallicity distribution function at [Fe/H]=-1.75, -1.45, -1.05, and -0.75, which correspond to about 55%, 30%, 10%, and 5% of our sample, respectively. Additionally, the most metal-rich stars are the most centrally located. Na and Al are correlated despite exhibiting star-to-star dispersions of more than a factor of 10, but the distribution of those elements appears to be metallicity dependent and are divided at [Fe/H]~-1.2. About 40-50% of stars with [Fe/H]<-1.2 have Na and Al abundances consistent with production solely in Type II supernovae and match observations of disk and halo stars at comparable metallicity. The remaining metal-poor stars are enhanced in Na and Al compared to their disk and halo counterparts and are mostly consistent with predicted yields from >5 M_sun asymptotic giant branch (AGB) stars. At [Fe/H]>-1.2, more than 75% of the stars are Na/Al enhanced and may have formed almost exclusively from AGB ejecta. Most of these stars are enhanced in Na by at least 0.2 dex for a given Al abundance than would be expected based on normal globular cluster values. All stars in our sample are alpha-rich and have solar-scaled Fe-peak abundances. Eu does not vary extensively as a function of metallicity; however, [La/Fe] varies from about -0.4 to +2 and stars with [Fe/H]>-1.5 have [La/Eu] values indicating domination by the s-process. A quarter of our sample have [La/Eu]>+1 and may be the result of mass transfer in a binary system.
The helium-enriched (He-enriched) metal-rich red giants of Omega Centauri, discovered by Hema and Pandey using the low-resolution spectra from the Vainu Bappu Telescope (VBT) and confirmed by the analyses of the high-resolution spectra obtained from
the HRS-South African Large Telescope (SALT) for LEID 34225 and LEID 39048, are reanalysed here to determine their degree of He-enhancement/hydrogen-deficiency (H-deficiency). The observed MgH band combined with model atmospheres with differing He/H ratios are used for the analyses. The He/H ratios of these two giants are determined by enforcing the fact that the derived Mg abundances from the MgI lines and from the subordinate lines of the MgH band must be same for the adopted model atmosphere. The estimated He/H ratios for LEID 34225 and LEID 39048 are 0.15+/-0.04 and 0.20+/-0.04, respectively, whereas the normal He/H ratio is 0.10. Following the same criteria for the analyses of the other two comparison stars (LEID 61067 and LEID 32169), a normal He/H ratio of 0.10 is obtained. The He/H ratio of 0.15-0.20 corresponds to a mass fraction of helium (Z(He)=Y) of about 0.375-0.445. The range of helium enhancement and the derived metallicity of the program stars are in line with those determined for Omega Cen blue main-sequence stars. Hence, our study provides the missing link for the evolutionary track of the metal-rich helium-enhanced population of Omega Centuari. This research work is the very first spectroscopic determination of the amount of He-enhancement in the metal-rich red giants of Omega Centauri using the MgI and MgH lines.
We present wide-field and high-precision BV and Ca & Stromgren by photometry of omega Centauri, which represents one of the most extensive photometric surveys to date for this cluster. The member stars of omega Cen are well discriminated from foregro
und Galactic field stars in the hk [=(Ca-b)-(b-y)] vs. b-y diagram. The resulting cleaned color-magnitude diagram (CMD) has allowed us to obtain an accurate distribution of the red horizontal branch (HB) and the asymptotic giant branch stars. We confirm the presence of several red giant branches (RGBs) with the most metal-rich sequence well separated from other bluer metal-poor ones. Our population models suggest that four populations with different metallicities can reproduce the observed nature of the RGB. The HB distribution is also found to be consistent with the multiple stellar populations of the RGB. From our population models, we propose that the most metal-rich population is about 4 Gyr younger than the dominant metal-poor population, indicating that omega Cen was enriched over this timescale. We identify, for the first time, a continuous and slanting RGB bump in the CMD of omega Cen, which is due to the metallicity spread amongst the RGB stars. Our photometry also reveals a significant population of blue straggler stars. The discovery of several populations and the internal age-metallicity relation of omega Cen provides good evidence that omega Cen was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing at the present time.
High spectral resolution and high signal-to-noise ratio optical spectra of red giants in the globular cluster Omega Centauri are analysed for stellar parameters and chemical abundances of 15 elements including helium by either line equivalent widths
or synthetic spectrum analyses. The simultaneous abundance analysis of MgH and Mg lines adopting theoretical photospheres and a combination of He/H$-$ratios proved to be the only powerful probe to evaluate helium abundances of red giants cooler than 4400 K, wherein otherwise helium line transitions (He {scs I} 10830 and 5876 AA) present for a direct spectral line analysis. The impact of helium-enhanced model photospheres on the resulting abundance ratios are smaller than 0.15 dex, in agreement with past studies. The first indirect spectroscopic helium abundances measured in this paper for the most metal-rich cluster members reveal the discovery of seven He-enhanced giants ($Delta$$Y=+$0.15$pm$0.04), the largest such sample found spectroscopically to date. The average metallicity of $-$0.79$pm$0.06 dex and abundances for O, Na, Al, Si, Ca, Ti, Ni, Ba, and La are consistent with values found for the red giant branch (RGB-a) and subgiant branch (SGB-a) populations of Omega Centauri, suggesting an evolutionary connection among samples. The He-enhancement in giants is associated with larger $s$-process elemental abundances, which correlate with Al and anticorrelate with O. These results support the formation of He-enhanced, metal-rich population of Omega Centauri out of the interstellar medium enriched with the ejecta of fast rotating massive stars, binaries exploding as supernovae and asymptotic giant branch (AGB) stars.
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