2008A&A...490.1135G

The young σ Orionis cluster is an important location for studying the formation and evolution of stars, brown dwarfs, and planetary-mass objects. Its metallicity, although it is a fundamental parameter, has not been well determined yet. We present the first determination of the metallicity of nine young late-type stars in σ Orionis. Using the optical and near-infrared broadband photometry available in the literature we derive the effective temperatures for these nine cluster stars, which lie in the interval 4300-6500K (1-3M_☉). These parameters are employed to compute a grid of synthetic spectra based on the code MOOG and Kurucz model atmospheres. We employ a χ²-minimization procedure to derive the stellar surface gravity and atmospheric abundances of Al, Ca, Si, Fe, Ni and Li, using multi-object optical spectroscopy taken with WYFFOS+AF2 at the William Herschel Telescope (λ/δλ∼7500). The average metallicity of the σ Orionis cluster is [Fe/H] = -0.02±0.09±0.13 (random and systematic errors). The abundances of the other elements, except lithium, seem to be consistent with solar values. Lithium abundances are in agreement with the ``cosmic'' ⁷Li abundance, except for two stars which show a log ε(Li) in the range 3.6-3.7 (although almost consistent within the error bars). There are also other two stars with log ε(Li)∼2.75. We derived an average radial velocity of the σ Orionis cluster of 28±4km/s. The σ Orionis metallicity is roughly solar.