Astronomy & Astrophysics

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Issue		A&A Volume 384, Number 3, March IV 2002
Page(s)		937 - 953
Section		Formation, structure and evolution of stars
DOI		10.1051/0004-6361:20020046

A&A 384, 937-953 (2002)
DOI: 10.1051/0004-6361:20020046

Lithium and H $\alpha$ in stars and brown dwarfs of $\sigma$ Orionis

M. R. Zapatero Osorio^{1, 2, 3}, V. J. S. Béjar⁴, Ya. Pavlenko⁵, R. Rebolo^{4, 6}, C. Allende Prieto⁷, E. L. Martín⁸ and R. J. García López^{4, 9}

¹ Division of Geological and Planetary Sciences, MS 150-21, California Institute of Technology, Pasadena, CA 91125, USA
² Mount Wilson Observatory, 740 Holladay Road, Pasadena, CA 91106, USA
³ Currently at: LAEFF-INTA, ESA Satellite Tracking Station, PO 50727, 28080 Madrid, Spain
⁴ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁵ Main Astronomical Observatory of Academy of Sciences of Ukraine, Golosiiv woods, Kyiv-127, 03680, Ukraine
⁶ Consejo Superior de Investigaciones Científicas, Madrid, Spain
⁷ McDonald Observatory and Department of Astronomy, University of Texas, Austin, TX 78712-1083, USA
⁸ Institute for Astronomy, Univ. of Hawaii at Manoa, Honolulu, HI 96822, USA
⁹ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain

(Received 24 August 2001 / Accepted 2 January 2002 )

Abstract
We present intermediate- and low-resolution optical spectra around H $\alpha$ and Li I $\lambda$ 6708 Å for a sample of 25 low mass stars and 2 brown dwarfs with confirmed membership in the pre-main sequence stellar $\sigma$ Orionis cluster. Our observations are intended to investigate the age of the cluster. The spectral types derived for our target sample are found to be in the range K6-M8.5, which corresponds to a mass interval of roughly 1.2-0.02 $M_{\odot}$ on the basis of state-of-the-art evolutionary models. Radial velocities (except for one object) are found to be consistent with membership in the Orion complex. All cluster members show considerable H $\alpha$ emission and the Li I resonance doublet in absorption, which is typical of very young ages. We find that our pseudo-equivalent widths of H $\alpha$ and Li I (measured relative to the observed local pseudo-continuum formed by molecular absorptions) appear rather dispersed (and intense in the case of H $\alpha$ ) for objects cooler than M3.5 spectral class, occurring at the approximate mass where low mass stars are expected to become fully convective. The least massive brown dwarf in our sample, S Ori 45 (M8.5, ~0.02 $M_{\odot}$ ), displays variable H $\alpha$ emission and a radial velocity that differs from the cluster mean velocity. Tentative detection of forbidden lines in emission indicates that this brown dwarf may be accreting mass from a surrounding disk. We also present recent computations of Li I $\lambda$ 6708 Å curves of growth for low gravities and for the temperature interval (about 4000-2600 K) of our sample. The comparison of our observations to these computations allows us to infer that no lithium depletion has yet taken place in $\sigma$ Orionis, and that the observed pseudo-equivalent widths are consistent with a cluster initial lithium abundance close to the cosmic value. Hence, the upper limit to the $\sigma$ Orionis cluster age can be set at 8 Myr, with a most likely value around 2-4 Myr.

Key words: circumstellar matter -- stars: abundances -- stars: late-type -- stars: low mass, brown dwarfs -- stars: pre-main sequence -- open clusters and associations: $\sigma$ Orionis

Offprint request: M. R. Zapatero Osorio, mosorio@gps.caltech.edu, mosorio@laeff.esa.es

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Lithium and H in stars and brown dwarfs of Orionis

Lithium and H $\alpha$ in stars and brown dwarfs of $\sigma$ Orionis