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We have obtained multi-fibre intermediate-resolution optical spectroscopy of94 photometric and proper motion selected low-mass star and brown dwarfcandidates in Upper Sco with AAT/AAOmega. We have estimated the spectral typesand measured the equivalent widths of youth and gravity diagnostic features toconfirm the spectroscopic membership of about 95% of the candidates extractedfrom 6.5 square degrees in Upper Sco. We also detect lithium in the spectrawith the highest signal-to-noise, consolidating our conclusions about theiryouth. Furthermore, we derive an estimate of our selections using spectroscopicdata obtained for a large number of stars falling into the instrument'sfield-of-view. We have estimated the effective temperatures and masses for eachnew spectroscopic member using the latest evolutionary models available forlow-mass stars and brown dwarfs. Combining the current optical spectroscopypresented here with near-infrared spectroscopy obtained for the faintestphotometric candidates, we confirm the shape and slope of our earlierphotometric mass function. The luminosity function drawn from the spectroscopicsample of 113 USco members peaks at around M6 and is flat at later spectraltype. We may detect the presence of the M7/M8 gap in the luminosity function asa result of the dust properties in substellar atmospheres. The mass functionmay peak at 0.2 Msun and is quite flat in the substellar regime. We observe apossible excess of cool low-mass brown dwarfs compared to IC 348 and theextrapolation of the field mass functions, supporting the original hypothesisthat Upper Sco may possess an excess of brown dwarfs. This result shows thatthe selection of photometric candidates based on five band photometry availablefrom the UKIDSS GCS and complemented partially by proper motions can lead to agood representation of the spectroscopic mass function (abridged).

Multi-fibre optical spectroscopy of low-mass stars and brown dwarfs in Upper Sco