Abstract
We report on the current status of an ongoing survey to select extremely metal poor (EMP) turn-off (TO) stars from Sloan Digital Sky Survey (SDSS) spectra, and determine their detailed chemical composition through high resolution follow-up. So far, 26 stars have been observed with UVES@VLT and X-SHOOTER@VLT, all but two showing an iron content below [Fe/H]=−3. Among them we detected the current record holder for the lowest total metallicity (SDSS J102915+172927, Z=10−5 Z⊙), four carbon-enhanced extremely metal poor objects (CEMP), as well as subsets with enhanced Ni and Mn. Lithium abundances or upper limits were derived, confirming the previously detected “meltdown” of the Spite plateau for metallicities below about [Fe/H]=−2.8. SDSS J102915+172927 in particular shows no detectable Li I 670.8 doublet, leading to an upper limit of A(Li)<1.1, hinting to an even deeper Li depletion in TO stars below [Fe/H]=−4. Spectroscopic follow-up is currently being prosecuted by the recently started ESO large program TOPoS, aiming to observe about 80 more EMP candidates.
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