Cool Stars 14 - Submitted Abstract # 90
This version created on 05 October 2006

FLAMES multi-object spectroscopy of low-mass stars in the "twin" young
clusters Sigma Ori and Lambda Ori


Giuseppe Germano Sacco, DSFA-Universita di Palermo
Roberto Pallavicini, INAF-Osservatorio Astronomico di Palermo
Sofia Randich, INAF-Osservatorio Astrofisico di Arcetri
Elena Franciosini, INAF-Osservatorio Astronomico di Palermo
Francesco Palla, INAF-Osservatorio Astrofisico di Arcetri

Multi-object spectroscopy of young clusters provides an excellent tool
to investigate critical aspects of star formation.  One of the main
open issues is the age distribution of stars.  Determination of
lithium abundances represents a secure method to derive ages of young
low-mass stars, allowing us to verify the existence of age spreads
inferred from HR diagrams.  The relationship between the mass
accretion rate through the disk and the mass of the central star, the
age and the enviroment constituites another critical issue.  In this
context, using FLAMES on VLT/UT2, we obtained spectra of 147 low-mass
candidate members of the two young (5 Myr) and coeval clusters Sigma
Ori e Lambda Ori.  Using radial velocities, Halpha and Li, we
identified 59 bona fide cluster members in the former and 45 in the
latter.  We derived Li abundances and found three Sigma Ori members
that show severe Li depletion, implying nuclear ages greater than
10-15 Myr, much larger than the cluster average age.  Nuclear ages for
Li-depleted stars are in reasonable agreement with isochronal ages
derived from the color-magnitude diagrams.  We measured both Halpha
equivalent width and the width at 10% of the peak (a good tracer of
accretion), findings that the fraction of Classical T-Tauri stars
(CTTs) and the accretion rates are larger in Sigma Ori than in Lambda
Ori.  Moreover, we study the accretion rate vs.  mass relation in 25
CTTs discoverd in the Sigma Ori cluster and compare our results with
those of the younger rho Ophiuchi region.

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