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| 2. |
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| 3. |
- Aoki, Wako, et al.
(author)
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LITHIUM ABUNDANCES OF EXTREMELY METAL-POOR TURNOFF STARS
- 2009
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 698:2, s. 1803-1812
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Journal article (peer-reviewed)abstract
- We have determined Li abundances for eleven metal-poor turnoff stars, among which eight have [Fe/H] < -3, based on LTE analyses of high-resolution spectra obtained with the High Dispersion Spectrograph on the Subaru Telescope. The Li abundances for four of these eight stars are determined for the first time by this study. Effective temperatures are determined by a profile analysis of H alpha and H beta. While seven stars have Li abundances as high as the Spite Plateau value, the remaining four objects with [Fe/H] < -3 have A(Li) = log(Li/H) + 12 less than or similar to 2.0, confirming the existence of extremely metal-poor (EMP) turnoff stars having low Li abundances, as reported by previous work. The average of the Li abundances for stars with [Fe/H]< -3 is lower by 0.2 dex than that of the stars with higher metallicity. No clear constraint on the metallicity dependence or scatter of the Li abundances is derived from our measurements for the stars with [Fe/H]< -3. Correlations of the Li abundance with effective temperatures, with abundances of Na, Mg, and Sr, and with the kinematical properties are investigated, but no clear correlation is seen in the EMP star sample.
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| 6. |
- Beers, Timothy C., et al.
(author)
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Broadband UBVRCIC photometry of horizontal-branch and metal-poor candidates from the HK and Hamburg/ESO surveys. I.
- 2007
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In: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 168:1, s. 128-139
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Journal article (peer-reviewed)abstract
- We report broadband UBV and/or BVRCIC CCD photometry for a total of 1857 stars in the thick-disk and halo populations of the Galaxy. The majority of our targets were selected as candidate field horizontal-branch or other A-type stars (FHB/ A, N = 576), or candidate low-metallicity stars (N = 1221), from the HK and Hamburg/ESO objective-prism surveys. Similar data for a small number of additional stars from other samples are also reported. These data are being used for several purposes. In the case of the FHB/ A candidates they are used to accurately separate the lower gravity FHB stars from various higher gravity A-type stars, a subsample that includes the so-called blue metal poor stars, halo and thick-disk blue stragglers, main-sequence A-type dwarfs, and Am and Ap stars. These data are also being used to derive photometric distance estimates to high-velocity hydrogen clouds in the Galaxy and for improved measurements of the mass of the Galaxy. Photometric data for the metal-poor candidates are being used to refine estimates of stellar metallicity for objects with available medium-resolution spectroscopy, to obtain distance estimates for kinematic analyses, and to establish initial estimates of effective temperature for analysis of high-resolution spectroscopy of the stars for which this information now exists.
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| 10. |
- Bessell, M.S., et al.
(author)
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On the oxygen abundance of HE0107-5240
- 2004
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In: Astrophysical Journal: Letters. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 612:1, s. 61-63
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Journal article (peer-reviewed)
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| 11. |
- Christlieb, Norbert
(author)
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Constraints on Big Bang Nucleosynthesis from observations of metal-poor stars
- 2008
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In: Journal of Physics G. - : IOP Publishing. - 0954-3899 .- 1361-6471. ; 35:1, s. 014001-
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Journal article (peer-reviewed)abstract
- An overview of observations of Li-6 and Li-7 in metal-poor stars is given, and possible consequences for Big Bang Nucleosynthesis (BBN) are discussed. Potential solutions for the discrepancy between the Li-7 abundance seen in metal-poor stars and the primordial abundance inferred from the value of the baryon density of the Universe, as determined from recent observations of the cosmic microwave background radiation with the Wilkinson microwave anisotropy probe, and standard BBN models, are discussed, and scenarios for the production of Li-6 in the early Universe are reviewed.
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| 12. |
- Christlieb, Norbert, et al.
(author)
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New Searches for R-Process Enhanced Stars
- 2001
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In: Proceedings of "Astrophysical Ages and Times Scales", ASP Conference Series. - : Astronomical Society of the Pacific, San Francisco, U.S.A.. - 1583810838 ; , s. 298-300
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Conference paper (other academic/artistic)abstract
- We discuss strategies for the detection of additional examples of highly r-process-enhanced, ultra-metal-poor stars, such as the two presently known examples of the class, CS~22892-052, and the newly discovered CS~31082-001. We expect that a quick, modera
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| 13. |
- De Jong, Roelof S., et al.
