Abstract

In this contribution we present the first results from a large observing campaign we are carrying out using VLT/Xshooter to obtain spectra of a large sample (~250 objects) of L-T transition brown dwarfs. Here we report the results based on the first ~120 spectra already obtained. The large sample, and the wide spectral coverage (300-2480 nm) given by Xshooter, will allow us to do a new powerful analysis, at an unprecedent level. By fitting the absorption lines of a given element (e.g. Na) at different wavelengths we can test ultracool atmospheric models and draw for the first time a 3D picture of stellar atmospheres at temperatures down to 1000K. Determining the atmospheric parameters (e.g. temperature, surface gravity and metallicity) of a big sample of brown dwarfs, will allow us to understand the role of these parameters on the formation of their spectra. The large number of objects in our sample also will allow us to do a statistical significant test of the birth rate and initial mass function predictions for brown dwarfs. Determining the shape of the initial mass function for very low mass objects is a fundamental task to improve galaxy models, as recent studies (van Dokkum & Conroy 2010) have shown that low-mass objects dominate in massive elliptical galaxies.