The lithium test in young brown dwarf candidates

We present high-, mid-, and low-resolution spectroscopy of the field brown dwarf candidate PC 0025 + 0447 (M9.5) spanning a 4 yr baseline (1994-1998), The strength of the emission lines and the amount of optical veiling are v ery variable. Our spectra taken at an epoch of low veiling allow us to dete ct the lithium resonance line, which was not detected when the veiling was high. The presence of lithium proves that PC 0025 + 0447 is a substellar ob ject less massive than similar to 0.06 M. and younger than similar to 1 Gyr . We also present mid-resolution spectra of three brown dwarf candidates in star-forming regions, namely rho Oph 162349.8 - 242601 (M8.5), V410 Tau X3 (M6.5), and V410 Tau X6 (M6). Lithium is detected in all of them, supporti ng their very young age. Of these three objects, only the one in rho Oph is sufficiently cool to warrant an unambiguous substellar status. The spectro scopic characteristics that PC 0025 + 0447 and rho Oph 162349.8 - 242601 ha ve in common are moderately strong Li I lines (pseudo-equivalent width simi lar to 1 Angstrom), persistent H alpha emission with equivalent width great er than 50 Angstrom, and weaker K I and Na I lines than field dwarfs of the same spectral type.