(author)
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4MOST : The 4-metre Multi-Object Spectroscopic Telescope project at preliminary design review
- 2016
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In: Ground-Based and Airborne Instrumentation for Astronomy VI. - : SPIE. - 0277-786X .- 1996-756X. - 9781510601956 ; 9908
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Conference paper (peer-reviewed)abstract
- We present an overview of the 4MOST project at the Preliminary Design Review. 4MOST is a major new wide-field, high-multiplex spectroscopic survey facility under development for the VISTA telescope of ESO. 4MOST has a broad range of science goals ranging from Galactic Archaeology and stellar physics to the high-energy physics, galaxy evolution, and cosmology. Starting in 2021, 4MOST will deploy 2436 fibres in a 4.1 square degree field-of-view using a positioner based on the tilting spine principle. The fibres will feed one high-resolution (R∼20,000) and two medium resolution (R∼5000) spectrographs with fixed 3-channel designs and identical 6k x 6k CCD detectors. 4MOST will have a unique operations concept in which 5-year public surveys from both the consortium and the ESO community will be combined and observed in parallel during each exposure. The 4MOST Facility Simulator (4FS) was developed to demonstrate the feasibility of this observing concept, showing that we can expect to observe more than 25 million objects in each 5-year survey period and will eventually be used to plan and conduct the actual survey.
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| 14. |
- Frebel, Anna, et al.
(author)
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Chemical abundance analysis of the extremely metal-poor star HE 1300+0157
- 2007
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 658:1, s. 534-552
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Journal article (peer-reviewed)abstract
- We present a detailed chemical abundance analysis of HE 1300+0157, a subgiant with Fe/H = -3.9. Froma high-resolution, high-S/N Subaru HDS spectrum we find the star to be enriched in C ([C/Fe](1D) similar to +1.4) and O ([O/Fe](1D) similar to +1.8). With the exception of these species, HE 1300+0157 exhibits an elemental abundance pattern similar to that found in many other very and extremely metal-poor stars. The Li abundance is lower than the Spite plateau value, in agreement with expectation for its evolutionary status. Of particular interest, no neutron-capture elements are detected in HE 1300+0157. This type of abundance pattern has been found by recent studies in several other metal-poor giants. We suggest that HE 1300+0157 is an unevolved example of this group of stars, which exhibit high C abundances together with low (or absent) abundances of neutron-capture elements (CEMP-no). Several potential enrichment scenarios are presented. The nondetection of neutron-capture elements including Sr, Ba, and Pb suggests that the carbon excess observed in HE 1300+0157 is not due to mass transfer across a binary system. Such a scenario is applied to carbon-rich objects with excesses of s-process elements. The normal observed Li abundance supports this interpretation. Most likely, the high levels of C and O were produced prior to the birth of this star. It remains unclear whether a single faint SN is responsible for its overall chemical pattern, or whether one requires a superposition of yields from a massive Population III object and a hypernova. These scenarios provide important information on the C production in the early universe and on the formation of C-rich stars in the early Galaxy.
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| 15. |
- Frebel, Anna, et al.
(author)
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Discovery of HE 1523-0901, a strongly r-process-enhanced metal-poor star with detected uranium
- 2007
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 660:2, s. L117-L120
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Journal article (peer-reviewed)abstract
- We present age estimates for the newly discovered, very r-process-enhanced metal-poor star HE 1523 - 0901 ([Fe/H] = -2.95) based on the radioactive decay of Th and U. The bright (V = 11.1) giant was found among a sample of bright metal-poor stars selected from the Hamburg/ESO Survey. From an abundance analysis of a high-resolution (R = 75,000) VLT/UVES spectrum, we find HE 1523 - 0901 to be strongly overabundant in r- process elements ([r/Fe] = 1.8). The abundances of heavy neutron-capture elements (Z > 56) measured in HE 1523 - 0901 match the scaled solar r- process pattern extremely well. We detect the strongest optical U line at 3859.57 angstrom. For the first time, we are able to employ several different chronometers, such as the U/Th, U/Ir, Th/ Eu, and Th/Os ratios to measure the age of a star. The weighted average age of HE 1523 - 0901 is 13.2 Gyr. Several sources of uncertainties are assessed in detail.
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| 16. |
- Frebel, Anna, et al.
