A Young Population of Brown Dwarfs

In a recent survey for faint red stars from a digital stack of Schmidt plates a number of candidate objects were identified. Parallax's for three of these objects have been reported showing them to have luminosities which interpreted within the available evolutionary models indicate them to be good brown dwarf candidates. The straightforward interpretation of their position in a colour magnitude diagram suggests that they are metal poor by 0.5-1.0 dex. However using standard spectral indices their spectra seem more consistent with the Pleiades brown dwarfs (PPL 15, Teide 1 and Calar 3) than with standard late-type M dwarfs. Our interpretation is that due to their selection by RF-IN colours which at values >3 preferentially selects objects with relatively low gravities. For late-type M dwarfs and brown dwarfs low gravities are expected to be a reliable indication of youth. We have constructed preliminary models for the number of brown dwarfs expected from the digital stack survey. We find that most of the objects expected are brown dwarfs less than 1 Gyr old and because of this it is important to make such models using a time-dependent scale height.