Medium-resolution optical spectroscopy of young stellar and sub-stellar M-dwarfs in the Cha I dark cloud

We obtained medium-resolution spectra of the bona-fide brown dwarf Cha H$\alpha$ 1, the five brown dwarf candidates Cha H$\alpha$ 2 to 6, two additional late M-type brown dwarf candidates, all originally selected by H$\alpha$ emission, and four previously known T Tauri stars, all located in Chamaeleon I. The spectral types of our targets range down to M8. We show their spectra and also list their IJK magnitudes from DENIS. All objects have radial velocities consistent with kinematic membership to Cha I and show Li 6708\AA absorption. Our Cha I brown dwarf candidates with lithium are young or sub-stellar or both. Cha H$\alpha$ 1, 3, 6, and 7 are certainly brown dwarfs: Either they are as old or older than the Pleiades and should have burned all their original lithium if they were late M-type stars, or, if they are younger than the Pleiades, $\le 125$ Myrs, they are sub-stellar because of the young age and the late spectral types ($\ge$ M7), according to three different sets of evolutionary tracks and isochrones. To classify Cha H$\alpha$ 2, 4, 5, and 8 as either stellar or sub-stellar, evolutionary tracks of higher reliability are needed.