Chemical abundances of late-type pre-main sequence stars in the σ Orionis cluster

Context. The young sigma 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. Aims. We present the first determination of the metallicity of nine young late-type stars in sigma Orionis. Methods. 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 4300-6500 K (1-3 M-circle dot). These parameters are employed to compute a grid of synthetic spectra based on the code MOOG and Kurucz model atmospheres. We employ chi(2)-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 (lambda/delta lambda similar to 7500). Results. The average metallicity of the sigma 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 cosmicLi-7 abundance, except for two stars which show a log is an element of(Li) in the range 3.6-3.7 ( although almost consistent within the error bars). There are also other two stars with log is an element of(Li) similar to 2.75. We derived an average radial velocity of the sigma Orionis cluster of 28 +/- 4 km s(-1). Conclusions. The sigma Orionis metallicity is roughly solar.