François, P.P.FrançoisCaffau, E.E.CaffauBonifacio, P.P.BonifacioSpite, M.M.SpiteSpite, F.F.SpiteCayrel, R.R.CayrelChristlieb, N.N.ChristliebGallagher, A. J.A. J.GallagherKlessen, R.R.KlessenKoch, A.A.KochLudwig, H. -G.H. -G.LudwigMonaco, L.L.MonacoPlez, B.B.PlezSteffen, M.M.SteffenZAGGIA, SimoneSimoneZAGGIA2022-07-152022-07-1520180004-6361http://hdl.handle.net/20.500.12386/32503Context. Extremely metal-poor stars are keys to understand the early evolution of our Galaxy. The ESO large programme TOPoS has been tailored to analyse a new set of metal-poor turn-off stars, whereas most of the previously known extremely metal-poor stars are giant stars. Aims: Sixty five turn-off stars (preselected from SDSS spectra) have been observed with the X-shooter spectrograph at the ESO VLT Unit Telescope 2, to derive accurate and detailed abundances of magnesium, silicon, calcium, iron, strontium and barium. Methods: We analysed medium-resolution spectra (R ≃ 10 000) obtained with the ESO X-shooter spectrograph and computed the abundances of several α and neutron-capture elements using standard one-dimensional local thermodynamic equilibrium (1D LTE) model atmospheres. Results: Our results confirms the super-solar [Mg/Fe] and [Ca/Fe] ratios in metal-poor turn-off stars as observed in metal-poor giant stars. We found a significant spread of the [α/Fe] ratios with several stars showing subsolar [Ca/Fe] ratios. We could measure the abundance of strontium in 12 stars of the sample, leading to abundance ratios [Sr/Fe] around the Solar value. We detected barium in two stars of the sample. One of the stars (SDSS J114424-004658) shows both very high [Ba/Fe] and [Sr/Fe] abundance ratios (>1 dex). Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme ID 189.D-0165.Equivalent widths of the Fe lines are only, and Tables A.1 and A.2 are also available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/620/A187">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/620/A187</A>STAMPAenArticle10.1051/0004-6361/201834375https://www.aanda.org/articles/aa/full_html/2018/12/aa34375-18/aa34375-18.html2018A&A...620A.187FFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation::PE9_6 Stars and stellar systems