  We present intermediate- and low-resolution optical spectra around Halpha and
LiI 6708 A 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 Msun 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 Halpha emission and LiI in absorption, which is
typical of very young ages. We find that our pseudo-equivalent widths appear
rather dispersed (and intense in the case of Halpha) 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, SOri 45 (M8.5, ~0.02 Msun), displays variable Halpha 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
LiI 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.
