FLAMES spectroscopy of low-mass stars in the young clusters σ Ori and λ Ori

Aims. We performed a detailed membership selection and studied the accretion properties of low-mass stars in the two apparently very similar young (1 - 10 Myr) clusters sigma Ori and lambda Ori. Methods. We observed 98 and 49 low-mass (0.2 - 1.0 M-circle dot) stars in s Ori and. Ori respectively, using the multi-object optical spectrograph FLAMES at the VLT, with the high-resolution (R similar to 17 000) HR15N grating (6470 - 6790 angstrom). We used radial velocities, Li and Ha to establish cluster membership and Ha and other optical emission lines to analyze the accretion properties of members. Results. We identified 65 and 45 members of the s Ori and. Ori clusters, respectively, and discovered 16 new candidate binary systems. We also measured rotational broadening for 20 stars and estimated the mass accretion rates in 25 stars of the s Ori cluster, finding values between 10(-11) and 10(-7.7) M-circle dot yr(-1) and in 4 stars of the. Ori cluster, finding values between 10(-11) and 10(-10.1) M-circle dot yr(-1). Comparing our results with the infrared photometry obtained by the Spitzer satellite, we find that the fraction of stars with disks and the fraction of active disks is larger in the sigma Ori cluster (52 +/- 9% and 78 +/- 16%) than in. Ori (28 +/- 8% and 40 +/- 20%). Conclusions. The different disk and accretion properties of the two clusters could be due either to the effect of the high-mass stars and the supernova explosion in the. Ori cluster or to different ages of the cluster populations. Further observations are required to draw a definitive conclusion.