| 1. |
- Feltzing, Sofia, et al.
(författare)
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Thin and Thick Disk Results for α-, r- and s-Process Elements
- 2004
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Ingår i: Carnegie Observatories Astrophysics Series, Vol. 4: Origin and Evolution of the Elements. ; 4
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Konferensbidrag (refereegranskat)abstract
- We present the first results from several ongoing studies of the stellara bundances, ages, and chemical trends in the long-lived dwarf stars in the solar neighborhood that belong to the thin and thick galactic disk.We confirm that the trends of α-elements in the thin and thick disk are distinct, and we also find that the thick disk show the typical signature of SN Ia contribution to the chemical enrichment of the interstellar gas out of which the later generations of thick disk stars formed. From a first study of the r- and s-process elements we reportpreliminary evidence that AGB stars also have contributed to the chemical enrichment in the thick disk. Furthermore, we investigate the individual as well as the collective ages of the thick disk stars andfind evidence of an overall age-metallicity relation in the thick disk.This has, to our best knowledge, not been shown before. All this indicate that the star formation in what is today's thick disk was extended over time and, at least for the stars that are today close tothe sun, the gas which the stars formed out of was well-mixed. These findings seem to rule out scenarios where the thick disk formed in a fast dissipative collapse.
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| 2. |
- J.G., Cohen, et al.
(författare)
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Abundances In Very Metal-Poor Dwarf Stars
- 2004
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Ingår i: Astrophysical Journal. - 0004-637X. ; 603:2, s. 1107-1135
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Tidskriftsartikel (populärvet., debatt m.m.)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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