6 Summary and conclusions

We have presented intermediate- and low-resolution optical spectra between 6100Å and 7000Å, covering H$\alpha$ and Li I at $\lambda$6708Å, for a total of 25 low mass stars and 2 brown dwarfs members of the $\sigma$Orionis young star cluster. Spectral types have been derived and are found to be in the interval K6-M8.5, which corresponds to masses from 1.2$M_{\odot}$ down to 0.02$M_{\odot}$ after comparison with state-of-the-art evolutionary models (Baraffe et al. 1998; Chabrier et al. 2000). We have measured radial velocities and pseudo-equivalent widths (pEWs) of the H$\alpha$ and Li I atomic lines and find that all our targets show remarkable H$\alpha$emission and Li I $\lambda$6708Å in absorption. All radial velocities (except for one object) are consistent with membership in the $\sigma$Orionis cluster as well as in the Orion complex. The distribution of H$\alpha$ and Li I pEWs against spectral type exhibits a large scatter at classes cooler than M3.5. This phenomenon occurs at the approximate mass, $\sim$0.25$M_{\odot}$, where low mass stars are expected to become fully convective. Some of our objects also show emissions of He I and forbidden emission lines of [O I], [N II] and [S II], probably indicating accretion from circumstellar disks. We infer that the likely rate of $\sigma$Orionis low mass members resembling classical TTauri stars is in the range 30-40%, suggesting that the cluster is only a few Myr old.

We note the intriguing case of the coolest object in our sample, SOri45, an M8.5-type brown dwarf with a mass estimated at 0.02$M_{\odot}$ (Béjar et al. 1999). It has a very intense, variable H$\alpha$ emission and lithium in absorption. Our tentative detection of forbidden emission lines of [N II] and [S II] suggests that SOri45 may have a cool, surrounding disk from which it is accreting. This brown dwarf also displays a radial velocity that deviates significantly from the cluster mean velocity.

We have also presented very recent computations of Li I $\lambda$6708Å curves of growth for low gravities ( $\log\,g=4.0$and 4.5), cool temperatures ( $T_{\rm eff}$ =4000-2600K), and lithium abundances in the interval logN(Li)=1.0-3.4. The distribution of our observed Li I pEWs appears to be well reproduced by the theoretical pEWs computed for the cosmic lithium abundance of logN₀(Li)=3.1. This leads us to conclude that lithium has not yet been depleted in the $\sigma$Orionis cluster. Therefore, after comparison to various lithium depletion curves available in the literature, we impose an upper limit to the cluster age of 8Myr, while the most likely age is in the interval 2-4Myr.

Acknowledgements

We are grateful to I. Baraffe and colleagues for making electronic files of their evolutionary models available to us, and to Louise Good for correcting the English language used in the manuscript. We also thank the staff at McDonald Observatory, especially David Doss, for their helpful assistance. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. Partial financial support was provided by the Spanish DGES PB98-0531-C02-02. YP acknowledges partial financial support from Small Research Grant of American Astronomical Society. CAP acknowledges partial financial support from NSF (AST-0086321).