| 1. |
- Beers, Timothy C., et al.
(författare)
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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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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 168:1, s. 128-139
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Tidskriftsartikel (refereegranskat)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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| 2. |
- Frebel, Anna, et al.
(författare)
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Chemical abundance analysis of the extremely metal-poor star HE 1300+0157
- 2007
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 658:1, s. 534-552
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Tidskriftsartikel (refereegranskat)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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| 3. |
- Frebel, Anna, et al.
(författare)
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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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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 660:2, s. L117-L120
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Tidskriftsartikel (refereegranskat)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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| 4. |
- Goswami, Aruna, et al.
(författare)
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A high-resolution spectral analysis of three carbon-enhanced metal-poor stars
- 2006
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 372:1, s. 343-356
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Tidskriftsartikel (refereegranskat)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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| 5. |
- Norris, John E., et al.
(författare)
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HE 0557-4840 : Ultra-metal-poor and carbon-rich
- 2007
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 670:1, s. 774-788
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Tidskriftsartikel (refereegranskat)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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