Optical spectroscopy of low-mass stars and brown dwarfs in the Pleiades

We present low-resolution spectra in the range 4600-9700 Å of a sample of low-mass stars and brown dwarf candidates in the Pleiades. We order the spectra into a temperature sequence, using a least-squares minimization procedure, and assign spectral types. The effectiveness of various proposed spectroscopic and photometric temperature indicators for low-mass stars is investigated. We find that the most effective spectroscopic temperature indicators are those based on molecular bands, especially TiO, rather than atomic transitions. We also find that the Pleiades objects may have surface gravities slightly lower than main-sequence objects of the same temperature, supporting their identification as pre-main-sequence cluster members. We investigate the effect of unresolved binarity on the temperature indicators, and conclude that ∼46 per cent of low-mass sytems in the Pleiades are multiple. Finally, we discuss how these results affect the status of our brown dwarf candidates, revising the number to 11 systems containing such candidates, of which three are single and the remainder binary.