EDP Sciences
Free access
Volume 488, Number 1, September II 2008
Page(s) 167 - 179
Section Galactic structure, stellar clusters, and populations
DOI http://dx.doi.org/10.1051/0004-6361:20079049
Published online 16 June 2008

A&A 488, 167-179 (2008)
DOI: 10.1051/0004-6361:20079049

FLAMES spectroscopy of low-mass stars in the young clusters $\sigma$ Ori and $\lambda$ Ori

G. G. Sacco1, 2, E. Franciosini1, S. Randich3, and R. Pallavicini1, 4

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

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 $M_{\odot}$) stars in $\sigma$ Ori and $\lambda$ Ori respectively, using the multi-object optical spectrograph FLAMES at the VLT, with the high-resolution ( $R\sim 17\,000$) HR15N grating (6470-6790 Å). We used radial velocities, Li and H$\alpha$ to establish cluster membership and H$\alpha$ and other optical emission lines to analyze the accretion properties of members.
Results. We identified 65 and 45 members of the $\sigma$ Ori and $\lambda$ 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 $\sigma$ Ori cluster, finding values between 10-11 and $10^{-7.7}~M_{\odot}$ yr-1 and in 4 stars of the $\lambda$ Ori cluster, finding values between 10-11 and $10^{-10.1}~M_{\odot}$ 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 ($52\pm9$% and $78\pm16$%) than in $\lambda$ Ori ($28\pm8$% and $40\pm20$%).
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 $\lambda$ Ori cluster or to different ages of the cluster populations. Further observations are required to draw a definitive conclusion.

Key words: stars: formation -- stars: pre-main sequence -- stars: late-type -- Galaxy: open clusters and associations: individual: $\sigma$ Ori, $\lambda$ Ori

© ESO 2008