(author)
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HE 1327-2326, an unevolved star with [Fe/H] < , -5.0. II. New 3D-1D corrected abundances from a very large telescope UVES spectrum
- 2008
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 684:1, s. 588-602
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Journal article (peer-reviewed)abstract
- We present a new abundance analysis of HE 1327-2326, which is currently the most iron-poor star, based on observational data obtained with the VLT Ultraviolet and Visual Echelle Spectrograph (UVES). We correct the one-dimensional (1D) LTE abundances for three-dimensional (3D) effects to provide an abundance pattern that supersedes previous works and should be used to observationally test current models of the chemical yields of the first-generation supernovae (SNe). Apart from confirming the 1D LTE abundances found in previous studies before accounting for 3D effects, we make use of a novel technique to apply the 3D 1D corrections for CNO which are a function of excitation potential and line strength for the molecular lines that comprise the observable CH, NH, and OH features. We find that the fit to the NH band at 33608 is greatly improved due to the application of the 3D-1D corrections. This may indicate that 3D effects are actually observable in this star. We also report the first detection of several weak Ni lines. The cosmologically important element Li is still not detected, the new Li upper limit is extremely low, A(Li) < , 0: 62, and in stark contrast with results not only from the Wilkinson Microwave Anisotropy Probe (WMAP) but also from other metal-poor stars. We also discuss how the new corrected abundance pattern of HE 1327-2326 is being reproduced by individual and integrated yields of SNe.
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| 17. |
- Goswami, Aruna, et al.
(author)
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A high-resolution spectral analysis of three carbon-enhanced metal-poor stars
- 2006
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 372:1, s. 343-356
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Journal article (peer-reviewed)abstract
- We present results of an analysis of high-resolution spectra (R similar to 50 000), obtained with the Subaru Telescope High Dispersion Spectrograph, of two carbon-enhanced metal-poor (CEMP) stars selected from the Hamburg/European Southern Observatory prism survey, HE 1305+0007 and HE 1152-0355, and of the classical CH star HD 5223. All these stars have relatively low effective temperatures (4000-4750 K) and high carbon abundances, which result in the presence of very strong molecular carbon bands in their spectra. The stellar atmospheric parameters for these stars indicate that they all have surface gravities consistent with a present location on the red giant branch, and metallicities of [Fe/H] = -2.0 (HE 1305+0007, HD 5223) and [Fe/H] = -1.3 (HE 1152-0355). In addition to their large enhancements of carbon ([C/Fe] = +1.8, +1.6, and +0.6, respectively), all three stars exhibit strong enhancements of the s-process elements relative to iron. HE 1305+0007 exhibits a large enhancement of the third-peak s-process element, lead, with [Pb/Fe] = +2.37, as well as a high abundance of the r-process element europium, [Eu/Fe] = +1.97. The second-peak s-process elements, Ba, La, Ce, Nd and Sm, are found to be more enhanced than the first-peak s-process elements Zr, Sr and Y. Thus, HE 1305+0007 joins the growing class of the so-called 'Lead stars', and also the class of objects that exhibit the presence of both r- and s-process elements, the CEMP-r/s stars. The large enhancements of neutron-capture (n-capture) elements exhibited by HE 1152-0355 and HD 5223 are more consistent with the abundance patterns generally noticed in CH stars, essentially arising from pure s-process nucleosynthesis. The elemental abundance distributions observed in these stars are discussed in light of existing theories of CH star formation, as well as the suggested formation scenarios of the CEMP-r/s group.
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| 18. |
- J.G., Cohen, et al.
