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 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, and a standard abundance analysis was performed. Results. We found that 2MASS J18082002-5104378 is an ultrametal-poor star with stellar parameters T-eff = 5440 K, log g = 3.0 dex, v(t) = 1.5 kms(-1), [Fe/H] = -4.1 dex. The star has [C/Fe] < + 0.9 in a 1D analysis, or [C/Fe] less than or similar to + 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.