Extremely metal-poor stars: the need for UV spectra
  Our current understanding of the chemical evolution of the Universe is that a
first generation of stars was formed out of primordial material, completely
devoid of metals (Pop III stars). This first population of stars comprised
massive stars that exploded as supernovae disseminating the metals they
synthesised in the interstellar medium. These massive stars are long dead and
cannot be observed in the local Universe. Among very metal poor stars
(metallicity below -2.0) we expect to find the direct descendants of these
pristine metal factories. The chemical composition of these stars provides us
indirect information on the nature of the Pop III stars, their masses,
luminosities and mode of explosion. The constraints are stronger if the
chemical inventory is more complete, more chemical elements and isotopic ratios
are measured for each star. Unfortunately the lower the metallicity of the
star, the weaker the lines. Access to the space UV spectral range gives us
crucial supplementary information. To start with, it allows access to some very
strong Fe lines that may allow to measure the abundance of this element in
stars for which this was not possible from the ground-accessible UV spectra.
The number of such stars is steadily increasing. Next the UV range allows us to
measure elements that cannot be measured from ground-based spectra like P, Ge,
As, Se, Cd, Te, Lu, Os, Ir, Pt, Au. In addition it is fundamental for measuring
other elements that can be accessed from earth, but with great difficulty, like
C, S, Cu, Zn, Pb. The Hubble space telescope, with its limited collecting power
made this possible only for very few stars. Old metal poor stars are cool, of
spectral types F,G,K, and their UV flux is low. The availability of a UV high
resolution spectrograph fed by a large area space telescope will open an
unprecedented window on the early evolution of our Galaxy.