(author)
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Abundances In Very Metal-Poor Dwarf Stars
- 2004
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In: Astrophysical Journal. - 0004-637X. ; 603:2, s. 1107-1135
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Journal article (pop. science, debate, etc.)abstract
- We discuss the detailed composition of 28 extremely metal-poor (EMP) dwarfs, 22 of which are from the Hamburg/ESO Survey (HES), based on Keck echelle spectra. Our sample has a median [Fe/H] of -2.7 dex, extends to -3.5 dex, and is somewhat less metal-poor than was expected from [Fe/H](HK, HES) determined from low-resolution spectra. Our analysis supports the existence of a sharp decline in the distribution of halo stars with metallicity below [Fe/H]=-3.0 dex. So far no additional turnoff stars with [Fe/H]<-3.5 have been identified in our follow-up efforts. For the best-observed elements between Mg and Ni, we find that the abundance ratios appear to have reached a plateau, i.e., [X/Fe] is approximately constant as a function of [Fe/H], except for Cr, Mn, and Co, which show trends of abundance ratios varying with [Fe/H]. These abundance ratios at low metallicity correspond approximately to the yield expected from Type II supernovae (SNe) with a narrow range in mass and explosion parameters; high-mass Type II SN progenitors are required. The dispersion of [X/Fe] about this plateau level is surprisingly small and is still dominated by measurement errors rather than intrinsic scatter. These results place strong constraints on the characteristics of the contributing SNe. The dispersion in neutron-capture elements and the abundance trends for Cr, Mn, and Co are consistent with previous studies of evolved EMP stars. We find halo-like enhancements for the α-elements Mg, Ca, and Ti, but solar Si/Fe ratios for these dwarfs. This contrasts with studies of EMP giant stars, which show Si enhancements similar to other α-elements. Sc/Fe is another case where the results from EMP dwarfs and from EMP giants disagree; our Sc/Fe ratios are enhanced compared to the solar value by ~0.2 dex. Although this conflicts with the solar Sc/Fe values seen in EMP giants, we note that α-like Sc/Fe ratios have been claimed for dwarfs at higher metallicity. Two dwarfs in the sample are carbon stars, while two others have significant C enhancements, all with 12C/13C ~7 and with C/N between 10 and 150. Three of these C-rich stars have large enhancements of the heavy neutron capture elements, including lead, which implies a strong s-process contribution, presumably from binary mass transfer; the fourth shows no excess of Sr or Ba.
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| 19. |
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| 20. |
- Lucatello, S., et al.
(author)
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The frequency of carbon-enhanced metal-poor stars in the galaxy from the HERES sample
- 2006
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 652:1, s. L37-L40
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Journal article (peer-reviewed)abstract
- We estimate the frequency of carbon-enhanced metal-poor (CEMP) stars among very metal-poor stars, based on an analysis of 349 stars with available high-resolution spectra observed as part of the Hamburg/ESO R-process Enhanced Star (HERES) survey. We obtain that a lower limit of 21% +/- 2% of stars with [Fe/H] <= -2.0 exhibit [C/Fe] >= +1.0. These fractions are higher than those that have been reported by recent examinations of this question, based on substantially smaller samples of stars. We discuss the source of this difference and suggest that in order to take into account effects that result in a decrease of surface carbon abundance with advancing evolution, a definition of CEMP stars based on a [C/Fe] cutoff that varies as a function of luminosity is more appropriate. We discuss the likely occurrence of dilution and mixing for many CEMP stars, which, if properly accounted for, would increase this fraction still further.
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| 21. |
- Norris, John E., et al.
(author)
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HE 0557-4840 : Ultra-metal-poor and carbon-rich
- 2007
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 670:1, s. 774-788
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Journal article (peer-reviewed)abstract
- We report the discovery and high-resolution, high-S/N spectroscopic analysis of the ultra-metal-poor red giant HE 0557-4840, which is the third most heavy-element-deficient star currently known. Its atmospheric parameters are Teff=4900 K, logg=2.2, and [Fe/H]=-4.75. This brings the number of stars with [Fe/H]<-4.0 to three, and the discovery of HE 0557-4840 suggests that the metallicity distribution function of the Galactic halo does not have a ``gap'' between [Fe/H]=-4.0, where several stars are known, and the two most metal-poor stars, at [Fe/H]~-5.3. HE 0557-4840 is carbon rich ([C/Fe]=+1.6), a property shared by all three objects with [Fe/H]<-4.0, suggesting that the well-known increase of carbon relative to iron with decreasing [Fe/H] reaches its logical conclusion (ubiquitous carbon richness) at lowest abundance. We also present abundances (nine) and limits (nine) for a further 18 elements. For species having well-measured abundances or strong upper limits, HE 0557-4840 is ``normal'' in comparison with the bulk of the stellar population at [Fe/H]~-4.0, with the possible exception of Co. We discuss the implications of these results for chemical enrichment at the earliest times, in the context of single- (``mixing and fallback'') and two-component enrichment models. While neither offers a clear solution, the latter appears closer to the mark. Further data are required to determine the oxygen abundance and improve that of Co, and hence more strongly constrain the origin of this object. Based on observations collected at ANU's 2.3 m telescope on Siding Spring Mountain, Australia, and European Southern Observatory, Paranal, Chile (proposal 276.D-5041).
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