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 , \sim 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 .