Invited review abstract

The aim of the project is to determine as accurately as possible the shape of the mass function across the stellar/substellar boundary in the young (5 Myr) and nearby (d = 145 pc) Upper Sco association. We have obtained multi-fibre optical spectroscopy of 94 photometric low-mass star and brown dwarf candidates in Upper Sco with the AAOmega spectrograph on the Anglo-Australian telescope. We have derived the spectral types, measured the equivalent widths of Halpha and gravity-sensitive features, estimated the effective temperatures and masses for each new spectroscopic member. Combining the current optical spectroscopy presented here with near-infrared spectroscopy obtained for the faintest photometric candidates, we confirm the shape and slope of our earlier photometric mass function. The mass function peaks at 0.2 M$_{\odot}$ and is flat in the substellar regime, corroborating the original hypothesis that Upper Sco may possess an excess of brown dwarfs compared to other young regions.