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 I and Calar 3) than with standard late-type M dwarfs. Our interpretation is that this is due to their selection by R-F-I-N 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 find some evidence that for a given spectral type dust formation is less prevalent in lower gravity objects. Ln addition to the spectral evidence the three stack objects whose parallax's have been measured show small tangential velocities which is a further 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. The main problem with the interpretation of finding young brown dwarfs from the plate stack is that their position in the HR diagram does not easily fit with the Pleiades sequence of brown dwarfs or the predictions of evolutionary models. The clear resolution of these issues probably requires further parallax measurements and follow-up observations of the new objects together with considerable work on high-temperature dust opacities and their inclusion in evolutionary models.