2MASS J18082002-5104378: The brightest (V=11.9) ultra metal-poor star
  Context. The most primitive metal-poor stars are important for studying the
conditions of the early galaxy and are also relevant to big bang
nucleosynthesis. Aims. Our objective is to find the brightest (V<14) most
metal-poor stars. Methods. Candidates were selected using a new method, which
is based on the mismatch between spectral types derived from colors and
observed spectral types. They were observed first at low resolution with EFOSC2
at the NTT/ESO to obtain an initial set of stellar parameters. The most
promising candidate, 2MASS J18082002-5104378 (V=11.9), was observed at high
resolution (R=50 000) with UVES at the VLT/ESO, and a standard abundance
analysis was performed. Results. We found that 2MASS J18082002-5104378 is an
ultra metal-poor star with stellar parameters Teff = 5440 K, log g = 3.0 dex,
vt = 1.5 km/s, [Fe/H] = -4.1 dex. The star has [C/Fe]<+0.9 in a 1D analysis, or
[C/Fe]<=+0.5 if 3D effects are considered; its abundance pattern is typical of
normal (non-CEMP) ultra metal-poor stars. Interestingly, the star has a binary
companion. Conclusions. 2MASS J1808-5104 is the brightest (V=11.9) metal-poor
star of its category, and it could be studied further with even higher S/N
spectroscopy to determine additional chemical abundances, thus providing
important constraints to the early chemical evolution of our Galaxy.
