Andreas J. Korn

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The chemical composition of the most metal-deficient stars largely reflects the composition of the gas from which they formed. These old stars provide crucial clues to the star formation history and the synthesis of chemical elements in the early Universe. They are the local relics of epochs otherwise observable only at very high redshifts; if totally(More)
We report on a dedicated effort to identify and study metal-poor stars strongly enhanced in r-process elements ([r/Fe] > 1 dex; hereafter r-II stars), the Hamburg/ESO R-process Enhanced Star survey (HERES). Moderate-resolution (∼2 Å) follow-up spectroscopy has been obtained for metal-poor giant candidates selected from the Hamburg/ESO objective-prism survey(More)
We report a detailed abundance analysis for HE 0107−5240, a halo giant with [Fe/H] NLTE = −5.3. This star was discovered in the course of follow-up medium-resolution spectroscopy of extremely metal-poor candidates selected from the digitized Hamburg/ESO objective-prism survey. On the basis of high-resolution VLT/UVES spectra, we derive abundances for 8(More)
Non-LTE line formation calculations of Fe  are performed for a small number of reference stars to investigate and quantify the efficiency of neutral hydrogen collisions. Using the atomic model that was described in previous publications, the final discrimination with respect to hydrogen collisions is based on the condition that the surface gravities as(More)
The measurement of the cosmic microwave background has strongly constrained the cosmological parameters of the Universe. When the measured density of baryons (ordinary matter) is combined with standard Big Bang nucleosynthesis calculations, the amounts of hydrogen, helium and lithium produced shortly after the Big Bang can be predicted with unprecedented(More)
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 T eff = 4900 K, log g = 2.2, and [Fe/H] = –4.75. This brings the number of stars with [Fe/H] < –4.0 to three, and the discovery of(More)
We present a homogeneous photometric and spectroscopic analysis of 18 stars along the evolutionary sequence of the metal-poor globular cluster NGC 6397 ([Fe/H] ≈ −2), from the main-sequence turnoff point to red giants below the bump. The spectroscopic stellar parameters, in particular stellar-parameter differences between groups of stars, are in good(More)
Emission-lines of magnesium at 12 µm have been observed in the spectrum of Procyon. We reproduce the observed, disk-averaged line flux from Procyon (as well as the observed intensity profiles from the Sun) by calculating the line formation, relaxing the assumption of Local Thermodynamic Equilibrium. We – 2 – find that the lines in Procyon are formed in the(More)
NLTE line formation calculations of Fe I in the solar atmosphere are extended to include weak lines in the visual spectrum of the Sun. Previously established atomic models are used to discriminate between different ways of treating colli-sional interaction processes. As indicated by the analysis of strong Fe I lines, the influence of deviations from LTE in(More)
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