Aims. We aim to derive the chemical pattern of the young clusters 25 Orionis
and lambda Orionis through homogeneous and accurate measurements of elemental
abundances. Methods. We present FLAMES/UVES observations of a sample of 14
K-type targets in the 25 Ori and lambda Ori clusters; we measure their radial
velocities, in order to confirm cluster membership. We derive stellar
parameters and abundances of Fe, Na, Al, Si, Ca, Ti, and Ni using the code
MOOG. Results. All the 25 Ori stars are confirmed cluster members without
evidence of binarity; in lambda Ori we identify one non-member and one
candidate single-lined binary star. We find an average metallicity
[Fe/H]=-0.05+/-0.05 for 25 Ori, where the error is the 1sigma standard
deviation from the average. lambda Ori members have a mean iron abundance value
of 0.01+/-0.01. The other elements show close-to-solar ratios and no
star-to-star dispersion. Conclusions. Our results, along with previous
metallicity determinations in the Orion complex, evidence a small but
detectable dispersion in the [Fe/H] distribution of the complex. This appears
to be compatible with large-scale star formation episodes and initial
non-uniformity in the pre-cloud medium. We show that, as expected, the
abundance distribution of star forming regions is consistent with the chemical
pattern of the Galactic thin disk.Comment: Accepted by Astronomy and Astrophysics. 14 pages, 10 figures, 7
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