SPECTROSCOPY OF NEW SUBSTELLAR CANDIDATES IN THE PLEIADES - TOWARD A SPECTRAL SEQUENCE FOR YOUNG BROWN DWARFS

We present optical and near-infrared spectroscopy (600-1000 nn) of eig ht faint (I > 18) very red IR - I > 2.2) objects discovered in a recen t deep CCD survey of the Pleiades cluster by Zapatero-Osorio, Rebolo, & Martin. We compare them with reliable cluster members like PPI 15 an d Teide 1, and with several held very late type dwarfs (M4-M9.5), whic h were observed with similar instrumental configurations. Using pseudo continuum ratios, we classify the new substellar candidates in a spect ral sequence defined with reference to held stars of known spectral ty pes. We also reclassify PPI 15 and Teide 1 in a self-consistent way. T he likelihood of membership for the new candidates is assessed via the study of their photospheric features, Ha emission, radial velocity, a nd consistency of their spectral types and I-band magnitudes with know n cluster members. Four of the new substellar candidates are as late o r later than PP1 15 (M6.5), but only one, namely, Calar 3 (M8), clearl y meets all our membership criteria. It is indeed an object very simil ar to the brown dwarf Teide 1. Out of the eight new substellar candida tes, our ''cautious'' membership analysis leaves only Calar 3 as a Ple iades brown dwarf with a high level of confidence. This object, togeth er with Teide 1, allows one to compare the spectroscopic characteristi cs of Pleiades brown dwarfs with those of old very cool dwarfs. The ov erall spectral properties are similar, but there are slight difference s in the NaI doublet (818.3 M1, 819.5 nm), VO molecular band (740 nm, and some spectral ratios, which are probably related to lower surface gravity in the young Pleiades brown dwarfs than in field stars. Finall y, we propose a way of improving future CCD-based brown dwarf surveys by using narrowband near-IR pseudocontinuum filters.