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

¹ INAF, Osservatorio Astronomico di Palermo, Piazza del Parlamento, 1, 90134 Palermo, Italy e-mail: sacco@astropa.inaf.it
² Consorzio COMETA, via S. Sofia, 64, 95123 Catania, Italy
³ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 50125 Firenze, Italy
⁴ INAF, Headquarters, viale del Parco Mellini 84, 00136 Roma, Italy

Received: 12 November 2007
Accepted: 17 May 2008

Abstract

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 σ Ori and λ Ori.

Methods. We observed 98 and 49 low-mass (0.2–1.0 ) stars in σ Ori and λ Ori respectively, using the multi-object optical spectrograph FLAMES at the VLT, with the high-resolution () HR15N grating (6470–6790 Å). We used radial velocities, Li and Hα to establish cluster membership and Hα and other optical emission lines to analyze the accretion properties of members.

Results. We identified 65 and 45 members of the σ Ori and λ 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 σ Ori cluster, finding values between 10^-11 and yr^-1 and in 4 stars of the λ Ori cluster, finding values between 10^-11 and 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 σ Ori cluster (% and %) than in λ Ori (% and %).

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 λ Ori cluster or to different ages of the cluster populations. Further observations are required to draw a definitive conclusion.