University of Hamburg

About: University of Hamburg is a education organization based out in Hamburg, Germany. It is known for research contribution in the topics: Population & Laser. The organization has 45564 authors who have published 89286 publications receiving 2850161 citations. The organization is also known as: Hamburg University.

Carbon-enhanced Metal-poor Stars. I. Chemical Compositions of 26 Stars*

Abstract: The chemical compositions of 26 metal-poor stars that exhibit strong CH and/or C2 molecular bands are determined. Twenty-two stars in our sample satisfy our definition for carbon-enhanced metal-poor (CEMP) stars based on the carbon abundance ratio ([C/Fe]) and the evolutionary status. In addition, we measure Na abundances for nine known carbon-enhanced stars. Combining our new sample with the results of previous work, we investigate the abundance and evolutionary status of a total of 64 CEMP stars. The following results are obtained: (1) All but one of the 37 stars with [Fe/H] ≥ -2.6 exhibit large excesses of barium ([Ba/Fe] > +0.5), while the other 27 stars with lower metallicity exhibit a large scatter in their barium abundance ratios (-1.2 < [Ba/Fe] < +3.3). (2) A correlation between the carbon and barium abundance ratios ([C/Fe] and [Ba/Fe]) is found in Ba-enhanced objects (comprising 54 stars), suggesting that the origin of the observed carbon excess in Ba-enhanced stars is nucleosynthesis in AGB stars, where the main s-process occurs. (3) The majority of the Ba-enhanced stars have -1.0 < [C/H] < 0.0, and a clear cutoff exists at [C/H] ~ 0, which we take as the limit of carbon enrichment by metal-poor AGB stars. The [C/H] values of Ba-normal stars are relatively low, with a wide distribution. (4) The difference in the distributions of evolutionary status between Ba-enhanced and Ba-normal CEMP stars suggested by our previous work is not statistically confirmed by the present, enlarged sample. (5) Excesses of Na are found in stars with extremely large enhancements of C, N, and Ba, suggesting efficient production of this element by AGB nucleosynthesis. The implications of these results on the origins of carbon in CEMP stars are discussed.

A reconstruction of global agricultural areas and land cover for the last millennium

Abstract: [1] Humans have substantially modified the Earth's land cover, especially by transforming natural ecosystems to agricultural areas. In preindustrial times, the expansion of agriculture was probably the dominant process by which humankind altered the Earth system, but little is known about its extent, timing, and spatial pattern. This study presents an approach to reconstruct spatially explicit changes in global agricultural areas (cropland and pasture) and the resulting changes in land cover over the last millennium. The reconstruction is based on published maps of agricultural areas for the last three centuries. For earlier times, a country-based method is developed that uses population data as a proxy for agricultural activity. With this approach, the extent of cropland and pasture is consistently estimated since AD 800. The resulting reconstruction of agricultural areas is combined with a map of potential vegetation to estimate the resulting historical changes in land cover. Uncertainties associated with this approach, in particular owing to technological progress in agriculture and uncertainties in population estimates, are quantified. About 5 million km2 of natural vegetation are found to be transformed to agriculture between AD 800 and 1700, slightly more to cropland (mainly at the expense of forested area) than to pasture (mainly at the expense of natural grasslands). Historical events such as the Black Death in Europe led to considerable dynamics in land cover change on a regional scale. The reconstruction can be used with global climate and ecosystem models to assess the impact of human activities on the Earth system in preindustrial times.

Search for neutral MSSM Higgs bosons at LEP

Abstract: The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for the neutral Higgs bosons which are predicted by the Minimal Supersymmetric standard model (MSSM). The data of the four collaborations are statistically combined and examined for their consistency with the background hypothesis and with a possible Higgs boson signal. The combined LEP data show no significant excess of events which would indicate the production of Higgs bosons. The search results are used to set upper bounds on the cross-sections of various Higgs-like event topologies. The results are interpreted within the MSSM in a number of “benchmark” models, including CP-conserving and CP-violating scenarios. These interpretations lead in all cases to large exclusions in the MSSM parameter space. Absolute limits are set on the parameter cosβ and, in some scenarios, on the masses of neutral Higgs bosons.

Oscillations and interactions of dark and dark bright solitons in Bose Einstein condensates

Abstract: Solitons are encountered in a wide range of nonlinear systems, from water channels to optical fibres. They have also been observed in Bose–Einstein condensates, but only now have such ‘ultracold solitons’ been made to live long enough for their dynamical properties to be studied in detail.

A novel method for efficient amplification of whole hepatitis B virus genomes permits rapid functional analysis and reveals deletion mutants in immunosuppressed patients.

Abstract: Current knowledge of hepatitis B virus (HBV) sequence heterogeneity is based mainly on sequencing of amplified subgenomic HBV fragments. Here, we describe a method which allows sensitive amplification and simplified functional analysis of full-length HBV genomes with or without prior cloning. By this method, a large number of HBV genomes were cloned from sera of six immunosuppressed kidney transplant patients. Two size classes of HBV genomes, one 3.2 kb and another about 2.0 kb in size, were found in all patients. The genome population from one serum sample was studied in detail by size analysis of subgenomic PCR fragments and sequencing. Regions with deletions and insertions were mapped in the C gene and pre-S region. Up to 100% of HBV genomes in all other immunosuppressed patients also had deletions in the C gene. Our results demonstrate the potential of the established method for the structural and functional characterization of heterogeneous populations of complete virion-encapsidated HBV DNAs and suggest that HBV genomes with C gene deletions can have a selective advantage in immunosuppressed patients.