Lithium and Hα in stars and brown dwarfs of σ Orionis ^*

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

Corresponding author: M. R. Zapatero Osorio, mosorio@gps.caltech.edu, mosorio@laeff.esa.es

Received: 24 August 2001
Accepted: 2 January 2002

Abstract

We present intermediate- and low-resolution optical spectra around Hα and Li i λ6708 Å for a sample of 25 low mass stars and 2 brown dwarfs with confirmed membership in the pre-main sequence stellar σ 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  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 Hα emission and the Li i resonance doublet in absorption, which is typical of very young ages. We find that our pseudo-equivalent widths of Hα and Li i (measured relative to the observed local pseudo-continuum formed by molecular absorptions) appear rather dispersed (and intense in the case of Hα) 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 ), displays variable Hα 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 λ6708 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 σ 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 σ Orionis cluster age can be set at 8 Myr, with a most likely value around 2–4 Myr.