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A long sought after goal using chemical abundance patterns derived frommetal-poor stars is to understand the Galactic chemical evolution (GCE) and topin down the nature of the first stars (Pop III). Here, we use a sample of 14metal-poor stars observed with the high-resolution spectrograph PEPSI at theLBT to derive abundances of 32 elements (34 including limits). We presentwell-sampled abundance patterns for all stars obtained using localthermodynamic equilibrium (LTE) radiative transfer codes and 1D hydrostaticmodel atmospheres. It is currently well known that the assumptions of 1D andLTE may hide several issues, thereby introducing biases in our interpretationas to the nature of the first stars and the GCE. Hence, we use non-LTE (NLTE)and correct the abundances using 3D model atmospheres to present a physicallymore reliable pattern. In order to infer the nature of the first stars, wecompare unevolved, cool stars, enriched by a single event (`mono-enriched'),with a set of yield predictions to pin down the mass and energy of the Pop IIIprogenitor. To date, only few bona fide second generation mono-enriched starsare known. A simple x^2-fit may bias our inferred mass and energy just as muchas the simple 1D LTE abundance pattern, and we thus carried out our study withan improved fitting technique considering dilution and mixing. Our samplepresents Carbon Enhanced Metal-Poor (CEMP) stars, some of which are promisingtrue second generation (mono-enriched) stars. The unevolved, dwarf BD+09_2190shows a mono-enriched signature which, combined with kinematical data,indicates that it moves in the outer halo and likely has been accreted onto theMilky Way early on. The Pop III progenitor was likely of 25.5M and 0.6 10^51erg(foe)/19.2M and 1.5foe in LTE/NLTE. Finally, we explore the predominant donorand formation site of the rapid and slow neutron-capture elements. Abridged

Mono-enriched stars and Galactic chemical evolution -- Possible biases in observations and theory